This site had an interview with Richard Varvill, the Technical Director and Chief Designer at Reaction Engines Limited. Reaction Engines Limited is a UK company that is developing a fully reusable launch system called Skylon.
There was a comment (link to the comment) by reader Goatguy which has a response from Richard Varvill
Below is a response to the specific points made by 'Goatguy':
1) SINGLE STAGE - Sounds great. But if they're ejecting a fuel tank, it isn't single stage. Let's remember to call technology standards for what they are.
Skylon does not eject a fuel tank. Skylon is single stage to orbit and has no expendable fuel tanks or equipment. Everything that takes off comes back apart from propellant and some cooling water.
2) 200 use - why there? Well, undoubtedly because the ceramic tiles that protect the present, beautifully ageing space-shuttle last for about 1/4 to 1/2 that many missions. They figure they can outdo the shuttle's re-useability.
The 200 flights is not determined by the lifetime of the TPS aeroshell, nor is the TPS material the same as the Shuttle. Skylon's aeroshell is a reinforced glass ceramic composite manufactured in thin sheets and corrugated for stiffness. The aeroshell is several hundred degrees cooler than the Shuttle during re-entry due to Skylon's lower ballistic coefficient. Re-entry insulation is provided by an internal multifoil insulation blanket.
The vehicle design lifetime is determined by economic considerations, representing a trade-off between development and operational costs. (ie: a longer design lifetime would reduce operational cost but increase the required development program cost and duration). Once introduced into service the vehicle lifetime and reliability will be gradually improved as the operational environment proves what the real life limiting factors are.
3) 400 x better? That takes quite a leap of faith! They're not going to cut the kg/kg fuel/payload ratio much - especially if they don't jettison the external fuel cells and go "2 stage" or "3-stage" in effect.
400x is the improvement in reliability compared to expendable rockets – which is mainly due to Skylon's ability to abort in all flight regimes and the position of each individual vehicle on the “bathtub curve” (as supplied each vehicle is quite a way down the “wear in” Weilbull Distribution since it is flight tested prior to delivery to the customer).
4) $5M per flight? OK, maybe ... especially since the Space One folks figured out (rightly I might add) that wings, turbojet-engines, standard JP1 and a whole lot of atmosphere (especially in a ramjet secondary) can gets you to Mach 6 or 7 (about 2 km/sec) without too much trouble. The 90-minute-orbit (250 km up, r = 7,125km, C = 44,700km, V = c/5400 = 8.3km/sec) then gets its first 2 km/sec off oxygen breathing "stage". But the rest (18x more energy) needs to come from non-air breathing rocketry.
Getting any single stage vehicle to Mach 6-7 from a standing start using air-breathing engines is not easy (and in fact has never been done). Although the rocket powered phase provides most of the energy the air-breathing phase is more technically difficult (from a propulsion perspective).
5) Looking at (4) more ... the whole reusable shuttle concept is vexed with "protection-and-accelleration costs". It has to be BIG enough to store the fuel to get from 2,000 m/sec to 8,300 m/sec. But BIG = WEIGHT, which equals more fuel. Which requires bigger shuttle. gah. And cuts down on the payload.
The above mass growth effect is encapsulated by the well known 'rocket equation'. Actually on Skylon most of the fuel (hydrogen) is used in air-breathing mode getting up to Mach 5, and hydrogen tankage accounts for most of the fuselage volume. Nevertheless the payload fraction always improves as the vehicle scale is increased due to the diminishing effects of fixed masses and minimum material gauges etc.
The rest of the discussion is at rather a tangent to the SKYLON concept. At this point in time we have no reason to alter our approach which 25 years of study and research have shown to be viable.
All the best
Response from Goatguy
I apologize for not having researched more on the Skylon technology before posting my series of questions and rebuttals. More informed (now), I see that the idea is dependent on several factors: the development of a cooled-air hydrogen turbojet engine, the ability of the ceramic-glass composites to deliver the necessarily high strength (and insulating R value)-to-weight ratio, the use of composites and ultralight weight materials for the 41,000 kg empty-weight fuselage, and the jettisoning of pressurized human-occupant cabins and all the associated support systems. I imagine that there would also be a call for minimalist (and light-weight) avionics and a lightly pressurized helium gas-fill to thwart oxygen-leak fire potential.
I also see that the thought-experiment of a space-plane with tanks that weigh nothing, engines that weigh nothing and fuselage that weighs nothing ultimately gains nothing in being multistage… as only the reaction-mass and the payload are accelerated. It is because conventionally the very large tanks, the rather large attendant turbopumps, thrusters, force-frame and control systems weigh so much that it makes sense to “multi-stage” them.
I do wonder Dr. Varvill though … is it possible that the cross-over between a single-stage and a 2 stage design – for a winged, aerodynamic lift vehicle – per the work done by the Scaled Composites SpaceShipOne folks … is such that the Skylon design could easily be kept as a winged lifter with a separable LOX/H2 stratosphere-to-space 2nd stage? Doing so would markedly lower the weight of the air-breathing vehicle: it wouldn’t need any thermal-barrier protection (or at least very little), so that that dead-weight would be reserved just for the secondary stage.
Thinking on this further – and maintaining the idea of a horizontal runway take-off approach – might there also be advantage in borrowing from the military? At least it is the American armed forced SOP to lash a couple or quad of modest-sized solid-rocket launch boosters to get the heaviest conventional aircraft up to take-off velocity when runway-length (combined with the vehicle mass and its jet thrusters) is too short for a safe takeoff. I know “technically” then that is a third stage … but it seems so obvious – attaining the first 400 m/sec over a shorter runway, which at this velocity would allow the craft to do an early abort utilizing a short return-to-spaceport loop (and rapid pumpless fuel dumping). Water cooled brakes … not needed, nor the water mass. Further, the high-pressure rocket casings wouldn’t have to be made from exotic materials, them being jettisoned after the first 30 seconds of acceleration. Cheap, robust, reusable.
Finally … I have to say that I am markedly more enthusiastic about the Skylon space-plane concept than I was when I wrote the comments (to which you graciously replied).