When the Shuttle designers chose a vehicle with no early launch abort system, this was because all things considered, they didn't think one would be needed. Contrast this with SpaceShipTwo which automatically orientates itself to re-entry attitude. My guess is you could be tumbling with a full electrical failure on SS2 and still make it back OK.

Hi folks, I share RobLynn's skepticism about Skylon, given the difference in the weight of a couple of SSME's and the Sabre's, and given the very low density of the propellant mix. It seems to me that the main reason that Skylon works on paper is because of it's extraordinarily low structural weight: 53 tonnes unladen mass minus 19 tonnes leaves just 34 tonnes for everything else, and that's for a vehicle thats 83 meters long - longer than any commercial aircraft now flying.

Doing a primitive all-rocket SSME comparison with Skylon C1 configuration:Skylon 1100m³ tanks, 56 tonnes in orbit, of which ~19600kg for 2 Sabre engines.http://www.spacefuture.com/archive/a_single_stage_to_orbit_thought_experiment.shtmlUsing 3x 3200kg SSMEs would reduce vehicle mass by 10 tonnes - so 46tonnes to orbit.Assuming 9200m/s deltaV and an ascent averaged Isp of 425s gives an initial mass of 418 tonnes needing 1040m³ of 358kg/m³ LOX+LH2. 3xSSME at 109% give lift off thrust to weight about 1.31. Low drag and High L/D may enable the trajectory to be optimised for lower delta V.That is 5% smaller fuel volume than Skylon.There are probably also some additional weight savings to be had (reduced wing and landing gear loads, possibly smaller rudder and canard area requirements with all rockets close to axis in a boattail, and reduced dynamic pressures and pitching moments during ascent).So if Sabre powered Skylon is feasible then does that also imply that VTHL SSTO is just as feasible? It would almost certainly have less risk and far lower development costs.A mountainside launch catapult giving a few hundred m/s might improve it to the point where it was greatly superior to Sabre Skylon.

OK, very primitive maths:The combined propellant tank volume is 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% to allow for a heavier T/O weight making structural wt 40.8 tonnes, 2XSSME'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 T"O 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.Call me a cynic, but I'm tempted to think that the weights work on paper because they have to work on paper, if they don't the Sabre's no longer have a function on a launch vehicle, and the entire enterprise is questionable.

The ESA evaluation had the TPS for the C1 as a carbon-silicon carbide (CSC) ceramic.

Using Skylon mass estimating relations the dry mass of your BOTE calculation is closer to 62mt including 4 SSME's required for thrust to weight and engine out, this puts your payload at about 5mt.This gives a payload mass fraction of 0.8% compared to to 4.3% for Skylon, a propellant fraction of about 90% versus 81%. About what you'd expect for all rocket versus Skylon.

What amazing TPS does this thing have that's cheaper, easier and better than the Shuttle?

How do you handle the forces from venting the tanks? Just do them in two opposite directions?(BTW, thanks for being so accessible on this site; it's great!)

Will you be able to release information on the progress of the precooler tests? Or will it have to wait for the full report?

Hi folks, I share RobLynn's skepticism about Skylon, given the difference in the weight of a couple of SSME's and the Sabre's, and given the very low density of the propellant mix. It seems to me that the main reason that Skylon works on paper is because of it's extraordinarily low structural weight: 53 tonnes unladen mass minus 19 tonnes leaves just 34 tonnes for everything else, and that's for a vehicle thats 83 meters long - longer than any commercial aircraft now flying.Can anyone tell me what the mass of the individual propellants is? For the lighter C1 version I've read that LH2 is 66 tonnes and LOX is 150 tonnes, do those ratios hold for the D1?

Many in the Reaction Engines team have worked on pure rocket proposals (including myself on the Delta Clipper) and we are sure we are in a much better place. But if someone wants to complete with a pure rocket system they are at liberty to do so.

Downix, those numbers are for the earlier C1 version of Skylon, payload for which was 12 tonnes to low equatorial orbit.

With regards to Andrew_W’s pure rocket alternative. The initial figures seem to leave out TPS, recovery system (wings parachutes or rocket relight) and landing gear all add many tonnes. You also need all the OMS and RCS kit and their propellants and the power generation system. Put all these in and past studies suggest you exceed a take off mass of 1000 tonnes for the class of payloads we are dealing with. The dry mass then works out more than SKYLON which parametrics suggest will be more expensive.

Part of the passion in pure rocket SSTO debates is very small changes in assumed achievable mass ratio makes massive differences in performance if 12 % TOM gets you 9.5 km/s, then at then 11% things are brilliant nearly 10 km/s so what is the problem but at 13 % you are looking at 9 km/s still well short of orbit. Thanks to the rocket equation such vehicle are incredibly sensitive, now as Alan point out (but the original observation was I think Bob Parkinson) if the world were 10% smaller it would be very different.

On the other side of the argument the main reason our structure masses are lower than conventional civil aircraft of comparable dimensions is the truss frame structure - like an airship.