http://www.physorg.com/news6341.html

This is a very good article, it points out many of the pathologies.

But I found your attack on the proposed Skylon vehicle really very bewildering.

The Skylon vehicle is one of very few vehicles that stands a very good chance of making orbit and returning, single stage, completely intact, and with a very decent payload fraction. The idea that you can discount a vehicle because you "don"t like the shape" of its very lightweight tankage and thermal system seems to me to be fundamentally flawed.

I interpret the lessons in the "Cold Equations" as stated as:
(1) it's very costly to use the fuel you carry to depend on for all your thrust - pushing on the atmosphere is much better, except when the heat gets to be unsustainable.
(2) An single stage to orbit design is totally doomed by the stated laws. Time to forget it.

So a 2-stage design is needed, _but_ it does not have to be a throw-away one. The first stage could carry the second stage, empty of fuel (except LH2?) up to a fueling point. Then it boosts the second stage up, and it uses hypersonic scramjets to get as far as possible before heating is a problem, then rockets are used for the remainer.
The main benefit of this design is the added thrust for the jets engines over rockets.
The disadvantage is the developement cost of the 2 stages.

Get Scotty to work on the transporter so we can leave the cold equations out there.

"Jeffrey F. Bell is a former space scientist and recovery space activist."

Space scientists study the space environment, just as meteorologists study the atmosphere and oceanographers study the oceans. That does not make them experts on the design of vehicles that move through those environments. That is the province of engineers, not scientists.

Bell is a prolific author of anti-spaceflight polemics, but his articles are full of sloppy errors. For example, thie current article mentions "Bristol Spaceplanes' Skylon design." There's no such thing. The Skylon design is from Reaction Engines Limited, based on work at the University of Bristol -- *not* Bristol Spaceplanes. Bell confuses two different companies, working on very different concepts, simply because they're both associated with the name "Bristol."

Bell entitles his article "The Cold Equations of Spaceflight," implying that his conclusions are the result of equations he has solved. Yet, his article does not present and solve even a single equation. He just offers his opinion, supported by nothing but his authority as a "former space scientist."

Bell does correctly state one result of the rocket equation: "that an SSTO booster using LH2 fuel and LO2 oxidizer needs a fuel mass fraction of around 0.92." From that single point, he makes a leap to the conclusion that such structures are impossible to build.

He does not present any strength-of-materials calculations to support that conclusion. In reality, such structures have been built in the past, in programs such as Have Region and Science Dawn, so we know that they can be built again.

When you build and measure, you know. Bell does not build or measure. He assumes.

Even without such lightweight structures, reusable vehicles would be possible with a simple workaround in the form of staging.

The laws of physics don't care what a "former space scientist and recovery space activist" think. The Cold Equations tell us it's possible to build reusable spacecraft, as do past experiments.

[> (1) it's very costly to use the fuel you carry to depend on for all your thrust - pushing on the atmosphere is much better, except when the heat gets to be unsustainable.

That's a "Bellism." In reality, the cost of rocket propellant required to reach orbit is quite affordable -- typically less than 1% of the cost of hardware.

> Then it boosts the second stage up, and it uses hypersonic scramjets to get as far as possible before heating is a problem

It doesn't work that way. A scramjet forces you to stay down in the atmosphere where you can get air, which makes heating much more of a problem. The most efficient way to get to orbit is to get out of the atmosphere as quickly as possible. Rockets are a very efficient way to do that. None of Bell's "unobtanium" required.

It is boobs like Bell and their lack of vision that have destroyed the U.S. space program. I point to the vision of Paul Allen and the genius of Burt Ratan (I think I spelled his name right) that put a man in sub orbital space flight with a program whose budget was only about $30 million.

It is to the point that NASA should be scrapped and a completely new organization should be created. A place were people like Bell cannot collect a pay check for saying it can't be done. Space exploration is not for the pessimistic.

The truth of the matter is that NASA has become just another government jobs program. The real heavy lifting and innovation will come from the place it always does, the private sector. When there is a profit to be made, someone will find a solution.

But hey what do I know about space I’m just a nuclear engineer.

I recently read your 'Opinion' piece at www.spacedaily.com in which you bemoan the unrealistic expectations of so-called 'SpaceCadets'. While some of your argument has merit, I must take issue with your unfounded assertion that the design for the Skylon Spaceplane is a 'particularly bad example'.

I'm not sure what you think it's a bad example of. You seem to be objecting to the fact that it's not built like an Atlas booster, where the outer skin and the fuel and oxidiser tanks are one and the same thing, and provide the structural rigidity. Missiles are built to be launched vertically, and not horizontally. Such a structure would experience at least five times the bending moments of a vertically launched vehicle, so a different structure is necessitated. Also, Skylon would be coming back, and the outer ceramic shell would function as a heat shield for re-entry - something that a throwaway missile doesn't require. Also, Skylon uses air-breathing engines for ascent to high altitude and wouldn't need to take a large mass of oxidiser with it - that's the so-called '0.92' rule out of the window. Finally, it's unmanned, so doesn't require all that stuff for keeping little pink bodies alive. The cold equations of spaceflight still apply, but you have to apply the correct equations. These are intelligent, practical engineers, and unfortunately they know what they are talking about, they do the sums, and hell - they can even make a profit out of it without any need for Unobtainium, or even Handwavium.

I'm disappointed that you feel that anyone who expresses a vision of space being reached by something other than increasingly advanced versions of the V2 can be denegrated as a 'SpaceCadet'. Perhaps you hadn't seen the designs for the piloted, winged versions of the V2/A4, or the A9/A10 combinations, or the Sänger-Bredt 'Silverbird'. Way back then, the German scientists whose technical brilliance and foresight was so sought after by the USA and Soviet Union alike, had come to the rational and obvious decision that the best way to get into space was with a reusable, controllable vehicle that didn't have to throw away 90% of itself in order to work.

We know that spaceplanes can work, and that it's only been a lack of political will fuelled by internal inter-service rivalries and back-stabbing that has pushed us boldly back into the 1960's era of cheap-shot tin cans on top of fireworks. How depressing, if that's the best we can do - the sad machines of a future without imagination, from a country that spends more on chewing gum, pet food or pornography than space exploration.

Maybe it would help to redefine the problem. How about if you tow the vehicle to 40000 feet (drop off ground gear) and load the RP-1 from the mother ship (Boeing 747) just before release and fire. The equations say this is feasible at around 10% vehicle weight, no show stopper. Keep reentry temp down with lower wing loading, protect any residual fuel in a small insulated interior tank while you vent off the main tanks. Very doable, cheap.

Jack Dixon

Jeffrey F. Bell is not making a very good case on many of his articles.

The most recent article he wrote was on this past weekends X-Prize event. He basically stated that the whole event was a joke. I would have to disagree. At least there are people who are still pushing the envelope and are interested in doing something. Instead of sitting on the side lines and heckling about it.

If Mr. Bell is no longer a space enthusiast, that is fine. But you all are right. He does not have all of his facts in order.

actually, one doesnt need to rely on rockets. I agree, that the best way to get to orbit is to get out of the thick intransigent atmosphere as quicky as possible, but why not use some of that turgid atmosphere as reaction mass ? simply the way the Skylon proposes to do this: air breathing hydrogen jet engines until one reaches Mach 5 and a reasonable altitude. then combine the rest of your LH2 fuel with some LOX you brough along for the very purpose.

the calculations on their website show the Isp of the airbreathing stage to be about 2000s. (this is a key figure in spaceflight - it means how much thrust will one kilo of (important) CARRIED propellant provide ? 2000s means every kilo of CARRIED propellent will provide 2000g Newtons of thrust, also known as 2000 kilograms-force, or 9080 lbs of thrust for all you naughty non-SI people !
I actually did do a Physics degree (got Honours too) and paid particular attention to the bit that spoke about Tsiolkovskii's equations of rocket motion.

I seriously think about the question of SSTO a lot. I reckon its not 'can it be done ?' which it can, but 'should we do it for a serious vehicle ?' and the answer is no: this is because a ridiculous amount of the vehicle must be hauled into orbit once it has surpassed its usefulness - just look at the 3 tonne SSMEs on the shuttle - a bit of a glass chisel once the orbiter is in space, cos theres no ET to power them - so why are they on the back of the orbiter ?

I think the question we should be considering more is 'can we make decent cheap fully re-usable space launchers ?' and ' can we make what we have got more efficient and cheaper to fly ?'. and the answers to those are 'Yes' and 'yes - lets start by tweaking the current operational systems a bit and see what we end up with'.

ta for yer time!

Adrian Mann, I hate to wee on your barbecue here, but the Skylon is intended to be manned - indeed piloted by pink stuff (like it!).

also, the bit about a nation that spends more on catfood and porn than space research ? hmm sounds like the UK - a country so far sighted they killed a good space program in 1971, before it had a chance really to fly (once into orbit doesnt count), and has refused to condone manned spaceflight, meaning any brit who wants to go to space either must buy a ticket (only for the loaded) or become a citizen of another country.