The Guild of Icarus: Aerospace Engineering and Aeronautical Club

[J. Wilhelm]

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

Isn't that one of Otto Lilienthal's gliders?  If so I believe it did fly!

https://en.m.wikipedia.org/wiki/Otto_Lilienthal

Lilienthal in mid flight, 1895


Like others, he did get the idea that the criss section if a wing was important
Illustration of the flight of a white stork by Otto Lilienthal in Der Vogelflug als Grundlage der Fliegekunst published in 1889

[J. Wilhelm]

What do you think of this set-up?



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

Normally I'd go crazy critical on a design like this one.  But given how relatively easy it is to actually generate lift if you understand the forces needed and in the presence of sufficient power, there is a chance this can happen.

The thing is that until very recently, engineers and scientists did not understand how some animals could generate enough power to move as swiftly as they could

There is an energy recycling method that evolution has incorporated into the motor control of vertebrates.  In general terms for any kind of flapping motion, whether you are generating lift by flapping wings or flapping fins - which admittedly are two very different methods of propulsion,  there is always a vortex "down wash" from which you can recuperate energy by "slapping " the vortex in the right way at the right time.

The fluid mechanics of flapping involves a type of non dimensional frequency called the Strouhal Number, St, used traditionally to characterize fluid mechanics phenomena such as Karman "vortex streets"  The Strouhal number has been linked by experiment to what scientists have found to be a natural frequency of flapping for each type of animal,  whether it flies or swims,  with the experiments  happening in the 1990s at universities like MIT and Caltech.

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

St = fL/U

The Strouhal number depends on the forward speed motion of the animal, U, the flapping frequency, f in Hertz,  and some characteristic scale, L, of the animal, all of which is related to the size and motion of the downwash vortex from flapping, and thus is a good predictor of optimum flapping frequency to recover as much energy from the downwash as possible.

A myriad of hummingbird and insect like drones has appeared in recent years, no doubt enabled by this science. Smaller animals (insects,  hummingbirds)  having the highest frequency,  and large animals like  condors having the slowest flapping frequency.   I wonder if this ornithopter enthusiast is aware of that  theory....

Hummingbird drones

http://www.youtube.com/watch?v=WRwPCzUpGLU

Under certain low speed conditions (which applies to most animal locomotion), air basically behaves like water, so adjusting for viscosity you may be able to reproduce the fluid dynamics of water in air, and viceversa (that is why there is such a thing as "water channels" as opposed to "wind tunnels."

Who in Brassgoggles remembers when we were posting about the robotic penguins built by a company called Festo?

Robotic penguins

Festo - AirPenguin

[Miranda.T]

Aaah! You know, dear Miranda?  The problem is that I’ve always been an aeronaut.
(snip)

Ah. and there is the source of part of my frustration. The promised cheap access to space that the shuttle promised to bring was supposed to give much wider access to scientists and engineers; you could be an aeronaut/astronaut without needing to qualify as 'the best of the best'. for well-known reasons the shuttle didn't do that, and I just don't see the current efforts from NASA or SpaceX really challenging the status quo.

(snip)
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)  
(snip)

Impressive!

(snip)
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)

As with the HOTOL project, I was disappointed when that programme was cancelled. Then the shuttle was deemed to be too unsafe to continue to fly. It looked like the idea of a cheap and regular access to space via a fully reusable vehicle had died, but them SpaceshipOne helped re-ignite interest in this (although it of course it is a bit of a cheat - sub-orbital, side-stepping the thorny issue of full re-entry).

(snip)
Aaargh! SKYLON seems to defy every single expectation! I guess they're just pushing the numbers as far as they can. (snip)

That is certainly a comment I've seen before in the context of Skylon - if it is to work at all, it wll be right on the edge. By that way, on the cotext of Columbia it was not so much the initial strike I was thinking of but the burn-though of the wing's structure on re-entry; the aluminium framework of course never stood a chance with the protective tiles lost.

(snip)
...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.
(snip)

Bamboo - I do like that idea. I was wondering if you could stopper the ends whether you could pressurise the inside channel by forcing compressed air into it to give extra rigidity. I don't know if the structure of bamboo could hold the pressurised air, though.

I have to say I'm thinking a peddle type arrangement and propeller; flapping is vastly too complicated. The propeller itself would have to be extremely lightweight. However, some of the images you posted... Maybe using hydrogen gas-bags to help with lift. The problem is that just slinging a bike with a propeller under a dirigible, the poor pilot would be too subject to the wind blowing then around (although maybe they'd stand a chance with two peddlers...  ), so possibly just use small gas-bags to help lift rather than fully provide it; I'm wondering about putting them inside the aerofoil section of the wings - no penalty in terms of wind pressure, and, if pressurised, could help give the wind shape and structure.

Yours,
Miranda.

[J. Wilhelm]

Indeed it was a great disappointment to see the X-33 canceled as well.  The X-33 incorporated all the most radical techniques available into one project,  then someone at Skunk Works committed the first cardinal sin of prototyping by actually setting the goal of using the testing model as the actual flight test model.  Which is never done,  because the structure is weakened by structural testing.  Also, they were supposed to complete the project in 3 years.  Both practices of which were something I had never heard of before.  I felt the project was doomed from the start. The X-33 was a 1/2 size prototype of Venture Star, the final goal.

Nevertheless the discussion at AIAA gave you the impression that they were serious on the nitty gritty of engineering (see attached papers). All the plans were finalised.  As the photo shows,  they were serious on the testing.  They fabricated the tanks from carbon fibre.  The problem is that one by one the technologies tested started failing.  The tanks started de-laminating in testing.  It had to do with non uniform curing after coming out from the autoclave - one of the largest in the world? The Venture Star would need a specially made autoclave.  And the welded aerospike ramp fell apart in testing,  They switched to aluminium tanks,  but by then the vehicle was so heavy they had to make design changes including an external fuel tank that looked like a pencil.  At that point the project was killed.

The project failed in part because of unrealistic goals set from the start, having less to do with the physics of it and more to do  with engineering procedure.  The technology could have made it, if they had taken the time to properly develop and test the technologies involved.  Not 3 years, but say 10, I'd say. With testing of several prototypes at every step of the way.

~ ~ ~

Flapping is in fact very complicated,  Flapping can be of several types and involves forward motion and tilting of the wing while flapping,  because of both drag and boundary layer separation and also to "recycle" the energy of the downward vortex as I explained.


~ ~ ~

Well,  cylindrical tubes are very strong by default. Filling them may or may not be necessary, depending on what you're doing with them.  For two-force members such as in a lattice, you'll probably not need filling.  To withstand bending moments and torsion as in a standard frame,  those elements could be filled with neoprene foam.

~ ~ ~

Two peddlers are absolutely necessary,  especially if your second peddlers name is Maximilian Meen. This effort could be part of a Great Race.  You know, it's Fate,  and all that jazz.  

http://youtube.com/watch?v=7vE3Z5-6ROc

[Miranda.T]

For those interested in such things, this link is to a PDF detailing the current state-of-play with respect to Skylon's development: http://forum.nasaspaceflight.com/index.php?action=dlattach;topic=36826.0;attach=1073534.

Yours,
Miranda.

[J. Wilhelm]

For those interested in such things, this link is to a PDF detailing the current state-of-play with respect to Skylon's development: http://forum.nasaspaceflight.com/index.php?action=dlattach;topic=36826.0;attach=1073534.

Yours,
Miranda.

Dear Miranda:

I finally gave myself the time to read the first paper you posted, the arodynamics asessment by the three NASA workers on the Skylon. Their assessment is pretty grim, worse than I've would have given.

One word of caution: these engineers chose to use a Direct Numerical Simulation (DNS) method (Finite Volume DNS package), but using the Euler Equations. I wouldn't have done that, even back in 1998, when computers were slower...

HOWEVER (and ignoring that their  technical communication skills are questionable), their findings are not, shall we say, encouraging? They really kill the design.

http://www.nas.nasa.gov/assets/pdf/papers/Mehta_Unmeel_Skylon_2015.pdf

Understanding what they wrote, I re-interpret here:

Spoiler (click to show/hide)

I spent quite an amount of time developing and using the exact same DNS method (Finite Volume, upwind), for my own research (the same paper I presented at AIAA - linked above), as this is bread and butter for supersonic flows.  But for my supersonic and hypersonic flows I used the whole Navier Stokes Equations (Conservation of Mass, Momentum, Energy and Entropy balance).

I say this because these three engineers chose to use Euler Equations which basically are an abbreviated form of the Navier Stokes Equations, which assume no heat transfer occurs in the flow field and no viscous effects are considered (hence no effects related to friction).

I was scratching my head for a while, wondering why the engineers would neglect friction when talking about hypersonic flows and  over simplify, using ideal gas law, all the while talking about temperature gradients in the order of thousands of degrees and temperatures on the skin. It made little sense to have used the Euler Equations in my mind.

But then I remembered that you can still treat the air flow outside of the boundary layers as inviscid, and shock waves can be mathematically treated as adiabatic (no heat transfer in radiation and conduction) affairs, even though it makes little sense at first glance (since shock waves in fact dissipate an enormous amount of heat).

But theoretical shock waves (Rankine Hugoniot jump relations) can be derived from the Euler equations, actually.  Shocks are like black holes, that is, they are a type of mathematical singularity. They operate on an infinitesimally thin line called the "shock front" and are like pushing the "reset" button for pressure and temperature.

So the word of caution here is that the engineers really are modelling only the flow of gases outside of the boundary layer (the layer with friction and maybe turbulence right next to the skin), and any temperatures from rocket combustion are treated as "boundary conditions," in laymen terms, mathematically imposed.

Any shock compression follows the Rankine Hugoniot jump relations, just a consequence of the Euler equations, and gas expansions are basically an isentropic consequence of ideal gas law. No radiation allowed, no conduction and no friction allowed. No chemical reactions either. Note hypersonic flows involve all of the above especially next to the skin.

So they are just following how the gas transports energy and changes pressure and temperature as it is "thrashed around" the fuselage, so to speak (this gives you an idea of how much energy you are really talking about when you speed up to Mach 15). This is, for lack of a better term, a "supersonic simulation," not a hypersonic one.

In the software, you set the engine temperature, ambient pressures and expected free-stream fluid velocities to set an initial condition for shocks to develop naturally. The simulations are good for "surrounding flows" and calculating drag, but otherwise, thermal effects on the skin and skin temperatures will be significantly off in the hypersonic range.

Their temperature assessments will be off, actually, as much as 100% (double) error by the time you get to Mach 15, compared to using the non-ideal gas corrections.  Mostly because real air molecules absorb energy, and because you allow heat to move around otherwise.  Also, they ignore chemical adsorption and radiative heat transfer to the skin (infra-red from the combustion process), which is really a huge amount of energy (but in all honesty, I would also would have ignored radiation in my calculations at least until I was a graduate student studying radiative heat transfer in participating media This is a necessary correction that NASA had to take care of for rocket engines on the Lunar Module, for example.

Anyhow, in simple terms (and in English) Their Cart3D temperatures are more than double what you might actually encounter if you bother to use the full Navier Stokes Equations and consider a non-ideal gas correction. When you correct the math you get much lower temperatures.

Correcting for non-ideal gas laws by assuming molecules can absorb energy, produces an effect known in Hypersonics as a "thermal relaxation effect" where Mother Nature "gives you a break," so to speak.  Without this "break" from Mother Nature, designing a Moon mission, would have been absolutely impossible and the Space Shuttle would simply not exist. I first learned about this thermodynamic correction in my first few classes in Intro to Hypersonics, back in 1997-8.

This is their "small letter disclaimer" on Page 15.

At M_∞=16.969, approximately 20 percent of the aft portion of the fuselage, including the empennage, is surrounded by fluid at very high static temperatures. As the rocket mode is used to raise M_∞ beyond 17, the percentage of fuselage engulfed with nacelle plumes will further increase and the thermal environment will become increasingly severe.

The thermal environment will also depend on how the SABRE nozzles are gimbaled. However, the plumes are so under-expanded that it is unlikely this will substantially alleviate the impingement effects. At M_∞= 12.189 for a perfect gas (γ= 1.4), the freestream total temperature is approximately 30 times T_∞= 231° K, and the freestream recovery temperature is roughly 26 times T_∞. However, based on real gas chemistry and the edge pressure = atmospheric pressure, the freestream total and recovery temperatures are, respectively, nearly 13 and 11 times the freestream temperature (as shown in Table 3). Here, the assumed recovery factor is 0.85.

Because Cart3D simulated temperatures are based on Euler equations for a perfect gas, they do not translate directly into fuselage skin temperatures. The surface equilibrium radiation thermal environment will differ when simulations are conducted with air and hydrogen/oxygen chemistry and account for viscous, plume radiation, and real gas (γ~ 1.3) effects. This level of physics will provide information such as surface temperatures, flow separation, and realistic effects of shock-shock/boundary interactions and vertical tail bow shock/boundary layer interactions. Nevertheless, the fundamental fluid phenomena will remain the same. High-temperature gas has the ability to emit significant radiation in the UV, visible, and IR regions of the electromagnetic spectrum, leading to potentially substantial heating of the aft fuselage surface. Radiative processes augment the convective heating.

By the way, above, "Total Temperature" (and Total Pressure) is synonymous to Stagnation temperature (T_o) (and Stagnation Pressure, P_o), the temperature (pressure) of a stream of air that comes to a stop at the nose or leading edge of a wing, and is usually the highest anywhere on the fuselage.

So take these NASA engineer's "exact" results with a big grain of salt. They are missing a lot of detail. But the bad news is that, even cut in half to correct for non ideal gas effects (never mind heat transfer), the temperatures around the places where the plumes hit the fuselage would be in the order of at least 10 times higher than ambient temperature. At T_o = 13*231 K = 3003 K = 2729 C on the leading edge of the vertical stabilizer, the conditions are already harsher than anything the Space Shuttle skin ever endured at 1533 K = 1,260 °C = 2,300 °F (The maximum operating temperature if RCC panels is 1922 K)...

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100016285.pdf

These workers are MOSTLY correct in that the plumes would expand greatly and basically cook the tail. The transport of energy and pressures around the fuselage skin would be very similar with or without heat transfer, with or without viscosity, because the plumes are just expanding and compressing gases in free-stream. The geometric dilation of the plumes with altitude and speed would be the same. As you go higher and faster and the static pressure is lower, but your nozzle pressure remains the same, higher, relative to the ambient, and thus you create shock waves trailing the nozzles, and expansion compression "diamonds."  

http://www.aerospaceweb.org/question/propulsion/q0220.shtml

http://www.allstar.fiu.edu/aero/rocket3.htm

The plume edges expand outwardly the higher up (and the faster) you go to get into orbit.  The nozzles become very "under-expanded" (over-pressured) at high altitude, engulfing the tail, with nozzle shocks intersecting the skin in front of the vertical stabilizer. That whole tail end of Skylon needs a very different thermal rating.  

Their Lift to Drag ratio discussion, I found much less interesting. Basically they're saying that at Mach 3, REL's lift and drag figures are too "optimistic,"  but at hight speeds (M=15) their figures are "pessimistic," because the plumes actually help you equalize the front-to back pressure distribution, thus reduce drag and increase lift - never mind that you just melted/ablated your tail off.  

So their use of "favourable" and unfavourable" terminology in the paper depends on what you're talking about. Favourable for L/D.  VERY unfavourable for heat transfer purposes. They flip flop on the terms (the term is more commonly  used for pressure gradients in engineering for  discussions).  Talking about plumes and L/D ratios is like talking about apples and bananas; they are related because both are fruits and grow on trees, but otherwise, makes little sense to equate lift to drag ratios to plume-widths. These are related only by way of the under-expansion of rocket nozzles which applies to this specific project (and rockets) but it's not obvious for airplanes in general.

And their "computations being superior to REL engineering practices" comments fully deserve to be in the "WTF"  thread. It seems they they hired a used car salesman to sell their "Cart3d" method  

~ ~ ~

Just because we're engineers and geeks doesn't mean we're good writers or presenters.  You should see some of the presentations in meetings. Professional engineers from industry are notoriously bad at conveying their message - only a couple of steps away from pointing and grunting at the screen.   That is part of the reason that industry started demanding that college students take courses in technical communication. So you don't end up confusing your audience.

~ ~ ~

In other news, I found these videos all of you might find interesting.

My God man! Look at that hair and those moustaches!  Did graduate students really look like that back in the 1980s? I started college in 1987 and I don't remember being that funky.  

Part 1 NOVA The Light Stuff (human-powered flight)

Part 2 NOVA The Light Stuff (human-powered flight)

Part 3: NOVA The Light Stuff (human-powered flight)

*Edited for scientific terminology correctness.

[Peter Brassbeard]

Out for a pleasant afternoon flight

[Miranda.T]

Dear Admiral Wilhelm,
That's some heavy fluid-dynamics you're cooking with there! That doesn't look too promising. It seems REL are moving towards putting together a full engine for ground-based tests in the next couple of years, so I guess once (if) that is working they can start to quantify these issues more concretely.

Yours,
Miranda.

[J. Wilhelm]

Dear Admiral Wilhelm,
That's some heavy fluid-dynamics you're cooking with there! That doesn't look too promising. It seems REL are moving towards putting together a full engine for ground-based tests in the next couple of years, so I guess once (if) that is working they can start to quantify these issues more concretely.

Yours,
Miranda.

What I'd like to see is for REL to continue research on the engine and instead of pushing for a payload bearing vehicle, concentrate on an traditional cylindrical rocket equipped with a SABRE air breathing engine for the sole purpose of testing the engine in the Mach 5-20+ range as well as the plume dynamics, and possibly the aerodynamics of an engine pod. That engine could be very important.

The progress of the more developed SABRE engine concept is being held back. By what I see as a hastily designed and secretive space plane airframe concept, which in my opinion requires a heavy conceptual overhaul,  vis a vis engine placement, further practical design of re-entry thermal protection systems and possibly engine nozzle geometry,  to actually become a viable SSTO vehicle.

Trying to do too much at the same time kills projects,  like it happened to the X-33. What is a perfectly viable project,  quickly becomes a white elephant in the eyes of investors, and the project as a whole ends up terminated.

Once the Sabre engine is developed, it will be much easier to sell the Skylon project to investors. A proven technology allows you to concentrate in areas that need developing. Then you can develop a new kind of space plane.

Consider the X-33.  Skunk Works was developing a new airframe (carbon composite), new skin (metal plates), new tanks (carbon fibre), and new engine (aerospike). Only the lifting body design was a derivative of past experience.

Result:  Each technology faced problems during developing. They ran out of time, and the whole design depended on all new technologies working.  When that didn't happen the design changed so much it was no longer an SSTO. The project was cancelled.

Now compare to the Space Shuttle:

Engines <== Saturn V moon rocket technology
Solid Rocket Booster  <== ICBM technology
Airframe technology <== traditional aluminium airframe technology with sparred wings, skin
External Tank corrugated (metallic thin shell pressure vessel) <== More Saturn V technology
~
Thermal Protection System <== All new technology
Hypersonic Lifting Body Design <== All new technology

They only had to concentrate on the aerodynamic design and thermal protection system.  The rest could be handled by industry subcontractors knowledgeable on their respective areas (Thiokol, Martin Marietta, etc.) Subcontractors upon subcontractors joined the team. Rockwell developed the Orbiter.

The "secret sauce" was that the skin allowed airframe designers to make a glider that actually looked like an airplane.  It had a delta wing.  A big one. With a huge vertical stabilizer, which implies slow speed positive control.

Result? A success. Just two new technologies were developed, the rest was sure-footed technology. The Space Shuttle itself was solid as a rock.  Outside of the two tragedies brought about by a defective O-ring, and a foam impact on the leading edge, that Orbiter was actually pretty good. A nightmare to maintain the tiles. And chunky-looking, but  a perfect design. It just worked.

http://www.youtube.com/watch?v=P2itpixEZ6o

Note: On ascent, upon orbital insertion, last measured Mach No. is 25 (12:04 min).

http://www.youtube.com/watch?v=PsHljHRM67c

[J. Wilhelm]

Out for a pleasant afternoon flight

Welcome aboard, Mr. Brassbeard! I see you have come to us 'a la Montgolfier!  Tried and true technology never dies.

[Miranda.T]

I think one has to see the shuttle as a qualified success.

On the plus side, it took more individuals into space than all other launch systems put together, it allowed the ISS to be built, it provided at platform for unprecedented advances in space science and completed audacious missions such as the Hubble repair. Oh, and it was the most complex transportation system ever built.

On the down side, operating costs were vastly more than promised (which many see as 'shackling' NASA to low Earth orbit by eating up large proportions of its budget), it's launch frequency and flexibility again never matched the promises, and 14 astronauts lost their lives due to it, which I believe is the single largest loss to any particular launch system (I think early Soyuz was 4 fatalities, but nothing since the early days); at the end, wasn't the estimate of catastrophic failure down to something like 1 in 50 flights?

Yours,
Miranda.

[J. Wilhelm]

I think one has to see the shuttle as a qualified success.

On the plus side, it took more individuals into space than all other launch systems put together, it allowed the ISS to be built, it provided at platform for unprecedented advances in space science and completed audacious missions such as the Hubble repair. Oh, and it was the most complex transportation system ever built.

On the down side, operating costs were vastly more than promised (which many see as 'shackling' NASA to low Earth orbit by eating up large proportions of its budget), it's launch frequency and flexibility again never matched the promises, and 14 astronauts lost their lives due to it, which I believe is the single largest loss to any particular launch system (I think early Soyuz was 4 fatalities, but nothing since the early days); at the end, wasn't the estimate of catastrophic failure down to something like 1 in 50 flights?

Yours,
Miranda.

It was 133 successful flights and 2 catastrophically failed flights. Naturally that was not their expected design failure rate. The foam impact can be considered a rocket launch failure, because of the need to have an external tank in the first place, and the failed O-ring is definitely a failure inherent to solid fuel ICBM rockets. 

The idea of the SSTO is to reduce costs, but with rocket engine technology, the risk is inherently high. Play with rockets and you'll get burned.  That is why developing the Sabre is so important.  It reduces system size and risk.

[Peter Brassbeard]

The difficulty with single stage to orbit is it shaves structural margins razor thin, especially once you account for recovering and reusing the stage.

[J. Wilhelm]

It may sound retrograde for me to say so,  but what we need to do is re-incarnate HOTOL in a non-payload version,  just for the purpose of testing and developing the SABRE.  It has the right geometry for the rocket nozzles plumes, in the current cylindrical geometry of Sabre, and it could have a delta wing in a Space Shuttle / Buran / Boeing X-37 re-entry hypersonic glider configuration.  

We already understand the fluid dynamics and thermal protection system of the Shuttle.   With minimal effort and existing materials we can develop an X-37 looking unmanned engine test bed.

As Ms Miranda wrote, the impression was that the original HOTOL was close enough in SSTO horizontal takeoff mode, and the only objection was the finding of having a heavy tail which in turn forced a body dynamic centre to be pushed very far back, which resulted in a small payload.  But what if there is no payload?

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

If we can get away with carrying fuel and horizontal takeoff,  then fine,  but even if we can't carry the fuel, we can use booster rockets or a second stage used to substitute for the low speed flight, or perhaps even a single stage carried piggyback on a 747. There are more design options available.

https://en.m.wikipedia.org/wiki/Boeing_X-37

~ ~ ~

Just having a little fun with Photoshop  

Basically, I'm picturing something like an X-37 with a SABRE engine mounted on top between two vertical stabilizers. Without a payload bay, this is basically a flying fuel tank with an engine. There should be no issue with the rocket plume in this case.  I'm unsure about the placement of the engine though.

Spoiler (click to show/hide)

Very often for wave riders with air-breathing engines, it is pictured that the engines including diffuser (inlet) and nozzle (outlet) would be placed on the windward side as opposed to the leeward side, with the windward side serving as lifting surface, as well as diffuser and nozzle (see X-30 NASP). For supersonic airplanes the same is done, like for example on the Concorde. There are several resons for that. At supersonic speeds there is some significant extra amount of compression that occurs at the bottom of the wing and belly of a lifting body. Also, it is more likely that the boundary layer will remain attached on the windward side. Turbulence and flow separation can be issues on the leeward side of the airplane, especially at high angles of attack and very low speeds. In rear-engined subsonic aircraft, like the Boeing 727 and Lockheed L-1011, a great deal of care was placed on raising the engine inlet away from the boundary layer, and sometimes the entire engine is moved high above the fuselage (eg. DC-10).

So the engine on top is a "no-no" under most normal conditions other than boat planes.  But this is not a normal condition; there is another problem.  One of the things that I did not understand from the Skylon, is how they are planning to protect the engine inlet cones (supersonic diffuser ramps) from the re-entry temperatures. What are they made of? In most hypersonic designs, the engine inlets are rectangular, so you can extend the thermal protection systems to the diffusers themselves (flat surfaces can easily made into supersonic diffuser ramps), even if that thermal protection is in the form of thick silica bricks or Reinforces Carbon Carbon. So the rule of thumb is rectangular inlet for most underwing designs.

This means that either we place the engine on top and accept loss of compression for ramjets between Mach 3 and 5 and possible subsonic turbulence, or we redesign the SABRE to a rectangular configuration to allow for passive thermal protection systems during re-entry, or we figure out how to use most of the cooling system of the SABRE to actively cool the engine cowl during re-entry.  Sticky problem.

I imagine that having as straight a path as possible from the heat exchangers/bleed concentric to the compressor and ramjets would be ideal, rather than tight elbow ducts as envisioned for the Rolls Royce RB545 engine in the original HOTOL concept. That design was cavalier enough to separate the heat exchangers far away from the compressor and actually have ducts passing air into the compressor "one storey above" the inlet, so to speak - I'm not sure how much ramjet action remains - if at all, after that. Hence, maintaining as much of the coaxial geometry on the SABRE is what I imagine.

The HOTOL reborn
Disclaimer: no measurements below are intended to be accurate in any shape of form.  This is just a rough concept.

[Miranda.T]

Dear Admiral Wilhelm,
The two-stage approach, with both being recoverable, winged vehicles, is definitely s potential solution here, and of course this is exactly the approach SpaceShipOne took to win the X-prize. The only reason I can see why it's not been taken up for orbital craft is the cost of developing and building two vehicles. Wasn't this one of the very early ideas for the shuttle? As you mentioned, that would have removed the need for external tank and SRBs, removing the reasons for both accidents; with this design, the shuttle could still be flying today. Swiss Space Systems are currently working in such a design - http://www.space.com/21605-project-soar-swiss-space-plane-gaining-steam-video.html.

There has been much debate over the configuration of the sabre test-bed - maybe they should look back at the HOTOL configuration for these. As a little aside, at one point they thought of assisting HOTOL's launch by using a rocket-sled launch ramp; shades of of Fireball XL5 https://www.youtube.com/watch?v=u7LMlr9cWO4

The difficulty with single stage to orbit is it shaves structural margins razor thin, especially once you account for recovering and reusing the stage.

Definitely; everything on Skylon, if it is ever to fly, will be pushed to the absolute limit. If only the Earth had the gravitation of, say, Venus, it would be a lot simpler...

Yours,
Miranda.

[J. Wilhelm]

The terrible beauty of Hurricane Patricia, the strongest-ever hurricane to hit the Americas on the Pacific side, as photographed by Astronaut Scott Kelly aboard the International Space Station

https://twitter.com/StationCDRKelly

Hurricanes are Mother Nature's own turbine engines, like a giant energy sink, converting the energy stored in warm air and moisture into mechanical energy.

As photographed from a geostationary weather satellite:

[Miranda.T]

It seems that REL are receiving an influx of cash and expertise: http://spacenews.com/bae-takes-stake-in-british-air-breathing-rocket-venture/.

Yours,
Miranda.

[J. Wilhelm]

It seems that REL are receiving an influx of cash and expertise: http://spacenews.com/bae-takes-stake-in-british-air-breathing-rocket-venture/.

Yours,
Miranda.

That would be good news, but unwritten are the strings attached to that money.  The article specifically states that "[BAE] had agreed to purchase a 20 percent equity stake in single-stage-to-orbit engine designer Reaction Engines Limited (REL) and would provide the company with 'industrial, technical and capital resources' for ground-based testing of a prototype".

The critical phrase is "ground-based testing of a prototype." Remember that Mach 5+ wind tunnels are practically non existent, and have extremely small test chambers. because it takes an enormous amount of energy to accelerate air to those speeds.  Moreover, once you do, the gases in the test chamber are not air, but rather a decomposing soup of molecules that have been super saturated with energy - and that means that heat transfer experiments are useless in ground based experiments outside of rocket sleds. True hypersonic testing usually involves mounting something on a sounding rocket an launching it, so the air at high Mach numbers is pristine and cool.

The SABRE engine, on the other hand employs a turbo-machinery based hybrid-cycle, and therefore you can simply reproduce the performance of half the cycle in ground based facilities.  Also, even for a ramjet, the flow inside the engine downstream from the diffuser is subsonic, so again, you can reproduce that more easily in ground facilities.

In other words, the funded testing pertains exclusively to SABRE and not to SKYLON.

Also, it seems that we, Americans, are of a single mind:

Quote from article above

The U.S. Air Force Research Laboratory subsequently looked at REL and SABRE and said the technology shows promise, with the caveat that it might be better suited to an less-challenging architecture employing two stages to reach orbit.

My agreement with their opinion is no accident, perhaps.  The Air Force Office for Special Research, AFOSR (a division of the same office as above) used to provide the funding for my undergraduate research project and that of two other graduate students in my office back in 1998, plus two graduate students at Purdue University (where the actual Mach 4 wind tunnel was).  That AIAA paper, above which I wrote, was funded by the AFOSR (note not all military projects are classified - many are open to the public including international students), and was based around testing for Mach 4 low altitude self propelled anti tank projectiles.  So this opinion by the Air Force doesn't surprise me.  

I'd be happy to participate in the development of a test vehicle if somebody made a little room for me.... But that's the $64K question, ain't it? 

[Miranda.T]

I'm determined to get Steampunk Sub-Orbital (SSO) to work. After having no volunteers to travel in my Jules Vern inspired multi-stage accelerated projectile (after all, I can't do it, I need to be back in mission control...), I've decided to try a different approach - an airship launched rocket-plane.

This is an idea that is kicked around in the 'real world' as a possible launch system, and seems right up Steampunk's street. The airship part should be possible; a bit of focused research to to develop an ultra-high altitude version, but how could a rocket be produced with Victorian/Edwardian technology? It is, after all, rocket science, and how long did it take brilliant engineers such as Goddard, Korolev and Von Braun* to get the technology working? Then I thought of a, if not primitive, then at least a more straightforward example of a rocket-powered craft - the Me 169 Komet. Could, with the appropriate knowledge, the materials available in the early 20th century be up to building something along the lines of the Komet? Then, with a high-altitude launch and assuming the evil fuel used didn't blow up or leak and kill the pilot, I wonder how high the craft could have been pushed?

Then there's all sorts of interesting questions over pressure suits for the pilot (and indeed coping with the pressure differential for the rocket's mechanism) and the mechanism of slowing the whole thing down for a safe descent (maybe air-brakes or a variant of what SpaceShip One used). Note there's no need for too fancy a material to protect the craft against heating on re-entry, as this is just sub-orbital.

Anyway, I shall throw it open to debate...

Yours,
Miranda.

* Just focussing on the engineering and ignoring actions taken during World War 2...

[J. Wilhelm]

You know? That is quite a workable idea.  Do you remember all those YouTube videos about students launching balloons with cameras,  and so forth? Why not use a balloon to lift a small rocket plane?  I bet I can accelerate to any Mach number I desire if the craft is small enough. If I could self level the rocket (dropped within the atmosphere for positive dynamic control) .   It would have to be a hypergolic fuel rocket (solid rocket) powered craft so no oxygen tanks are required.

For Victorian technology I'd envision stratosphere capable rigid airships like the USAS Orca,  but weather balloons to carry experiments to low orbit altitude, and then drop launch self leveling rockets from that altitude. The thing is it has to be a rocket and not a plane,  because practical aerodynamic knowledge was sparse.

[Miranda.T]

The Komet's rocket seemes to have been a bit of a hybrid between a solid fuel which gave oxygen for a liquid one ('T-Stoff' and 'C-Stoff' - or possibly the other way aorund!). The reason I was thinking rocket plane was to allow it to be dropped and then fired, with the control surfaces taking it from level to rising flight. If we could stretch the era's knowledge base to rocketry, could we stretch to a bit more understanding of aerodynamics?

Yours,
Miranda.

[J. Wilhelm]

The Komet's rocket seemes to have been a bit of a hybrid between a solid fuel which gave oxygen for a liquid one ('T-Stoff' and 'C-Stoff' - or possibly the other way aorund!). The reason I was thinking rocket plane was to allow it to be dropped and then fired, with the control surfaces taking it from level to rising flight. If we could stretch the era's knowledge base to rocketry, could we stretch to a bit more understanding of aerodynamics?

Yours,
Miranda.

Well there are several ways to do this depending on how high you climb and how far you're willing do drop before attaining positive attitude control.  The last thing you want is an uncontrolled tumble.  But the problem is that the higher you climb before launching the rocket or rocket plane, the less chance you have of using any aerodynamic control at all, so you have to make a choice what kind of vehicle you want.

For a controlled descent to Earth, the latter method is preferable.  Perhaps this is where a mid-atmospheric (115 km altitude) vehicle starts resembling Burt Rutan's Spaceship One? I'd expect that there'd be an initial drop whereby if you let it drop, you may attain aerodynamic control, and the fire the rocket to a maximum Mach number of 3.  The terminal speed altitude being key here, as the speed of sound (and hence the Mach number) is very sensitive to the altitude, and as the density of the atmosphere greatly increases, so this would determine your maximum speed.

But for a high altitude hypersonic trial, preferably I'd try to not have a long drop and instead try to gain rocket powered (hypergolic) attitude control right away.   Then the ascent would be dominated by rocket propulsion, with a subsequent burn designed to attain the maximum speed possible. At this stage the vehicle looks more like the X-15 maintaining a stable flight in the Mach 5 range (speed= 5000 km/hr speed of sound=1000km/h at 145km~90 mi of altitude. Then the descent would be as a glider, still far too slow compared to an orbital re-entry vehicle like the Space Shuttle, but very much more like the descent of the German WWII Era Silbervogel, using a lifting body design to create a cyclical lift ("bouncing") from the tenuous atmosphere.

[Miranda.T]

I was thinking the craft would be a bit X-15 like. One thing though - presumably the airship is hydrogen-filled to get maximum lift, so you wouldn't want to be igniting the rocket too soon after drop...

Yours,
Miranda.

[J. Wilhelm]

I was thinking the craft would be a bit X-15 like. One thing though - presumably the airship is hydrogen-filled to get maximum lift, so you wouldn't want to be igniting the rocket too soon after drop...

Yours,
Miranda.

That's an interesting question, given that the oxygen content of a rarefied atmosphere is very low... To be honest I don't know the answer as to exactly how severe the risks are.  Even  without combustion (in which case the lingo is that this would be a detonation, not an explosion)  you naturally don't want to puncture the gas bag; you want the airship to be away from the rocket plume.

[MWBailey]

There's bound to be a graph or table somewhere detailing the amount of oxygen necessary for hydrogen to combust...

Not a graphic, but a yahoo answers page on the subject:

https://answers.yahoo.com/question/index?qid=20100522105309AA1bWkS