Betruger wrote:Paging Dr 93143
Uh...
...okay. Let's do this.
GIThruster wrote:They haven't even begun to work on anything but the engine, and the real work has all been in the heat exchanger.
We discussed this a bit in
this thread, but I suppose a little rehash never hurt anybody, and there were things that weren't said...
REL has done extensive work on the airframe. If you read the papers, it's obvious that it's much more than an artist's conception. The structure and TPS concept and layout is quite detailed. The truss framework has been designed to maximal load cases using a safety factor of 1.5. TPS samples have been tested, and Pyromeral Systems Inc. has been engaged regarding upgrading Pyrosic to System 2 standards. CFRP truss elements have been tested, and titanium composite truss elements have obsoleted the CFRP ones (a visitor to their booth at a recent show got to hold one, and accidentally threw it at the ceiling because it was so light - it was rated for 15 tonnes in compression). Local solutions for high heating rates have been developed, including a promising effusion cooling concept that seems to have replaced a liquid metal cooling solution on part of the leading edge of the wing which was subject to a shock-shock interaction. The whole airframe shape has been tested to Mach 12 in a wind tunnel, and high-fidelity reentry simulations have been done.
Shape-wise, Skylon seems to be a counterexample to von Tiesenhausen's Law of Engineering Design
(#30). Everything about the external shape is an obvious result of engineering decisions. The fuselage is a modified Sears-Haack body, and the reasons for the modifications are clear (trim, tank volume vs. payload width, area ruling, OMS accommodation). The wing is a standard delta in low-wing configuration. The engines are mounted in nacelles at the wingtips to solve the centre-of-gravity problem HOTOL had without cooking the fuselage with the exhaust, and they're curved because of a mismatch between the desired angle of attack and the desired angle of thrust. The intakes are a simple shock-on-lip design that allow the bypass ramjet trick while eliminating spillage drag and providing just enough pressure recovery to maintain the compressor face at a nearly constant pressure over the whole trajectory; the translating spike design also allows them to be closed for reentry. The canards vs. tailplanes are less clear to me, but the D1 went with canards after messing with rear stabilizers for a while, so plainly it was an engineering decision, probably related to trim. The tailfin is apparently sized for engine out, if I'm reading the ESA report correctly... and that's the whole thing. No V-tails or cranked deltas or waverider shapes here...
Logistics have also been looked at. They envision a standard payload canister that would eliminate the need for clean-room conditions when loading. The User Manual has specifications for payload CoM limits, power supply, acoustic and thermal environments, and so on. Propellants are loaded subcooled to allow substantial holds without the need for venting. The OMS and fuel cells use hydrogen and oxygen, just like the main engines but stored in separate insulated tanks - no toxic hypergols. The 2-day turnaround time is currently driven by TPS inspection, and could potentially be reduced. And Mark Hempsell recently left REL to start a consulting firm to try to sell a universal docking standard that doesn't suck, which could be used on everything from satellite deployment and refueling mechanisms to moon base modules.
Skylon C1 was only a proof of concept, but still involved a lot of design work and bench-scale risk reduction. In contrast, the D1 design effort was/is an attempt to come up with a high-resolution design that is essentially guaranteed to meet its payload target. It is reportedly "consistent with" AIAA standards for mass growth margins, and incorporates a 2.5-tonne performance margin on top of that. It benefits heavily from the new SABRE 4 thermodynamic cycle, which wastes a lot less hydrogen than previous iterations; it turns out this broke their old trim strategy from C1 (which involved draining the rear hydrogen tank first to counter forebody lift at high Mach numbers) and they had to rework it, resulting in some subtle changes in the shape. Last I heard it looked like about a 325-tonne vehicle (with margins) with a 15-tonne payload (also with margin). Oh, and apparently due to the better engines and larger wings, it's now expected to be capable of intercontinental self-ferry on straight hydrogen...
As for the engine, it's not just the precooler they've been working on. They've done experimental work on contra-rotating helium turbines, air cooling and LOX cooling of rocket combustors, high-temperature SiC secondary heat exchangers and their manufacture, a preliminary inlet design that was tested in a wind tunnel... even a nonessential like the expansion-deflection nozzle has been bench-tested and is shortly getting a dedicated sounding rocket (the Valkyrie) to try it out over a Skylon-like trajectory with Skylon-like exhaust (the thing is an ammonia/nitrous/LN2 triprop!). The thermodynamic cycle itself is on its fourth major revision since the Rolls-Royce RB545, and at least two detailed computational models of the previous iteration are publicly known to exist, one at REL and one at the von Karman Institute.
GIThruster wrote:What would it fly in? Can it be dropped into an existing frame? Last I saw of SABRE they were still using art work with landing gear that could not possibly support the mass of the craft. It's easy to get SSTO numbers if you cheat on things like the requirements for taking off.
Okay, let's see your calculations.
Skylon is not "art work". It's the product of a decades-long low-level design effort that
has involved an attempt to get the landing gear mass down, largely because the HOTOL team had such a bad experience with the launch trolley idea. The big issue was apparently braking energy during an abort, which the water cooling solves. And the ESA did look at the airframe design, including the landing gear, and concluded that it made sense.
From erblo on the other thread:
ESA assessment report wrote:6.9 Undercarriage
A water-cooled under carriage is assumed as, without it, the brakes were originally sized by the abort case fully laden. Nitrogen (N2) is used for inflation. High pressures are required for takeoff however; for landing, the vehicle only needs around 5 bar, 66kg of N2 is required for take off and it is foreseen to vent to the lower pressure level for the trip to orbit (The effect of a failure to depressurise is not assessed but the mass penalty will be small).With water cooling, the brakes are now sized by the landing to 25.8 kg per wheel.
From you, on the other thread:
GIThruster wrote:That landing gear looks like it came off the Concord, not off a cargo plane which is what the mass of a loaded Skylon would be like. It needs landing gear like a C-17 or C-130--short and heavy with huge tires. If you put the kind of weight necessary for a fully loaded Skylon on the kind of gear they have portrayed, it would snap like a twig; and the people who did the drawing know this.
Actually, Concorde is 68% of the mass of Skylon C1 and 4/5 of its length. In other words, the static compression loads in geometrically similar landing gear (scaled with vehicle length) would be on average maybe 6%
higher for Concorde. There are dynamic loads and bending moments to consider, of course, and of course the Skylon gear isn't just a scale-up of the Concorde gear, but this plainly isn't the sort of situation where someone with little or no real engineering experience or training could simply glance at the design and
know that it's wrong.
Remember, the vehicle is mostly full of liquid hydrogen, and the heavy stuff (engines, LOX tanks, payload bay) is all clustered around the wings and main gear. It's not as heavy as it looks.
It
is a big aircraft for such a low footprint. That's why they need a specially strengthened runway. Cargo planes can't require that, so they make sure not to overload the ones they want to be allowed to use.
...
Skylon D1 actually seems to have enough margin to triple the mass of the undercarriage, assuming nothing else goes wrong. But I don't expect them to have to do that.
ladajo wrote:If they don't test a full up engine by the end of the year, it will be front half of next year.
Unless you have information I don't, I wouldn't expect the SABRE to actually be ready for testing that soon. Last I heard they were targeting 2017 for stand testing. I'm not sure they'd be able to get a test stand ready this year anyway - it's a big engine, and loud.
It
is encouraging that they got enough investment this time around to make them decide (provisionally) to forgo the subscale demonstrator, which wouldn't have been representative in some ways, and go whole hog...
Oh yeah, that reminds me:
GIThruster wrote:SABRE is billions away from happening
Actually, it looks like an investor commitment of less than £400M is enough to go for the full-scale prototype...