The Guild of Icarus: Aerospace Engineering and Aeronautical Club

[J. Wilhelm]

Welcome to the Guild of Icarus, I am your host J. Wilhelm under the assumed title of "Keeper of the Guild, Maestro Dio*." The Guild of Icarus is a club for all people interested in all matters related to Aeronautical and Space Flight, including engineering as well as aviation.

My interest in the subject arises from my studies at the University of Texas, where I obtained the degrees of BSc and MSc in Aerospace Engineering (Concentrations in Atmospheric Flight and Hypersonic Aerothermodynamics, respectively).

My Final Aircraft Design Project
A 73 passenger S/VTOL tilt-Propfan jetliner. Project Zarquon (1996)
(rough draft, measurements accurate, exact wing position undetermined)

Alas, I never have been able to practice my knowldege on account of devastating events in my life which were completely out of my control. I have now lost well over a decade of my life after I left school (not counting my 3rd attempt at a PhD in 2005). As I ponder now in my middle age whether I can even go back to this field, I now have decided to at least start exercising my neurons, and naturally I need an outlet. This is another reason for starting The Guild.

A third reason for starting the guild, is completeness. If I just pepper monologues on the subject all over Brassgoggles, you will be getting a very skewed image of what flight is. I am heavy on the science, but I should note that Pilots and Engineers effectively "speak two different languages," as designing a machine, and flying it are two completely different things. I have found out that we have at least a couple of pilots in the forum. I'd be very interested to see what they can bring to the table.

My only experience with practical flight is limited to a very old mechanical Link-GAT-1 flight simulator (Cessna 150) that I used to study flight performance at college back in the 1990s. Honestly, if you saw the flight simulator, you might think that there was a coin slot on the side and a sign which read "Rides 25¢" and that we stole it from a local supermarket.  

~ ~ ~

So let the games begin.... Make a suggestion for a topic! There will be no bars on the subject - the discussion can be as technical or whimsical as you want, just remember that if you have some knowledge others lack, then you assume the role of the teacher, and your responsibility is to make the subject tractable to all who read this thread. Do you have science you want to discuss? Bring it forth. Do you have nice fantasy illustrations you want to show? Bring them forth. This is a place for people who like flight.  It will all be appreciated.

Naturally lighter than air flying machines such as airships and balloons are included , but if you read the history on aeronautics, you may find out that the field is actually very old, as human beings have been trying to achieve flight for a very long time, indeed!

It is not at all impossible that heavier than air machines could have taken flight before the balloon and airship, though we, Steampunk, are rather obsessed with airships. I was very surprised as a young student to discover that even before heavier than air flight was achieved, scientists were already delving into areas of study that we only associate with technology from the 20th. C, more akin to what we see in the movie "The Right Stuff."

As it happens, this being a Steampunk forum, Victorian Era scientists were busy developing the science that people would use to develop supersonic flight. They could not implement the technology, because the truth is that the development of Fluid Mechanics as a field in Mechanical Engineering, was a slow, tortuous and haphazard affair, more like Frankenstein Monster of events, mostly made in the lab, that did not make much sense until after the turn of the 19th. C.  But all the tools for flight were already in place by the middle of the 19th. C.  I'll rant more about that the next time I return...

Schlieren Photography of a bullet in supersonic flight, by Ernst Mach, 1888
The photo shows a leading bow shock wave, and a trailing shock wave

*EDITED to keep consistency with the title used in Last Exile

[GCCC]

Tagging this to follow the thread.

[Miranda.T]

Dear Admiral Wilhelm,
I do like your VTOL design; in horizontal flight would the forward jets have been passive to avoid 'cooking' the fuselage? I know this is a question that has recently been raised for Skylon, and it may be that if that craft is ever made there will need to be some active cooling (i.e. dumping water over the skin) to avoid the sabre engines scorching its rear half.

Anyway, my thoughts on Steampunk supersonic flight. As shown in your picture, a bullet shape is nice and stable (if spinning) in flight, so let's go Jules Verne (or possibly Gerald Bull - https://en.wikipedia.org/wiki/Gerald_Bull) and do a sub-orbital by putting our passengers in a glorified shell. But what of the problem of squishing them to a thin red smear by the launch force? Well, I was watching a documentary a little while ago about one of Hitler's V-weapons that never saw service - a supergun that would have used staged explosions along the launch tube to accelerate the projectile. So, I'm thinking a massive launch tube (laid along the side of some convenient mountain), our intrepid aeronaut's capsule accelerated at multiple stages to keep down to say 4g max. I'll have to play with some numbers when I get a few moments to see if it might be feasible.

Yours,
Miranda.

[GCCC]

Regarding the above, I've been trying to ascertain how Verne's astronauts survived their (literal) moonshot. Would it have been possible (hypothetically, of course) for the astronaut's compartment to have remained stable/stationary through some sort of gyroscopic (or other) method while the outer shell did all the spinning, thus sparing the organic material inside from becoming wet little pancakes?

[J. Wilhelm]

Dear Admiral Wilhelm,
I do like your VTOL design; in horizontal flight would the forward jets have been passive to avoid 'cooking' the fuselage? I know this is a question that has recently been raised for Skylon, and it may be that if that craft is ever made there will need to be some active cooling (i.e. dumping water over the skin) to avoid the sabre engines scorching its rear half.

Anyway, my thoughts on Steampunk supersonic flight. As shown in your picture, a bullet shape is nice and stable (if spinning) in flight, so let's go Jules Verne (or possibly Gerald Bull - https://en.wikipedia.org/wiki/Gerald_Bull) and do a sub-orbital by putting our passengers in a glorified shell. But what of the problem of squishing them to a thin red smear by the launch force? Well, I was watching a documentary a little while ago about one of Hitler's V-weapons that never saw service - a supergun that would have used staged explosions along the launch tube to accelerate the projectile. So, I'm thinking a massive launch tube (laid along the side of some convenient mountain), our intrepid aeronaut's capsule accelerated at multiple stages to keep down to say 4g max. I'll have to play with some numbers when I get a few moments to see if it might be feasible.

Yours,
Miranda.

Fortunately,  there is enough distance between the engine cowls and the fuselage, especially at speed. Jet engines can get very close to the fuselage (remember the de Havilland Comet?).  I knew about the active hydrogen cooling for the skin, and the helium cooling for the compressor,  but never heard about issues with nozzle exhaust heat transfer to the fuselage... Do you have a specific source that details this design problem?  

The SABRE engine is basically a hydrogen + liquid oxygen rocket, with the exception that below a certain speed, you are compressing air from the environment to get the oxygen. Conventionally I'd think there is a huge difference between the heat transfer in the vicinity of a rocket nozzle exhaust and a turbo jet exhaust. In the rocket nozzle you're also having to deal with intense radiative heat transfer.

SABRE engine

For the VTOL tilt Propfan that was one of the concerns we needed to address early on.  But the real risk is to the tarmac upon takeoff   That is a problem my team didn't tackle, and we just assumed the landing facilities (pad) would be redesigned accordingly. We also assumed that the use of Propfans would reduce the hot jet exhaust.

A propfan is basically a supersonic propeller,  either directly attached to the turbine rotor (Unducted Fan),  or attached by gears like a regular turboprop. The idea was to "feather" the props at altitude when all of that thrust wasn't needed. The complications, of course,  included turbulence from the propellers,  but otherwise it helped you to extract as much heat from the turbine as possible and channel it to the propfan.  Cooler and more efficient thrust.

GE Unducted Fan

We had to comply with several US Federal Aviation Requirements including FAR 25 for jetliner,  and FAR 29 for heavy rotorcraft (helicopters). It needed to survive an engine failure (nearly impossible to achieve in tilt rotor craft), and I needed to come with a climb and descent path that satisfied both types of craft. I didn't care much for the airport facilities as I was too busy designing a flight path just to satisfy FAR statutes (no other team needed to do this but was a condition imposed by the professor as FAR were part of the test).  In the real world,  FAA requirements would have to be adjusted to tilt rotorcraft to make more sense.

This project was my baby, so to speak, and I had two other ten members with me leading the group.

~ ~ ~ ~ ~ ~


Actually I'm sure the ballistic missile approach has been suggested before, but in combination of rockets for more delicate payloads :

From the same Wiki article on Gerald Bull

The ultimate goal of the program was the Martlet-4, a three-stage 16.4" rocket that would be fired from a lengthened gun at Barbados and would reach orbit. In 1964 Donald Mordell was able to convince the Canadian government of the value of the HARP project as a low-cost method for Canada to enter the space-launch business, and arranged a joint Canadian-US funding program of $3 million a year for three years, with the Canadians supplying $2.5 million of that. Another 16.4" gun, mounted horizontally, was being tested at the Highwater range, and was extended by cutting the breech off the end of one gun and welding it to the end of another to produce a new gun over 110 feet long. The extension allowed the powder to be contained for a longer period of time, slowing down the acceleration and loads on the airframe, while also offering higher overall performance. Once the system had been tested at Highwater, a second barrel was shipped to Foul Bay, attached and strengthened with external bracing to allow it to be raised from the horizontal. This gun was extensively tested in 1965 and 66.

What you want is to achieve orbital speed without exceeding the human ability to take acceleration.  Rockets might still have dominated the thinking, though. It's one of those things that could have been done earlier, and the stability issue being practically solved by a cannon bore or helical rail.

[J. Wilhelm]

Regarding the above, I've been trying to ascertain how Verne's astronauts survived their (literal) moonshot. Would it have been possible (hypothetically, of course) for the astronaut's compartment to have remained stable/stationary through some sort of gyroscopic (or other) method while the outer shell did all the spinning, thus sparing the organic material inside from becoming wet little pancakes?

Rotational inertia will demand a much slower rotation rate for a larger diameter cylinder and mass, so I don't think that centripetal acceleration would reduce the occupants to wet little red pancakes. The pancake syndrome comes mostly from the linear acceleration needed to reach muzzle velocity. There would be however, severe problems with orientation, and balance in the human body,  and as you suggested,  you need to come up with a solution like an inner chamber perhaps.

Satellite Launch

[Miranda.T]

(snip)
Fortunately,  there is enough distance between the engine cowls and the fuselage, especially at speed. Jet engines can get very close to the fuselage (remember the de Havilland Comet?).  I knew about the active hydrogen cooling for the skin, and the helium cooling for the compressor,  but never heard about issues with nozzle exhaust heat transfer to the fuselage... Do you have a specific source that details this design problem?  
(snip)

Details are here

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150015818.pdf with a discussion here http://forum.nasaspaceflight.com/index.php?topic=36826.msg1415833#msg1415833.

As to the Vern inspired capsule, I was always just so impressed as to how close he was to the actual Apollo launches - pretty much the same launch location, his capsule was about the size of Apollo, the transit time to lunar orbit was correct; impressive prescience. As for the capsule spin, with clever design a little but of (simulated) microgravity could be beneficial, although as you say looking out of any windows would be very disorientating...

Yours,
Miranda.

[J. Wilhelm]

(snip)
Fortunately,  there is enough distance between the engine cowls and the fuselage, especially at speed. Jet engines can get very close to the fuselage (remember the de Havilland Comet?).  I knew about the active hydrogen cooling for the skin, and the helium cooling for the compressor,  but never heard about issues with nozzle exhaust heat transfer to the fuselage... Do you have a specific source that details this design problem?  
(snip)

Details are here

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150015818.pdf with a discussion here http://forum.nasaspaceflight.com/index.php?topic=36826.msg1415833#msg1415833.

As to the Vern inspired capsule, I was always just so impressed as to how close he was to the actual Apollo launches - pretty much the same launch location, his capsule was about the size of Apollo, the transit time to lunar orbit was correct; impressive prescience. As for the capsule spin, with clever design a little but of (simulated) microgravity could be beneficial, although as you say looking out of any windows would be very disorientating...

Yours,
Miranda.

Thank you.  I downloaded the AIAA paper (My technical communication professor back in college would have a field day with these workers' writing skills). I'll be taking a look at it later today.

It looks from the abstract that they're talking about the rocket plume, the performance of SABRE and it's effect on Skylon's fuselage.

"Underexpanded" in this case refers to the pressure and speed of the rocket nozzle exhaust.  A rocket nozzle is basically a converging-diverging duct where the speed of gases increases from subsonic at the combustion chamber, before reaching the "throat" of the duct (at which point the flow travels at the speed of sound), and subsequently accelerate throghout the "Bell",  which is what everyone sees in a rocket engine, such that the gases have reached supersonic speeds.

As the gases travel from the combustion chamber through the throat into the bell,  they're expanding (literally),  and in doing so,  they're accelerating.  There's a 1-1 relationship between diverging nozzle diameter (as function of distance along axis of nozzle) and the supersonic Mach number and pressure of the gases.  So you design the bell to expand the gases,  ideally to equalize the gas pressure to ambient pressure (giving you a particular Mach number at the outlet, so there's a given maximum bell diameter for a particular barometric "design"  altitude,  and at any other altitude the bell either exhausts gases over the ambient pressure (under expanded), or below ambient pressure (over expanded).  If underexpanded or overextended,  you get trailing expansion and compression (shock) waves, which waste energy into the environment.

In this case they're categorizing the exhaust as unexpectedly Underexpanded,  but mysteriously they're linking that to "favorable" lift to drag ratio of the body?   so their writing skills are not so good. The abstract sucks  

In reading the article I'll find out exactly what they mean  

[Miranda.T]

If I remember correctly, there was work ongoing to try to optimise the nozzle shape, but given the range of speeds and conditions that's clearly not a trivial task. At one point I think they were looking at a variable nozzle geometry but if I'm interpreting the current reports correctly that idea seems to have been dropped.

Yours,
Miranda.

[J. Wilhelm]

If I remember correctly, there was work ongoing to try to optimise the nozzle shape, but given the range of speeds and conditions that's clearly not a trivial task. At one point I think they were looking at a variable nozzle geometry but if I'm interpreting the current reports correctly that idea seems to have been dropped.

Yours,
Miranda.

This is just the nature of the beast, I'm afraid. I still have to find the time to read the article, but on the surface, and falling to classical theory, conventional bell (converging diverging) nozzles are naturally "tuned" to one pressure only and you usually design at which optimal altitude you'll get the cleanest output. Variable nozzles exist, but they're heavy.

My impression so far:

Superficially it sounds to me like they wanted to adjust the optimal design altitude on the rocket nozzles (which would be easy enough to fix), but it may turn out that that no amount of tinkering with the design altitude of the nozzle is good enough to prevent excessive heat transfer from the rocket plume to the fuselage, for the simple fact that the optimal design altitude range (cleanest output) is just very narrow.

Like a radio tuning to a radio station, which filters the radio frequencies to a narrow band and only one radio station passes through. Similarly, the rocket nozzle restricts the output pressure to a very narrow range with a single design altitude.  It's the nature of the beast.

I guess you could attempt to "widen" the peak performance range of the nozzles, to accommodate the fact that your fluid dynamics around the fuselage is a bit "off" ...  by way of "more favourable lift to drag ratios,"  which to me simply sounds like a very indirect, roundabout way of saying that the layers of air next to the fuselage are faster that expected, and thus thinner than expected (possibly a thinner boundary layer), thus allowing the rocket plumes to get closer to the fuselage. I won't know until I read more of the paper. The boundary layer over the fuselage is no "protection" against rocket plumes.

This is the result of the design geometry they chose. The side by side engine pod design is to blame here IMHO.  They need to re-design the engine pods AND re-position the engines.

The X-33 prototype by Skunk Works used an "aerospike" in the back, instead of a bell. If you're willing to sacrifice maximum performance at a particular altitude (that is at a particular ambient pressure), you can actually get a wider range of altitudes by - literally- turning the bell nozzle inside out.

Lockheed Martin Skunkworks X-33

In an aerospike, you let the atmosphere act like a "soft wall" to the expanding plume of gases, dowstream from the "throat" of the nozzle, using just a central cone or ramp to provide the "bell" curved shape to expand the gases (remember expansion <=> speeding up), and in doing so you get a lower maximum peak performance at design altitude, but a relatively high performance spread over a much wider range of altitudes (like the radio circuit having a "wide" tuning circuit and accepting several radio stations at once).  Such that the overall performance of the rocket is higher, from launch to the maximum design altitude.

X-33 Aerospike Engine
The multiple nozzles along the edge actually are the "throat" of the converging diverging nozzle
M~1 at that row of nozzles and the gases speed up as they hit the ramp.

The problem with aeropikes is that when it comes to exhaust plumes, they are much worse. The "soft air wall" around the plume does a very poor job of containing the plume itself and does not block radiative (infra-red) heat at all.  Aerospikes allow wider plumes, and thus the fuselage would get a harder hit. So aerospikes are not the solution for side by side engine pods.  

The engine needs to be placed centrally and in the back of the vehicle like on the X-33. Or if I ignore the concentric bleed design, and imagine converting the engine geometry to a planar orientation, taking the SABRE compressors and aligning them along the aerospike ramp, in rectangular engine pods, orienting the  aerospike ramps along the back such that they direct the plumes away from the fuselage (say in the "up/down" directions). That could greatly reduce the risk to the fuselage. There is a price to pay, as the rectangular geometry for supersonic expansion or compression ramps is always less efficient than conical ones.

I don't want to lose the lay reader on what we're talking about, though... we are reaching a critical point with a lot of physics and I haven't even read the article yet  

For those who are a bit lost, but would like to search, here is a "glossary" or "summary" of the physics subjects covered so far, which you can Google on the Internet:

Spoiler: ShowHide

1: Compressible Flow physics:

Expansion of gases (lower pressure, lower temperature) = speeding up flow, typ. using "Nozzles"
Compression of gases (higher pressure, higher temperature) = slowing down flow, typ. using "Diffusers"

2: Acceleration of gases to supersonic flow in "converging-diverging nozzles" (rocket engines):

Converging Nozzle = for accelerating subsonic flow (Happens at combustion chamber)
Throat = point where Mach = 1, if enough mass is rammed at throat ("choked nozzle"),
Diverging nozzle (bell) = for accelerating supersonic flow.

Normally for subsonic flow a converging (closing) nozzle speeds air), but the reason the supersonic nozzle first converges and then diverges is because once you reach a pint where M = 1, the air is being "thrown around so hard" that it actually starts yielding to the forces by expanding when you demand it (e.g. opening cone), and compressing when you demand it (e.g. closing cone). You can actually have supersonic air turn a sharp corner of more than 90 degrees quite easily.

Otherwise if M->0 like in very slow biplanes, or auto-mobiles, air behaves more like water, actually neither expanding nor compressing, with constant density ("Incompressible Flow")

For rocket bell nozzles: There is an equivalence Pressure <=> Mach number so outlet pressure determines Mach number and is tied to the nozzle outer diameter. The more you open the bell, the faster the flow accelerates and the lower the pressure becomes.

If outlet pressure is not the same as ambient pressure, then you get more shocks and "expansion-compression diamonds" that imply loss of energy and wider, less contained plumes of gases (the problem I assume they are dealing with in the paper - I have yet to read that).

3. General fluid physics:

Pressure drag: the distribution of pressure around your vehicle determines how much Lift and Pressure Drag you have.  You can't make Lift without making Drag. One implies the other. More lift=more drag.

Net Drag = Pressure Drag + Viscous Drag, Pressure Drag is generally bigger, but Viscous drag is dangerous because of frictional heat in high speed flows (read below).

Boundary layer: the air layers right next to the fuselage "stick" to the surface: ("no slip condition") so that the layer in direct contact with the body is not moving at all, and only in a certain thickness of air above the fuselage, these layers are moving, yet slower than the "free stream" flow away from the fuselage, and the layers inside the boundary layer are always are slipping, that is "shearing" between one another.

Shearing in boundary layer = friction between air layers => viscous drag => extra heat by friction.

All viscosity is "made apparent" ONLY inside the boundary layer.  Outside the boundary layer, you may assume air is inviscid. All viscous drag is generated in the boundary layer.

Also the boundary layer is the universal point of origin of turbulence. The boundary layer can be "tripped" leading to turbulence.

Turbulent Flow acts like "more viscous flow." It has an "apparent viscosity" which is higher than the natural viscosity of non-turbulent ("laminar") flow.  This means turbulence leads to more air layer shearing inside the boundary layer, which means more viscous drag. So generally, turbulence is not wanted, but it's nearly impossible to avoid in larger objects (depends on the scale of the fuselage. Larger objects are more likely to have turbulent boundary layers).

Sometimes though, turbulence is the "lesser of two evils" and you may want to trigger it on purpose (to fill a void in air which acts like a vacuum <=>  a technique to reduce pressure drag).

~ ~ ~

[Miranda.T]

(snip)
The engine needs to be placed centrally and in the back of the vehicle like on the X-33. Or if I ignore the concentric bleed design, and imagine converting the engine geometry to a planar orientation, taking the SABRE compressors and aligning them along the aerospike ramp, in rectangular engine pods, orienting the  aerospike ramps along the back such that they direct the plumes away from the fuselage (say in the "up/down" directions). That could greatly reduce the risk to the fuselage. There is a price to pay, as the rectangular geometry for supersonic expansion or compression ramps is always less efficient than conical ones.
(snip)

Ah, but there is no choice in having the engines neat the centre of gravity for this type of craft (or so REL believe). What killed the HOTOL project (https://en.wikipedia.org/wiki/HOTOL) back in the '80s was the choice of rear-mounted engine; it caused (in simulation) so massive a turning moment they would have needed to add so much weight to compensate that the payload to LEO would have been down to about two tonnes. Also they were (apparently) having issues with HOTOL's separation of centre of pressure and centre of gravity. So, the solution seems to be engines mounted near the COG and they'll just have to find some fix for the fuselage heating.

Yours,
Miranda.

[J. Wilhelm]

(snip)
The engine needs to be placed centrally and in the back of the vehicle like on the X-33. Or if I ignore the concentric bleed design, and imagine converting the engine geometry to a planar orientation, taking the SABRE compressors and aligning them along the aerospike ramp, in rectangular engine pods, orienting the  aerospike ramps along the back such that they direct the plumes away from the fuselage (say in the "up/down" directions). That could greatly reduce the risk to the fuselage. There is a price to pay, as the rectangular geometry for supersonic expansion or compression ramps is always less efficient than conical ones.
(snip)

Ah, but there is no choice in having the engines neat the centre of gravity for this type of craft (or so REL believe). What killed the HOTOL project (https://en.wikipedia.org/wiki/HOTOL) back in the '80s was the choice of rear-mounted engine; it caused (in simulation) so massive a turning moment they would have needed to add so much weight to compensate that the payload to LEO would have been down to about two tonnes. Also they were (apparently) having issues with HOTOL's separation of centre of pressure and centre of gravity. So, the solution seems to be engines mounted near the COG and they'll just have to find some fix for the fuselage heating.

Yours,
Miranda.

My fault. I think we are talking about two different things here. The X-33 is a rocket glider, it ascends like a rocket through the atmosphere without using aerodynamic lift along the way, and returns to earth as an unpowered hypersonic glider, and so stability issues are limited to dynamic axis and engine thrust vector alignment...

Whereas the HOTOL and SKYLON are space planes or better stated space airplanes, with "airplane" being the operative word here. This there are stability concerns regarding the position of the dynamic centre (as a function of speed) in relation to the centre of gravity for horizontal flight into orbit.

This is basically an Aircraft Sizing problem (preliminary design). For positive stability, the dynamic centre (centre of all forces including drag, lift and thrust) must lie behind the gravitational centre, which in this case as you say moves aft a little too far back, pushing the dynamic centre too far back (and it moves back further at greater speeds).  For example, in the drawing of my Zarquon Tilt Propfan above, the wing hadn't been - at the time I made that drawing - fixed in the right location.

I think the HOTOL designers had to go through the surprise that their structure was grossly unbalanced after positioning the lifting surfaces (similar to Space Shuttle) , and needed a lot of actuator force and structure to balance it right during flight (I have NOT read any literature on what actually happened - I'm just guessing here based on what I know and the clues you give me).

However, having stated that above, I have seen greater hurdles overcome, and this seems a problem specific to a particular design with particular engine setup and chosen fuselage/wing materials.

Note that in Hypersonic aircraft designs there is an equivalence between the shape or "style" of the vehicle, and the materials used for the airplane's skin. The reason being that a sharp, elongated idealised supersonic, needle/arrow shape design, while being less draggy, and reducing the intensity of the bow shock wave (less pressure / temperature at the nose), also implies that the boundary layer flow over the skin is much faster - which itself implies an increased frictional (viscous/ shearing) drag and associated heat - which in hypersonics is the real killer here.

One problem with the SKYLON, as you and I touched at the Queer Geer some time ago, is that the elegant supersonic aerodynamic design of the SKYLON suggests little concern for heat transfer problems on the skin, and moreover, it suggests an all metal skin, because it incorporates a straight razor-thin wing / engine pod pylon, and the style of the vertical stabilizer also implies all-metal construction.

The long geometry of the SKYLON also exacerbates the problem of pitch and yaw stability in flight.  Let me explain, the way I view it, with the engine pod pylons serving as straight-edge wings, this design is a "relative" of the 1950s-60s era super-high-efficiency straight-wing supersonic designs (e.g. F-104 Starfighter, X-15 rocket plane, Douglas X-3 Stiletto), and so this design suffers from pitch-yaw stability problems at lower speeds by default (elongated needle shapes with straight wings all share that problem). Maintaining stability with a very heavy aft engine might truly exacerbate the inherent low-speed stability problems, to the point you make a very heavy aircraft, just to not tumble out of the sky.

Douglas X-3 Stiletto Supersonic Reasearch Airplane, 1952
https://en.wikipedia.org/wiki/Douglas_X-3_Stiletto

Douglas X-3 Stiletto: 1950s U.S. Experimental Jet Aircraft

Lockheed F-104 Starfighter, Mach 2 interceptor jet, 1956
https://en.wikipedia.org/wiki/Lockheed_F-104_Starfighter

North American X-15 (Hypersonic Rocket Plane M=6.7, 1959)
https://en.wikipedia.org/wiki/North_American_X-15

If the pods can't be placed toward the rear, then I'd suggest they must be rectangular in geometry, incorporate a ramp aerospike (directing plumes up and down), and perhaps be a variation of the  "wing rooted design" where the whole engine pod is part of a Straight or Delta wing. Looking more like the engine setup used in the Concorde, but with rectangular aerospikes.

Honestly, I'd abandon the straight wing design.  You might find that solves a lot of the problems with stability and weight related to stability not to mention heat transfer problems - naturally at the expense of added wing weight, but the lesser of two evils.  I do fear however that the Stiletto-style fuselage may be impractical in the end due to heating concerns unless you can extend the cooling from the engines to the fuselage. Perhaps go back to a wave rider configuration like the National Aerospace Plane (NASP), but using a variant of SABRE?

https://en.wikipedia.org/wiki/Rockwell_X-30

Early 1980s concept for the Rockwell X-30 NASP
The conventional supersonic design requires extremely high-heat resistant metals (which we don't have)

Later Waverider version of Rockwell X-30 NASP
Design has little or no wings, and fuselage acts as engine inlet diffuser, engine outlet nozzle (aerospike), as well as lifting surface by riding on the "shock layer" (highly compressed layer of decomposed air just behind the shockwave's "shock front" (a/k/a Bow Shock)

From https://en.wikipedia.org/wiki/HOTOL

During development, it was found that the comparatively heavy rear-mounted engine moved the center of mass of the vehicle rearwards. This meant that the vehicle had to be designed to push the center of drag as far rearward as possible to ensure stability during the entire flight regime. Redesign of the vehicle to do this required a large mass of hydraulic systems, which cost a significant proportion of the payload, and made the economics unclear.[5] In particular, some of the analysis seemed to indicate that similar technology applied to a pure rocket approach would give approximately the same performance at less cost.

The last sentence is critical.  Seems to suggest that vertical ascent would be better using an air-breathing rocket configuration

Seriously anything worthwhile is very difficult.  All of these approaches are feasible, but require larger than acceptable compromises. I think the most feasible part of SKYLON is actually the SABRE engine, to be honest.

I'd pursue either

1) An air-breathing rocket glider similar the X-33 but in a shape closer to the Space Shuttle (read below) with a centrally aft-located aerospike

OR

2) An air breathing wave-rider airplane like the late X-30 NASP,

AND then for either system above develop a modular Inconel mesh re-inforced silica glass fibre brick skin system.  Basically a cross between the X-33 exchangeable metal plates/mesh skin, and the Space Shuttle silica fibre bricks. That skin will be light enough and heat resistant enough to give you a bluntish design that will easily resist re-entry temperatures and still give you large enough control surfaces like the Space Shuttle.

OR

3) If and only if you can solve the skin heating problem, attempt a Delta wing idealised supersonic design, like the early version of the X-30 NASP, but abandon the straight-wing approach, and use rectangular air breathing rockets with rectangular aerospikes incorporated into the Delta wing. This would be the closest to SKYLON. But the materials and skin cooling technology are a question mark to me.

[Banfili]

I am going to follow this. At last my recent physics course is coming in handy! I am interested through my dad, who was a pilot, and used to fire up our imagination with planes, and stars and hypothetical trips into space. I have a book of interest but will have to dig it out from the space warp in which it has taken up residence, swot up a little, and join in the conversation. Thank you, J Wilhelm - I toff my Tim Tam to you!

[Miranda.T]

(snip)
The long geometry of the SKYLON also exacerbates the problem of pitch and yaw stability in flight.  Let me explain, the way I view it, with the engine pod pylons serving as straight-edge wings, this design is a "relative" of the 1950s-60s era super-high-efficiency straight-wing supersonic designs (e.g. F-104 Starfighter, X-15 rocket plane, Douglas X-3 Stiletto), and so this design suffers from pitch-yaw stability problems at lower speeds by default (elongated needle shapes with straight wings all share that problem). Maintaining stability with a very heavy aft engine might truly exacerbate the inherent low-speed stability problems, to the point you make a very heavy aircraft, just to not tumble out of the sky.

Douglas X-3 Stiletto Supersonic Reasearch Airplane, 1952
https://en.wikipedia.org/wiki/Douglas_X-3_Stiletto

Douglas X-3 Stiletto: 1950s U.S. Experimental Jet Aircraft

Lockheed F-104 Starfighter, Mach 2 interceptor jet, 1956
https://en.wikipedia.org/wiki/Lockheed_F-104_Starfighter

(snip)

Of course, one difference between Skylon and those aircraft is that Skylon will have a very simple flight profile (in atmosphere); take off, arc up to orbit, de-orbit and then arc back to landing; no sudden or energetic manovers as the above aircraft would have needed to make. And of course Skylon is 'fly-by-wire' without even a human pilot - allowing even an inherently unstable shape to stay in the air (isn't this the case for the {piloted} Typhoon Eurofighter?)

(snip)
One problem with the SKYLON, as you and I touched at the Queer Geer some time ago, is that the elegant supersonic aerodynamic design of the SKYLON suggests little concern for heat transfer problems on the skin, and moreover, it suggests an all metal skin, because it incorporates a straight razor-thin wing / engine pod pylon, and the style of the vertical stabilizer also implies all-metal construction.
(snip)

Putting aside the heating during ascent issue, are you thinking of re-entry here? The designers seem to believe this isn't an issue actually due to the shape. Compared to the shuttle, on re-entry Skylon would be a lot less dense (lots of empty space in its fuel tanks) and so should hit a much lower terminal velocity and hence rate of heating. They think this can be managed with a skin coating (and possibly some active cooling via water on leading edges) without resorting to those troublesome tiles. They did have a clever material invented by a French company in mind (there was a clip of it being heated to some ridiculously high temperature to no effect), but there is the slight complication they have gone into receivership.

Yours,
Miranda.

P.S. on a not unrelated note, a nice video showing how the Sierra Nevada Corp hope their Dream Chaser space plane will operate http://www.space.com/30782-dream-chaser-space-plane-2016-tests.html.

[J. Wilhelm]

Of course, one difference between Skylon and those aircraft is that Skylon will have a very simple flight profile (in atmosphere); take off, arc up to orbit, de-orbit and then arc back to landing; no sudden or energetic manovers as the above aircraft would have needed to make. And of course Skylon is 'fly-by-wire' without even a human pilot - allowing even an inherently unstable shape to stay in the air (isn't this the case for the {piloted} Typhoon Eurofighter?)

Snip

Putting aside the heating during ascent issue, are you thinking of re-entry here? The designers seem to believe this isn't an issue actually due to the shape. Compared to the shuttle, on re-entry Skylon would be a lot less dense (lots of empty space in its fuel tanks) and so should hit a much lower terminal velocity and hence rate of heating. They think this can be managed with a skin coating (and possibly some active cooling via water on leading edges) without resorting to those troublesome tiles. They did have a clever material invented by a French company in mind (there was a clip of it being heated to some ridiculously high temperature to no effect), but there is the slight complication they have gone into receivership.

Yours,
Miranda.

P.S. on a not unrelated note, a nice video showing how the Sierra Nevada Corp hope their Dream Chaser space plane will operate http://www.space.com/30782-dream-chaser-space-plane-2016-tests.html.

Regarding stability I'm afraid it's not that easy, dear Miranda. Stability in this case refers  to the natural resistance of a craft to yaw, pitch and roll in the sense of not tumbling at the slightest atmospheric perturbance. The vehicles I mentioned are stable alright, but they are definitely much less stable as they slow down to subsonic speeds to approach the ground.

After re-entry, landing approach is the 2nd deadliest phase of the mission, and a common problem for high speed vehicles that have a small wing area,  such as straight wing supersonic designs and no wing hypersonic designs such as lifting bodies.  This is a major headache, actually. The Delta wing was invented to combine swept wing theory and Supersonic airfoil sections with a greater area for lift and stability.  It resolved the problem of stability at low speeds.

~ ~ ~

The thermal issue is not just for re-entry, but horizontal ascent (and descent) as well.  Unless I'm misreading the ascent / descent flight path (what exactly is that arc?), generically, a space plane must travel a much greater distance within the atmosphere before reaching the 17500 mph speed necessary for low-orbit insertion. Maybe the friction effects during ascent are limited compared to reentry but then you have the issue of flight time duration.

The X-33, and Space Shuttle don't have to suffer from high hypersonic effects for as long as the X-30, HOTOL, or SKYLON would; their only hurdle is to slow down in re-entry, and ascent is largely a supersonic affair.  The rocket gliders' trajectory is steep, and thus much shorter,  and the maximum speed is at an altitude where the atmosphere is very rarefied. The space airplane will endure heat for longer periods of time.

The only thing I can think of is a relatively slow subsonic or supersonic ascent to high altitude and then a quick jump upwards. But that doesn't explain reentry. EDIT: HOLY COATINGS, BATMAN! Upon reading more on the SKYLON, I was surprised to find that they plan to use a ceramic coating. Actually this is a relief, because it's an admission that there are in fact serious thermal issues being dealt with, including chemistry (read below).

~ ~ ~

On shape: Actually the slender shape doesn't help in a re-entry setting or high speed climb cruise even.  It exacerbates the problem. Greatly. Again, long skinny shapes are low-drag affairs,  but the speed of the air next to the skin (boundary layer) is higher, so friction becomes a problem.  Supersonic aircraft don't need to worry about that. But for hypersonic aircraft this is the biggest challenge. It seems the SKYLON design with zero edge blunting just plows through these difficulties. We haven't seen that in actual production vehicles... well, ever.

~ ~ ~

On terminal speed: Regardless of the mass of the vehicle,  re-entry still involves slowing down from orbital speed to - as you say- some terminal speed but high in the atmosphere. But the density of the atmosphere is very low, so either way, no matter your orientation, you will take some time to slow down,  all the while still reaching very high Mach numbers and associated plasma related ablation and chemical reactions due to air decomposition in the shock layer's extreme temperatures downstream from the bow-shock.

Metals, as a general rule and assuming a Mach number range anywhere between 5 and 15 (20-25 for lunar/interplanetary capsules) , will melt, burn (oxidize), and chemically react (radical "adsorption" which is an exothermic reaction - you get more heat),  and even vaporize all at the small time!

Now SKYLON claims a maximum operating Mach number of 27 with the SABRE engines (is that on ascent?). That's higher than the Apollo capsule (M = 24.5 - sounds very high to me). So these skin issues are serious and using metals like in the X-33 is a tremendous complication.  So ceramic it is for the SKYLON.

On descent, SKYLON must slow down very fast with active cooling and a non-chemically reacting skin if they plan to use the fuselage more than a few times. I imagine the SKYLON would reach terminal velocity without any need for power as the re-entry challenge is to dissipate energy, so it can't be that far away from the Space Shuttle or X-33, except that their design doesn't involve a "burning belly"   angle of attack. Instead, the design as shown,  suggests the axis of the aircraft is always aligned to the free stream. Pure hypersonic controlled flight at every location along the flight path.

NOW HERE'S THE CLAIM: with a "low ballistic coefficient" (large size/low mass), the SKYLON slows down sooner, without Mach number figures, they claim a reduction to HALF the "maximum temperature in the Space Shuttle of 1,260 °C (2,300 °F).."

Note that in the Space Shuttle, the black High Temperature (HRSI) and white Low Temperature (LRSI) Reusable Insulation tiles, handle operating temperatures of up to 1260 °C (2300 °F), and 1200 °F (649 °C), respectively.  

The only difference between the two tiles, besides size is a cover which gives the black color (borosilicate glass) and white color (aluminium oxide). The black tiles dissipate heat from hypersonic effects much faster by design (this is a thermal effect of the black color itself). The white ones also protect from hypersonic effects but double function by retaining a bit of body heat and reflecting solar radiation by design, to manage heat while in orbit (Actually this is true.  White is like a two-way "closed-door" barrier to heat reflecting any incoming or outgoing radiation. Black on the other hand is like an open door for electromagnetic radiation emitting and absorbing infrared and visible light more efficiently than any other color).

The  nose and wing leading edges of the Space Shuttle are made of a "disposable" material Reinforced Carbon Carbon (RCC), designed to wear out much sooner (ablate), so we really can't compare that to the coating on SKYLON.

https://en.wikipedia.org/wiki/Space_Shuttle_thermal_protection_system

Space Shuttle's Thermal Protection System

What exactly is the re-entry and descent profile for the SKYLON?  You got me thinking  now. What is their "secret sauce?"    There is more than one way to skin a cat, but right now, I'm not knowledgeable on the particular mission design for the SKYLON.

I don't know about the French fellas' special material. To be honest, what I know dates to the turn of the Millennium, as that is when I was active, scholarly speaking. And we know that the French ceramic material will handle about the same temperature as the white LRSI tiles in the Space Shuttle. I haven't seen details on method of installation either. No details. A continuous coating in ceramic seems implausible to me, on account of possible thermal stresses and difficulty in manufacture. You need to allow for thermal expansion and compression of the skin. and structure of the body.

Why I rant against metals: From what I know, metals tend not to do well under such conditions, and the Inconel panels of the X-33, the highest temperature metal skin ever proposed, were designed for 50 cycles of life  about one half the lifespan of the Space Shuttle's silica tiles.

Lockheed Martin's X-33 Inconel thermal protection tile system

The metal panels in the X-33 slowly turn into "potato crisps"  by warping and scorching before being replaced. The warping issue actually produced points of heat concentration at the posts wher the panels are bolted to a composite lattice, and thus gave many headaches to designers according to the engineers I talked to during a presentation by NASA at the 1998 AIAA Meeting and Exhibit in Reno,  Nevada, (in which I presented my undergraduate research as part of a student contest) . That discussion left an impression in my mind.

In contrast, the silica fiber and ceramic bricks are chemically inert, avoiding the chemical reactions (and heat) from radical adsorption. In the U. S.we went to an extreme amount of effort in the 70s to develop the silica bricks, just  to be able to have a reusable rigid non warping skin for bigger wings and control surfaces, which would afford stability at lower speeds and in landing.

On the X-33, engineers compromised on rigidity and lifespan, thinking the disposable metal panels would be cheaper to make and much easier to replace (the real cost of that silica brick skin and a big reason for the Shuttle being over budget was maintenance of the bricks, actually).

[Miranda.T]

(snip)

Regarding stability I'm afraid it's not that easy, dear Miranda. Stability in this case refers  to the natural resistance of a craft to yaw, pitch and roll in the sense of not tumbling at the slightest atmospheric perturbance. The vehicles I mentioned are stable alright, but they are definitely much less stable as they slow down to subsonic speeds to approach the ground.

(snip)

On shape: Actually the slender shape doesn't help in a reentry setting or high speed climb cruise even.  It exacerbates the problem. Greatly. Again, long skinny shapes are drag affairs,  but the speed of the air next to the skin (boundary layer) is higher, so friction becomes a problem.  Supersonic aircraft don't need to worry about that. But for hypersonic aircraft this is the biggest challenge. It seems the SKYLON design with zero edge blunting just plows through these difficulties. We haven't seen that in actual production vehicles... well, ever.

(snip)

Metals, as a general rule and assuming a Mach number range anywhere between 5 and 15 (20-25 for lunar/interplanetary capsules) , will melt, burn (oxidize), and chemically react (radical "adsorption" which is an exothermic reaction - you get more heat),  and even vaporize all at the small time!

Now SKYLON claims a maximum operating Mach number of 27 with the SABRE engines (is that on ascent?). That's higher than the Apollo capsule (M = 24.5 - sounds very high to me). So these skin issues are serious and using metals like in the X-33 is a tremendous complication.  So ceramic it is for the SKYLON.

(snip)

On descent, SKYLON must slow down very fast with active cooling and a  reacting skin if they plan to use the fuselage more than a few times. I imagine the SKYLON would reach terminal velocity without any need for power as the reentry challenge is to dissipate energy, so it can't be that far away from the Space Shuttle or X-33, except that their design doesn't involve a "burning belly"   angle of attack. Instead, the design as shown,  suggests the axis of the aircraft is always aligned to the free stream. Pure hypersonic controlled flight at every location along the flight path.

(snip)

REL seem to be aware of the heating during ascent issue, and think that this will not last long enough to be a problem; another reason why they point out Skylon could not be used as a hypersonic airliner (at last one that stayed in the atmosphere). As far as I know, Skylon is supposed to reenter in a shuttlecock manner (i.e. belly first).

Metals are indeed bad news in this context - witness the Columbia disaster. In terms of the choice of material, I guess it's a playoff between peak temperature and time of heating. If you have a lower terminal velocity, you need to withstand a lower peak temperature but do so for a longer time; you do indeed need to get rid of that gravitational potential energy somewhere.

Yours,
Miranda.

[CloudWolf]

greetings all. Mr. Wilhelm i doubt you'll recall but we briefly discussed such topics a few times, i found the conversations enjoyable. i am currently studying engineering and intend to study aerospace engineering at university, i hope to be able to engage here despite my admittedly poor knowledge i have a passion for the subject.
sincerely
CloudWolf (aka Josh/Claudia)

[J. Wilhelm]

greetings all. Mr. Wilhelm i doubt you'll recall but we briefly discussed such topics a few times, i found the conversations enjoyable. i am currently studying engineering and intend to study aerospace engineering at university, i hope to be able to engage here despite my admittedly poor knowledge i have a passion for the subject.
sincerely
CloudWolf (aka Josh/Claudia)

Ah! Cloudwolf, welcome aboard. Claudia! Lovely name. (And how coincidentally appropriate for a Last Exile reference!)  I'm sure you'll enjoy the thread, but I'm afraid the conversation got very technical very fast in a very haphazard way. Welcome and come drink from the Firehose of Knowledge! 

Thankfully I have not needed to resort to ant maths - yet). I'm sure you're trying to get all of this stuff right now - if you are anything like me at the time. 

Some sage advice on the college programme, if I may:

Spoiler: ShowHide

Assuming you choose Atmospheric Flight as your concentration in the degree, for your purposes on this discussion in this thread, I'd tell you that basically AE students will cover subjects like propulsion and supersonic flow in the first 4 years of the degree. Hypersonics come later. actually. So actually I'm exercising my undergrad and grad nuggets simultaneously (two walnut sized bundles of neurons somewhere in my brain for sure). Actually there's a 3rd nugget running aorund there when I need it.

In the US, we have Bachelors' degrees (4 years) as well as Masters' degrees (2 additional years), in Europe and Latin America the degree may also exist as a Mechanical Engineering degree, as a single 6-year programme.

Now, I have to state that you will learn compressible flow for the purposes of calculating the fluid physics as a consequence of shock and expansion waves around the aircraft, and also the physics of converging-diverging nozzles (wind tunnels and rocket engines), so at least you will get a double dose of basic supersonics - once for the Propulsion class, and second for the Compressible Fluid Dynamics class (in other words "supersonic gas dynamics") - don't worry plain water - incompressible fluid dynamics ("subsonic aerodynamics") is hard enough already as a subject. Not to mention, classical Viscosity theory and intro to turbulence.  You will be busy.  Very busy. 

The isentropic Converging Diverging nozzle

Often teachers will "cheat", and start teaching you about compressible flows in the propulsion class BEFORE you actually take the Compressible Flow class. This is because the amount of theory needed to study converging diverging nozzles is just a small fraction of the other class, and it's a good introduction, so you get a "feel" for what supersonic flows are like. So rocket nozzles may likely be your introduction to the subject.

BUT be warned that as an undergraduate, you will be assuming that gases are ideal gases, and you can't consider the case when the temperature is so high that the enthalpy of the air is severely changed and the air becomes so hot that is starts decomposing. Any chemical reactions on the skin or the gases downstream from the bow-shock require a bit of thermochemistry that is very difficult to squeeze into a 4 year programme.

Basically, the first for years take you into the 1950's as far as flight technology goes.  To go past the 1950s into contemporary technology, like the Space Shuttle, you need those extra 2 years.

By the way, the same applies for advanced Turbulence theory; While on my first Masters degree, I actually took Combustion and Turbulence electives in the Mechanical engineering department, which led me eventually to my second Masters degree in Mechanical Engineering (Combustion and Heat Transfer).

Maybe top-5 colleges like MIT and Caltech manage to stick these hypersonics, turbulence and advanced combustion subjects in there for undegraduates - but otherwise, a class on Hypersonic Aerodynamics and advanced Turbulence theory are strictly graduate level stuff. *snort*

 

So 'member for the first 4 years of your education your maths will be off for flows above say Mach 3 (at any altitude), and any talk about Hypersonics, i.e. chemical dissociation, including chemical reactions at the skin (adsorption of radicals), and the relationship between vehicle's shape, and the materials used, strictly are outside of that scope.

If you want to really get good at the chemistry bit for re-entry and propulsion purposes, get the second Masters in Combustion and Heat transfer. Note Colleges frown on getting the two Masters together (Aerothermodynamics and Combustion/Heat Transfer) as they are "too close" to one another, they say.

But I asked for permission from the college admistration and I'd say that my overall knowledge was incomplete if I didn't have a good grasp on the cutting edge of turbulence and combustion (how can you not at least know about turbulence  ??). The two second Masters degress took me into cutting edge research - and in fact my 2nd Thesis was based on Turbulence research (turbulent water tunnel simulations of drag reduction techniques)... In all about 5 years of hypersonics and water tunnels Aaah! good memories 

Anyhow, depending in which direction you go, those Mech. bloaks will teach you Turbulence, Flame theory, and advanced Heat Transfer which will enable you to work in gas turbine engines (yes, it takes two Masters degrees to have all the tools for designing a turbine engine    At least here in America)

So it takes a total of 6 years to get to know intimately the theory we're discussing on the SKYLON skin, chemistry and body shape. I took those 6 plus another 3 for the 2nd Masters   - 9 years total. My motivation as a teenager was learning every aspect of the Space Shuttle - it took me three degrees to answer all the questions 

[Miranda.T]

It Strikes me I'm cluttering this thread with my obsession for space-planes* (notice I don't say unhealthy obsession, as I have a healthily wide and diverse range of obsessions  ). Anyway, that is not the reason the good Admiral started this thread, so I thought I should throw in an alternative line of discussion, which is: Steampunk human powered flight.

Now, there were many varied and fantastical Victorian designs for such a thing (plenty of images here https://www.google.co.uk/search?q=victorian+human+powered+flying+machine&hl=en-GB&gbv=2&prmd=ivns&tbm=isch&tbo=u&source=univ&sa=X&ved=0CBQQsARqFQoTCM3uscqGu8gCFYUXPgodfA8Nsw, but it took until the latter part of the 20th century for the ambition to be realised (https://en.wikipedia.org/wiki/Human-powered_aircraft). But of course, these aircraft took full advantage of modern lightweight materials (aluminium, synthetic skins for the frame, more recently carbon fibre). The question is, with the right design could this possibly have been achieved using materials available in the Victorian era?

I'll get the ball rolling; my (admittedly a bit unhinged) thought is to use nature's solution, and build the framework using bones from birds - already evolved to be strong and light. Of course, two issues present themselves. Size - given that a roc might be a bit difficult to find, a latticework structure would need to be built up from smaller bones, although 'smaller' is a relative term - we'd be talking say albatross. Which then brings me to the second issue - to collect enough bones, how many albatross' would you need to hunt? (Hear the rime of the ancient mariner...)

Anyway, if anyone else has a better idea than this (probably not difficult), please feel free to suggest away!

Yours,
Miranda.

*I think this particular obsession comes from the frustration that we seem to be going backwards in terms of humanity's ambitions for space. Orion/SLS, Dragon/Falcon etc. just feel like a throwback to the 60s and 70s, notwithstanding claims for possible reusability. The shuttle always looked to me as the right way to go, but its design was too compromised by too many 'intersted parties' making demands on it and budget limits. So, I'm really hopeful that Skylon, Dream Chaser** et.al. will get things back on track.

** Although this is, ironically for my argument, based on work from the 1950s & 60s (the Dyna-Soar project).

[J. Wilhelm]

Aaah! You know, dear Miranda?  The problem is that I've always been an aeronaut.

If you have an obsession on space planes then I'm barking-mad and screaming from a cell deep inside the metal sanatorium. After all, I dedicated 9 years of my life and spent well over $30000 of my money to specifically answer the question "how does the Space Shuttle work?"

Now tell me again about the degree of your obsession?      You're bragging about being evil while talking to the devil himself! Ha, ha, ha!

REL seem to be aware of the heating during ascent issue, and think that this will not last long enough to be a problem; another reason why they point out Skylon could not be used as a hypersonic airliner (at last one that stayed in the atmosphere). As far as I know, Skylon is supposed to reenter in a shuttlecock manner (i.e. belly first).

Aaargh! SKYLON seems to defy every single expectation! I guess they're just pushing the numbers as far as they can. I'm not seeing the thermal shuttlecock re-entry on it, but probably because the skin - quite frankly from my POV - is simply not developed yet beyond a concept.  All they have is the ceramic. I do buy the part of the corrugated shell, though, but read below.....

I need to see re-enforcements for leading edges, separate segments with different thermal resistance coefficients, cooling systems, etc, before I see this neat diamond-wing supersonic design do a "bellyflop" re-entry.  The technical jargon is to say that the thermal characteristics of the windward surface ("belly") are different from the thermal characteristics of the Leeward side. So you expect some variance in thermal protection like the space shuttle has...

X-33 Hypersonic Aerodynamics (1999)

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040087108.pdf

Discussion on transition to turbulence for the X-33 (1998)

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980037670.pdf

In 1998 NASA and Lockheed made like - I don't know- more than 10 presentations on the X-33 in Reno. Pfft! probably way more, they had an entire section of the conference just for the X-33. Students were having their own competition among the professionals.

This was the paper I was presenting on my research with Dr. Goldstein in the same conference. I had previously won the regional segment of the contest (two year cycle - talk about an ego boost for a 4th year student)  

http://arc.aiaa.org/doi/abs/10.2514/6.1998-7

*Sigh* The X-33 was like that though... it broke every single rule in the text-book, and the engineers made you believe it would fly. But the data was really out, available in the open and talked about in detail in the AIAA conferences. Long before the experiments were over we were already talking about the complications of the skin.

Giant autoclave-cured carbon fibre fuel tanks (which kept de-laminating), copper aerospike (yeah, I know let's not go there), metal skin (potato crisps / my pizza-baking sheet in the oven).  It's like mental torture. The only thing proven thing on the concept was the lifting body Dyna-soar pedigree it had. But at least there was the talk about it...

*snip*
Metals are indeed bad news in this context - witness the Columbia disaster. In terms of the choice of material, I guess it's a playoff between peak temperature and time of heating. If you have a lower terminal velocity, you need to withstand a lower peak temperature but do so for a longer time; you do indeed need to get rid of that gravitational potential energy somewhere.

I actually found this AIAA paper written for the 18 th AIAA International Space Planes and Hypersonic Systems and Technologies Conference in Tours, France, 26 September 2012...

AIAA Conference Paper
https://www.aiaa.org/WorkArea/DownloadAsset.aspx?id=14414

SKYLON Website:
http://www.reactionengines.co.uk/space_skylon_tech.html

Not that I'm sold on it though...  The SKYLON is like a Pandora's box. It's very radical. Everywhere I look I find something surprising an unxpected. Just when I'm getting exited about the French thermal coating I read this...

Skin (kg/m^2) 1.537 (0.5mm SiC/glass ceramic, corrugated 2x20mm)

The aeroshell is being described as a thin corrugated ceramic skin no more than 1/2 mm in thickness (Page 34 of AIAA paper)  if I read that correctly.     I imagine the skin is being held up by posts on to a lattice, similar to the X-33, and weighing about 8.7% of the total dry weight of 42.3 metric tonnes. The weight of the skin is about the same as the weight of the composite internal structure (space frame), and both are very light indeed.

I'm happy that it's a ceramic, for reasons already explained, and the corrugated part seems very similar to the thin-shell strengthening method use for the Space Shuttle's external tank. The floating lattice ius something that the X-33 had.

But that thickness for a ceramic shell is odd and it's a bit of a let down. Naturally the fear is that assuming you can sustain vibration, and thermal expansion, and everything else, that it will be strong enough to not be breached by impact. More importantly, for consequences look at the Space Shuttle Columbia tragedy.  

My fear is that a super thin brittle aero-shell is an inherent risk, if impacted by anything at those speeds (I guess an unmanned vehicle is a good idea). Though admittedly the SKYLON is highly unlikely to encounter debris flying high during ascent, the same way a chunk of foam from the External Tank detached an accelerated to an impact of 1 metric tonne onto the leading edge of the Columbia (there was no metal involved actually).

The fear really is more at low speeds and low altitudes where it is far more likely to get hit by something dense.  Bird strikes and a Concorde-style tarmac-debris strike can't be ruled out for a horizontal flight takeoff! This is now an aeroplane, after all.  It needs to fly low to the ground.

https://en.wikipedia.org/wiki/Space_Shuttle_Columbia_disaster

https://en.wikipedia.org/wiki/Air_France_Flight_4590

[J. Wilhelm]

It Strikes me I'm cluttering this thread with my obsession for space-planes* (notice I don't say unhealthy obsession, as I have a healthily wide and diverse range of obsessions  ). Anyway, that is not the reason the good Admiral started this thread, so I thought I should throw in an alternative line of discussion, which is: Steampunk human powered flight.

Ah! Perfect.  Then I'll post this here for practical inspiration, while I go run some errands, this Sunday.  I need to shift my brain from AIAA conferences to Leonardo da Vinci, and as it turns out college life will cover both subjects quite nicely

I had posted about the Igor Sikorsky Human Powered Helicopter Competition in another thread.  This is a Da-Vinci-esque competition that has been running for decades and a kind of Holy Grail for college students

AeroVelo Wins the 33-Year Old AHS Igor I. Sikorsky Human Powered Helicopter Competition

Human-Powered Helicopter: Straight Up Difficult:

[GCCC]

It Strikes me I'm cluttering this thread with my obsession for space-planes* (notice I don't say unhealthy obsession, as I have a healthily wide and diverse range of obsessions  ). Anyway, that is not the reason the good Admiral started this thread, so I thought I should throw in an alternative line of discussion, which is: Steampunk human powered flight.

Now, there were many varied and fantastical Victorian designs for such a thing (plenty of images here https://www.google.co.uk/search?q=victorian+human+powered+flying+machine&hl=en-GB&gbv=2&prmd=ivns&tbm=isch&tbo=u&source=univ&sa=X&ved=0CBQQsARqFQoTCM3uscqGu8gCFYUXPgodfA8Nsw, but it took until the latter part of the 20th century for the ambition to be realised (https://en.wikipedia.org/wiki/Human-powered_aircraft). But of course, these aircraft took full advantage of modern lightweight materials (aluminium, synthetic skins for the frame, more recently carbon fibre). The question is, with the right design could this possibly have been achieved using materials available in the Victorian era?

I'll get the ball rolling; my (admittedly a bit unhinged) thought is to use nature's solution, and build the framework using bones from birds - already evolved to be strong and light. Of course, two issues present themselves. Size - given that a roc might be a bit difficult to find, a latticework structure would need to be built up from smaller bones, although 'smaller' is a relative term - we'd be talking say albatross. Which then brings me to the second issue - to collect enough bones, how many albatross' would you need to hunt? (Hear the rime of the ancient mariner...)

Anyway, if anyone else has a better idea than this (probably not difficult), please feel free to suggest away!

Yours,
Miranda.

*I think this particular obsession comes from the frustration that we seem to be going backwards in terms of humanity's ambitions for space. Orion/SLS, Dragon/Falcon etc. just feel like a throwback to the 60s and 70s, notwithstanding claims for possible reusability. The shuttle always looked to me as the right way to go, but its design was too compromised by too many 'intersted parties' making demands on it and budget limits. So, I'm really hopeful that Skylon, Dream Chaser** et.al. will get things back on track.

** Although this is, ironically for my argument, based on work from the 1950s & 60s (the Dyna-Soar project).

I would think that gliders (wind powered, so still not human-powered) would not be that much of an issue... Surely a team of horses could get the thing aloft, proportions, design, and materials all being appropriate, of course.

But Miranda proposes human-powered flight... First, trying to emulate birds flapping their wings is right out, as my admittedly limited understanding of such things allow. A human is just never going to generate the power to flap wings that, to be sufficient to carry their weight aloft has to be stupidly long in the first place, not to mention that flapping is only part of the trick anyway (I'm told some folks got together to do the math to make Pegasus work; I don't remember the exact figures, but you wouldn't be able to get anywhere near the horse because of the wingspan). I vaguely recall reading that humans, which are mammals to begin with, need to stop trying to emulate birds, but instead emulate bats (see DaVinci's glider; also, the wing shape, not the flapping).

So okay; no flapping = fixed wing design, + bat-like wing design. The membrane should be no problem; I would think someone could have figured out how to do this, with a certain type/x number of layers/treatment, whatever, in silk. I don't see a thin-enough leather working, just because of the weight of it. Now, for the framework...

...Miranda has suggested Frankensteining together x number of bird bones since these are already the correct structure. We can't follow bat bones here; they're not hollow, and are relatively thin and fragile. But you know what else is hollow and fairly strong? Bamboo. Why not try that? If I remember correctly, bird bones, while hollow, still have a sort of matrix of supporting bone within the larger cavity. If that's the case, and the bamboo required it to support this project, could the hollow bits of the bamboo have some sort of supporting matrix, itself made of bamboo or bird bone or something else? If I do not remember correctly, the previous two sentences are moot.

I know 90% of what I just proposed is impractical/unfeasible/impossible, but I know others on this thread could tell me why it's wrong and/or how it could work.

Still, from a completely practical standpoint, I would think gliders would be the way to go, but again, it's not technically human-powered, then.

On your mark:  Read, set, MATH!

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What do you think of this set-up?



http://www.dcgeorge.com/ProjectFalcon_f.html

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Or this?





http://voyagesextraordinaires.blogspot.com/2010/03/fortress-explorations.html

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Assuming this were a glider (or, what the heck, even if it's not), what is wrong/right with this design (bottom left only)?



http://www.darkroastedblend.com/2008/11/those-magnificent-men-and-their-flying.html