Mark Thomas will be a speaker at this conference in May:http://www.aerosociety.com/Events/Event-List/2197/What-Price-Speed
Read elsewhere that the hydrogen is subcooled to 16K. If the tank pressure is reduced to 1 atm during flight then NPSH drops to about 0.7atm minus all the losses on the way to the pump inlet. The turbopump guy is not going to be very happy about this.
Part of the subcooling is for pump design reasons. The rest is so the vehicle can sit on the runway for two hours without venting. And I imagine the densification doesn't hurt... (Just realized I probably overestimated the amount of kerosene/LOX that would fit into the tank volume, though I suppose you could densify that too...)
Carryover from another thread:http://forum.nasaspaceflight.com/index.php?topic=40066.msg1521829#msg1521829
Quote from: 93143 on 04/21/2016 04:35 amQuote from: Nilof on 04/20/2016 11:46 pmMeanwhile, much more "traditional" SSTO proposals can get similar or higher margins and less payload uncertainty with way fewer new technologies and a more reasonable development budget...Analysis or citation please. This statement is preposterous on its face.Actually, if a Skylon vehicle used dense propellants, conventional high t/w rocket engines and was launched vertically, it would place somewhere between 2 and 4 times more payload into orbit for its empty weight (obviously swapping out the SABRE engines for rocket engines). I've done this calculation for several different iterations of the published Skylon empty mass, so don't wish to bother to hunt up the link here on NASAspaceflight at the moment. But using published info on Skylon anyone can repeat the calculation.The calculation illustrates Skylon's real technical risk (beyond the engines working as planned). No surprise, but its mass fraction is quite high, higher than I'd be comfortable proposing for a LOX-hydrocarbon vehicle, even VTOL. It just looks low due to the larger amount of LH2 it employs. But PMF calculations always must be normalized to propellant density, not weight, since that is the proper metric.
Quote from: Nilof on 04/20/2016 11:46 pmMeanwhile, much more "traditional" SSTO proposals can get similar or higher margins and less payload uncertainty with way fewer new technologies and a more reasonable development budget...Analysis or citation please. This statement is preposterous on its face.
Meanwhile, much more "traditional" SSTO proposals can get similar or higher margins and less payload uncertainty with way fewer new technologies and a more reasonable development budget...
After running through the mass numbers for Skylon that Reaction Engines offers, I'm left believing that they've got an engine that will theoretically get them to orbit in one stage, but only by them making unrealistic assumptions about the structural weight of the rest of the vehicle, total dry mass 53 tonnes, engine thrust 270 tonnes, engine T/W 14 therefore mass of engines ~19 tonnes, therefore mass of the rest of the vehicle ~34 tonnes. 34 tonnes for a winged vehicle that's 83 meters long, carries all its landing gear to orbit, it has a propellant volume around 1400^3 meters, it uses cryogenic propellants and it has to endure re-entry. I know they're promoting Skylon as having revolutionary construction materials and methods, but it seems to me they've had to make some excessively optimistic assumptions about the structural weight to get the numbers to come together so they can continue with their pet project - the engines.Looking at it another way: The combined propellant tank volume by my math (with a few assumptions on the current LH2:LOX ratio) would have enough volume to hold 500 tonnes of LH2/LOX at a 1:6 ratio, lets allow structural weight growth of 20% for the heavier take-off weight making structural wt 40.8 tonnes, 2 SSME's (or easily maintained equivalent) is + 6.4 tonnes, so total unladen weight is 47.2 tonnes, add a P/L of 15 tonnes and also the 500 tonnes LOX/LH2 and you get a take-off weight of 562.2 tonnes, at engine shut off weight is 62.2. Mo/M1 is 9.03, delta V at Ve 4500 m/s is 9907m/s.
While I was obviously too optimistic about the achievable exhaust velocity from SSME's lifting from the ground, I see no reasons in the rebuttals to Gary's comments to revise my skepticism of Skylon, that version of Skylon had a fuselage physically larger than an A380 that weighs less than 34 tons. Why aren't we building all out jet aircraft using the aeroshell that RE plans? Think of the fuel savings that could be had by cutting the weight of aircraft by over 50%!
Quote from: Alf Fass on 05/12/2016 06:46 amWhile I was obviously too optimistic about the achievable exhaust velocity from SSME's lifting from the ground, I see no reasons in the rebuttals to Gary's comments to revise my skepticism of Skylon, that version of Skylon had a fuselage physically larger than an A380 that weighs less than 34 tons. Why aren't we building all out jet aircraft using the aeroshell that RE plans? Think of the fuel savings that could be had by cutting the weight of aircraft by over 50%!So you don't really know anything, but you're sure the number have been fudged. Is that about it?
And you really can't figure out any reason all aircraft are made from this material?Passenger aircraft don't have to deal with skin temperatures of 1100c. They can get by with Aluminum and composites. The skin material is likely to be more expensive and certainly has less usage history than either, so they won't use it as there is no compelling need for it.
You've also ignored the fact that such a lightweight structural concept maybe possible because LH2 has such low density and the load is quite evenly distributed within the whole fuselage.
Rather like a soda can, which weighs 11g and carries around 330g, but can support at least nine more sitting on top of it. IE a load of 3.3Kg sitting on an 11g, a 300:1 payload to structure ratio.
But you can't scale that performance up. The problem plays to the strengths of the materials used to create a solution that seems to be impossible. Much like Skylon's structural concept.
rockets on the drawing board often don't turn out to be as good as the designers hoped
Quote from: Alf Fass on 05/13/2016 01:37 amrockets on the drawing board often don't turn out to be as good as the designers hopedLike?
So the vehicle has bulkheads in it to keep the H2 load evenly distributed even when the acceleration of the vehicle would make it unevenly distributed?
If the light weight Skylon fuselage is possible, it will be easy enough to build the tanks with that system to get to orbit with a reusable SSTO vehicle carrying the same PL, with the same vehicle dry mass, by using just plain old rocket engines.
Quote from: Alf Fass on 05/13/2016 01:37 amIf the light weight Skylon fuselage is possible, it will be easy enough to build the tanks with that system to get to orbit with a reusable SSTO vehicle carrying the same PL, with the same vehicle dry mass, by using just plain old rocket engines.Skylon is a detailed design optimized for a specific load envelope and mixture ratio with hydrogen in the bulk of the volume and almost all the heavy stuff in the middle where the lift and thrust loads are. You can't just hijack it for rocket mixture ratios, never mind different propellants or a different vehicle configuration, and expect the dry mass to stay the same. Mark Hempsell already pointed this out.
I didn't say fudged, rockets on the drawing board often don't turn out to be as good as the designers hoped, you and I are both in the situation of not really knowing anything, you're just trusting that this paper plane will work as well as its promoters imagine.
And you know these facts from what?
And if an ant were as big as an elephant it could lift 10 tons
If the light weight Skylon fuselage is possible,
it will be easy enough to build the tanks with that system to get to orbit with a reusable SSTO vehicle carrying the same PL, with the same vehicle dry mass, by using just plain old rocket engines.
Anyway, I updated my calculations for Skylon's operators and producers business case with very, very conservative assumptions. Assumptions for the operator: Skylon acquisition price: 2.3 BN Eur. per Unit (4x original REL value)Assumptions for the builder:Skylon development costs: 20 bn Eur (about 25% higher than REL estimates)
Quote from: francesco nicoli on 05/13/2016 01:45 pmAnyway, I updated my calculations for Skylon's operators and producers business case with very, very conservative assumptions. Assumptions for the operator: Skylon acquisition price: 2.3 BN Eur. per Unit (4x original REL value)Assumptions for the builder:Skylon development costs: 20 bn Eur (about 25% higher than REL estimates)This is a most interesting post but I think you're starting numbers are a bit off. Despite being a UK company REL normally quote their development budget and Skylon pricing in US dollars, whcih I think ithe international standard for the launch services business.IIRC their current development budget (including the Skylon Upper Stage) is $12Bn, that's 10.61Bn euros, so 25% higher would be about 13.27Bn. 20Bn Euros is about 88% higher. IIRC this is derived from the ESA cost model, which is very conservative, given it predicted the A380 cost to be about $3Bn higher than it actually was. The LSE analysis performed for ESA used a price of $2Bn for a Skylon, which is about Bn1.77 Euros. Your figure is therefor about 30% higher than the LSE estimate, which I think was conservative to begin with.