Talk-Polywell.org

I've yet to see any real figures or design for Skylon. They're working on the engine, not the ship. None of the graphics of the ship are actual representations as the landing gear is always conveniently tiny or absent. In actuality, for the design to work at all, there is a huge portion of the dry mass that needs to be devoted to the landing gear, because it has to support the weight of the entire, loaded ship. This is why X-33/Venturestar was vertical launch, to reduce the mass of the landing gear. If Skylon ever makes it to actually engineering the design of the ship, my guess is they'll be forced to go vertical launch for the same reasons.

SABRE is billions away from happening and Skylon is billions behind that. I just can't imagine sans government support it will ever fly.

I've yet to see any real figures or design for Skylon. They're working on the engine, not the ship. None of the graphics of the ship are actual representations as the landing gear is always conveniently tiny or absent. In actuality, for the design to work at all, there is a huge portion of the dry mass that needs to be devoted to the landing gear, because it has to support the weight of the entire, loaded ship.

I remember reading about the Skylon landing gear and why they think they can do that now (the old HOTOL was to have a sled for takeoff and only used the gear for landing).
IIRC, they think that they will get away with high pressure tires and water cooling for takeoff. The water will be dropped after take off.

That's interesting news but as I said, they haven't done any real engineering of the airframe yet. High pressure tires and water cooling aside, look at this pic:

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.

GIThruster wrote:...they haven't done any real engineering of the airframe yet.

I disagree.
On the other hand, I'd have to say that the engine has to come first.
Its the limiting factor.
The engine is the thing around which all else must be engineered.
So, in that way, they're doing it right.

Didn't Hempsell himself say that this was because Skylon would only land light?

zDarby wrote:

GIThruster wrote:...they haven't done any real engineering of the airframe yet.

I disagree.
On the other hand, I'd have to say that the engine has to come first.
Its the limiting factor.
The engine is the thing around which all else must be engineered.
So, in that way, they're doing it right.

but what use is a super-fast engine if you don't have an awesome-looking aircraft? priorities, man!

That's interesting news but as I said, they haven't done any real engineering of the airframe yet. High pressure tires and water cooling aside, look at this pic:

From what I understand they have.

Well I just watched the vid and it was a huge disappointment. It must have been nerfed after Darby posted the link. There is something like 10 minutes of people talking about stuff other than the topic, and finally when the topic arrives the vid is cut off. There is no data here at all about the progress of Skylon except that the British government is funding the research. There is no detail of any design work on Skylon as opposed to SABRE.

And just saying again, the art work won't fly. The thing is an enormous fuel tank. It has extremely high wing loading. It needs to be going very, very fast to take off and with so much weight it will require a huge mass fraction for the landing gear. That is not what is in the art work.

There is no detail of any design work on Skylon as opposed to SABRE

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Of course they have done that. ESA even signed off on the design of Skylon, not just the engine.
Bond and his team are not some rookies. They have been in aerospace for a loooong time. Bond was involved with the original HOTOL as well (among many other things).

That's funny because I distinctly recall Bond and his team are all from Rolls Royce. They're accustomed to designing turbine engines, not spacecraft.

Just sayin', I've been following for several years and I have seen no evidence that any serious work has been done on Skylon itself. There's plenty of rough figuring and art work, but obviously they are not portraying the concept realistically with stilt-shaped landing gear that looks like it came off Concord. There is no way to support the kind of weight we're talking about with the gear in the art work.

I can't say I've seen much detail on the landing gear (or any other important subsystems apart from the SABREs), but I too think that they have done their homework:

First, the undercarriage was one of the big problems of HOTOL and went trough numerous iterations, so I think they know they should take a careful look at it.

Also, the numbers from

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.

suggest to me as a layman that they have at least roughly specified the tires and brakes. This might have been all that ESA was asked to look at but otherwise I think they might have written something more like "the wheel bogies look good but the rest of the landing gear should be rescaled to x times the current load...".

GIThruster wrote:That's funny because I distinctly recall Bond and his team are all from Rolls Royce. They're accustomed to designing turbine engines, not spacecraft.

Just sayin', I've been following for several years and I have seen no evidence that any serious work has been done on Skylon itself. There's plenty of rough figuring and art work, but obviously they are not portraying the concept realistically with stilt-shaped landing gear that looks like it came off Concord. There is no way to support the kind of weight we're talking about with the gear in the art work.

Concorde's max take-off is 180t, Skylon's is ~400t, not a big difference considering that Concorde had to be able to land when full and Skylon does not, and that an airliner has to land tens of thousands times, while Skylon only a few hundred.

Besides, this is mostly a flying hydrogen tank. It is quite heavy, but at the same time low density and thus really huge. The wheels might just seem small compared to the airframe size.

That's funny because I distinctly recall Bond and his team are all from Rolls Royce. They're accustomed to designing turbine engines, not spacecraft.

They are not "all" from Rolls Royce. Many of the engine development team, including himself did also work for Rolls Royce. IIRC, Bond is some 80 years old now. He has been in this business for a long time. He was one of the principal people behind the HOTOL.

Jded wrote: Concorde's max take-off is 180t, Skylon's is ~400t, not a big difference considering that Concorde had to be able to land when full and Skylon does not, and that an airliner has to land tens of thousands times, while Skylon only a few hundred.

Besides, this is mostly a flying hydrogen tank. It is quite heavy, but at the same time low density and thus really huge. The wheels might just seem small compared to the airframe size.

C5 M
Empty weight: 380,000 lb (172,370 kg)
Loaded weight: 769,000 lb (348,800 kg)
Max. takeoff weight: 840,000 lb (381,000 kg)
Just about what a Skylon would weight .
I don,t think the engineering landing gear would be the problem

Skylon's max takeoff weight was about 325 tonnes, last I heard of the in-progress D1 design (last year). The C1 used in the pictures was 275 tonnes. It's mostly liquid hydrogen, in nonstructural tanks in an airship-style structural frame covered in a nonstructural composite skin; it's a lot lighter than it looks. And the LOX tanks are amidships, bracketing the payload bay, so the nose gear load fraction should be fairly low too.

Yes, they've done engineering on the airframe. They're well into the design of the D1 configuration, if they haven't finished it yet, and it apparently looks very similar to the C1 even though they started from scratch instead of iterating on the old design.

The design shown in the pictures isn't a fully-detailed buildable design, but it's not just an artist's conception.

They have done design work on the landing gear, as they have with the rest of the airframe. They say they can get down to the 1.5% mass fraction seen in the B-58's landing gear, but at the cost of high tire loads requiring specially strengthened runways similar to those used by the B-36. This is with water cooling for rejected takeoffs, the water being dumped before the climb to orbit.

One reason a cargo plane has so many tires is that it can't afford to require special runways.

GIThruster, you say you have been following this for a while now. To me it seems that you haven't been following it very closely. Your criticisms amount to argument from ignorance.

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Regarding Musk's comments, he was talking about a two-stage vehicle, or perhaps air-launch. He showed no sign of knowing what SABRE or REL even were, or that he was being asked about an approach to SSTO.

The SABRE, if they can get it to work, enables SSTO with robust margins (the required mass ratio is less than 5) and high payload fraction. D1, last I heard, was predicted to get 15 tonnes to LEO with a 325-tonne initial mass, which is 4.6% - that's half again as large as the Delta IV's wet payload fraction. Based on the old C1 data (C1 turned out to be optimistic with the tech level they were working with, leading to the C2 scale-up, but D1 incorporates advances that seem to more than make back the shortfall), the dry payload fraction looks to be similar to that of the Delta IV.

For TSTO, it's completely plausible that airbreathing isn't worth it. For SSTO, with all the advantages that accompany it, a working SABRE would be a game-changing enabling technology.