Интересное письмо на AROCKET
[AR] Propulsion technology of larger scale - why do science teachers lie to the children ?
I have had the privilege to have worked with most forms of rocket propulsion in the past 25 years. Solid, hybrid and liquid. Most likely because I did this as an amateur, I have never been
forced to become a specialist with limited knowledge of alternatives to my speciality. For the past five years I have even had the very special luck to be a full time, 24/7 rocket amateur,
with no need of wasting time on some stupid non space relevant job. I earn my living making lectures on my favourite topic - space flight.
Of all the forms of propulsion, it is my experience that a bi fuel, regeneratively cooled, cryogenic rocket engine is by far the easiest to build, and operate, at the scale relevant to space flight.
It is also by far the best suited to whatever we may hope of future "new space" developments. Low cost, non toxic, relatively safe to handle - LOX / RP-1 or in our case LOX / ethanol beats all other technologies in simplicity of fabrication, performance, development cost and cost of hardware.
This is NOT what my science teacher told me when I asked for advice on how to go to space. He said solid. He even said ammonium perchlorate. The rocket amateurs I meet, when growing up, said solid. They too said ammonium perchlorate, and added - too dangerous, so use potassium nitrate. Anything else was totally, impossible madness. As mad and impossible as deciding to build a thermonuclear powered moped.
All these knowledgeable people all knew exactly was I wanted to do - build rockets relevant to space flight. And that means BIG rockets. And they recommended potassium nitrate based solids.
25 years later, and many hundreds of static tests later - I know what works at the relevant scale. Bi fuel liquids. Period.
At Copenhagen Suborbitals we have a solid propellant that work rather well. Its a sixty years old type JATO propellant based on potassium perchlorate, bitumen and lube oil. Its called galcit 61-C. We have a nice, large, mechanized, oil bath heated, batch mixer, and make series of 3.5 kg solid fuel test engines. They burn at 70 - 90 bars, have never failed, and deliver some 170 - 180 sec ISP. The propellant has a high density of 1.78. It is far cheaper, and much less dangerous then than typical ammonium perchlorate / Al / HTPB. It can be mixed without vacuum and still be quite reproducible. Okay - really nice. Solids just don´t get much simpler. If operated inside its narrow temperature firing limits - its really very nice. Since hundreds of heavy aeroplanes was using it for rocket assisted take off, its fair to assume that most of those JATO´s worked, and few exploded. Our entire galcit 61 C experience supports this theory.
After a day of solid oxidizer prep, melting fuel-binder, casting, measuring density, and what do we have - I can make 15 kg of this propellant. Yet, on the back of my mind, I don´t know for sure everything is well, because proper quality check would require x-ray apparatus costing at lot of dollars that I don´t have. Statistic´s may render X-ray not needed - but you don´t fire a hundred big rockets just to gain statistics. The same would be even more true if I was working with ammoniumperchlorate / Al / HTPB. Cleaning an test engine after a burn is a nasty, stinky, disgusting job I might add. For further reading on CS solids, take at look at our report download section on COPENHAGEN SUBORBITALS - OPEN SOURCE AND NON PROFIT SPACE
, see resource, see galsit 61-C.
Now, we shift to bi fuel liquid, in the simple pressure blow down cycle. The TM65 test engine is an example of this. This engine burns a LOX / ethanol / water mixture - at at cost of 1/5th of the solid propellant mentioned above, and 1/15th of the cost of ammoniumpercholorate / Al / HTPB. It burns it at much lower pressure - 12 bars, yet gives slightly higher ISP, and a potential system mass ratio that is far better. After all a tank rated at 20 bars, is far lighter than an solid motor casing rated at 90 bars, with an un-cooled heavy nozzle.
Every nut, bolt, valve, weld, end cap, tube, and sensor, that goes into this machine is of the shelf, ordinary industrial commodities - any worn out russian tractor factory can build it. All of the skills needed are shared with shipbuilding, construction work, house plumbing, or car maintenance - and the like. If I make a phone call for ammonium perchlorate - or propellant grade HTPB all the red lights at FSB, CIA, KGB, PET, NKVD and MOSAD starts flickering - and the materials are so strategic, duel use, and dangerous that even the paperwork will stop you. Going liquid - you can bye all you need without anybody asking any questions, and no permits of any kind is needed. Go ahead, build your test stand - and come back when you want to fly. ( And that is when trouble starts, and you go out to sea, Gotcha ! )
But it gets even better.
Any problem you encounter during your developments has been solved decades ago during the V2, Viking, and Redstone rocket developments. Countless millions of engineers hours have already been used to debug the whole system. So, does your engine melt - add film cooling - ohh, does it oscillate, well add injector baffles or simply copy the injector of the Redstone´s North American A7 rocket engine. In short, read the fucking manual:
<div class="file_pdf"><a href="http://www.aggregat4.de/pdf/Gerätebeschreibung_A4.pdf">Gerätebeschreibung_A4.pdf</a></div>
and:My Army Redstone Missile Days, Page 12 Appendix A
Comparative, in depth detail materials, on a large scale solid system - lige e.g. the Polaris - is simply not avalible. And if it was, it could not be followed because key materials could
not be obtained. The very shape of the oxydizer crystals may change the mixing and combustion properties of a solid propellant in dramatic and destructive ways.
When making liquid fuel - two technicians at Suborbitals mix and pump 500 kg on board in about two hours. Ethanol is ethanol, water is water, LOX is LOX. An injector hole is a hole. After burnout the engine is clean, undamaged, not even a bit of soot is found on it. You can refuel, and fire again in minutes. Our record is 45 minutes from ignition to ignition.
What is wrong with those science teachers ? What was wrong with the rocket club ? What did they miscalculate ? They knew it was was not about 4th of July rockets when I asked how to go to space.
I have taken part in launching a 1,7 ton LOX based rocket from sea. Yes, its an operational challenge, and yes, a Polaris might be simpler in operational terms. But the factory making the Polaris
is by no means simple.
Does the hybrid give the perfect compromise ? Well, not really. It does solve the oxidizer problems, but LOX hybrids have complex combustion problems that are not solved by myriads of engineers 60 years ago. No extensive manuals exists. Its like not bad, just half bad - as in hybrid. Other energetic oxidizer's like H202 or N20 makes it far easier and the CS rocket workhorse -the HATV hybrid
is an option in the small and medium size. But the cost and availability of propulsion grade H202 is still an issue compared to LOX. N20 is simply to low density to be rally fun, and like H202, it can be
Ergo, LOX bi-fuel liquid it is.
The V2 and Redstone manuals have answers to one of my central problems with bi fuel liquid engines. How to feed the propellants - they say use pumps. Use simple,
steam turbine driven centrifugal pumps. Making a high pressure tank is not that hard, especially if the mass ratio is not very critical. However - in the simplest pressure blow
down version the pressure drop during the propellant flow is very big - we start at 20 bars, and 20 % into the burn, we are operating at 10 bars. Adding a high pressure tank, and a
high pressure control valve, a heat exchanger and so on is clearly an option. But - in any case it takes a LOT of very expensive helium. In Europe, its not about cost, its simply
about obtaining this gas. The V2, Viking, Redstone - even Soyus - use NO helium. Zero. The textbook says pumps. Okay, so at CS we are testing a turbine pump, not surprisingly being
powered by T-stoff ( H202 80 % ) just to see if the answers in the manuals are wrong - or may even hold water to this extent.
The old men said use solids. Busted. Definetly busted.
The old men says, if you insist on liquids, don´t even think of adding even a crude 1950ties style turbine pump.
I say, well fine, lets put it to the test.
In a few days, TM65 will go through its final test as a pressure feed bi fuel engine. Next time we fire it, it will have twin pumps and steam turbines. I want to see if we can make that work.
I don´t believe the old grumpy men, I believe there must be some truth in what the even older men found out at the dawn of the space age. After all its because of them, we are here.
Let´s try. I believe.