shynung

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  1. To reduce glints of sunlight reflection. Also to absorb stuff like LIDAR emissions. A radio-wave-absorbent coating directly underneath the Vantablack takes care of the RADAR emissions absorption. The ship's skin would have cooling ducts inside, to keep temperatures down. The ship is double-hulled, so that the cold outer skin does not receive thermal energy via conduction from e.g. hab modules. Liquid hydrogen boils at 22 K, that of helium is 4 K. Detecting a 22K object against space radiation background (2.73 K) is very difficult - 4 K, nearly impossible. The expanded-gas exhaust is expanded just enough so that their temperatures when exiting the nozzle is comparable to the ship skin's temperature. The heating chamber is closed off at most times by a shutter with a cooled outer skin. The nozzle base will only touch hot gas intermittently, and are cooled by the heatsink. Also needed mentioning is that the heatsink is only used until it reaches its boiling point. Spent heatsink is ejected overboard at its boiling temperature (22K for hydrogen, 4 K for helium). The main exception is the propulsion system, which does release puffs of gas at high temperatures, but this is mitigated by the exhaust gas being cooled by expansion in the nozzle. The main point is, it does not try to 'hold the heat in', but rather release it by boiling a cold liquid, using the enthalpy of vaporization to cool down without raising the coolant's temperature. Also, the process is not very efficient on the heatsink, as noted. A 1kW source of heat needs 8 kg/hour of liquid hydrogen to cool down to 22 K. Consequently, the ship's mass at launch will mostly be heatsink.
  2. Atomic Rockets page on stealthy ships To sum up, it is possible to make a spaceship nearly invisible. Spaceships are detected mainly by their thermal signature emitted from radiators, engines, exhaust plume, etc. A design mentioned in the linked article discusses a stealthy ship that minimizes its thermal signature by using liquid hydrogen/helium as single-use heatsinks - it is boiled to absorb heat from the ship itself, then expanded in an expansion chamber (to lower its temperature), the resulting gas being used as a cold-gas thruster. The ship itself is shaped like a long, thin cylinder, one end constantly facing the sun, to reduce sunlight reflection. A combination sun-shield and concentrating lens (here using fresnel lens), both to shield the ship from the sun's heat (keeping it cold enough to be stealthy), and use it to power a solar-thermal rocket, using the gas from heatsink boiloff as coolant. The solar thermal rocket pulses the propellant ejection, instead of letting it flow freely. This is to ensure that the propellant is almost as hot as the heating element before it is released to the nozzle, to improve specific impulse. The hot hydrogen gas from the solar thermal rocket is further cooled by a large nozzle assembly (not depicted in the diagram) by expansion. To make the ship even harder to detect, it can be coated with Vantablack, a special substance which absorbs up to 99.965% of light in the visible spectrum.
  3. The staged NERVA design @DDE referred to is the Boeing IMIS.
  4. @MaverickSawyer Common NTR design usually includes a protective coating on the reactor elements to prevent the propellant from eating them. However, coatings that work against reducing propellants (hydrogen, ammonia, methane) is useless against an oxidizing propellant (oxygen, water, carbon dioxide), and vice versa. Designing a reactor that can withstand both is a pretty challenging engineering task.
  5. NTRs can run on almost any propellant. A solid-core NTR at 3200 K can attain a respectable 410 seconds of ISP on water, according to the Atomic Rockets site.
  6. Expanding @DerekL1963 's response on catalysts, I think we should consider using a starting slug, something like a solution of potassium iodide in water, to decompose only the initial charge of the HTP. The rest of the undecomposed HTP can sustain combustion by itself.
  7. What I mean is, maybe we should start off using a traditional hybrid in the first place, to save R&D trouble off whoever ends up using our plans later on.
  8. Our goal is a paper design of an amateur-accessible orbital rocket, right? I agree with the others that we should start with a proven design, to lighten the load on engine R&D. We can scale the design as needed later on.
  9. @sevenperforce Throwing another idea here. If restartability is not a concern, we can do away with the catalyst bed if we inject a solution of calcium permanganate into the initial charge of HTP entering the combustion chamber. It acts as a decomposition catalyst, igniting the first load of HTP. The rest of the HTP can sustain the combustion without needing decomposition themselves.
  10. @wumpus The original idea was having the oxidizer tank inside the fuel tank, so the design can use only one ullage tank, among other things. And yeah, it'd need a great amount of tests to get it right. Using jellied propellant without any bladder system means there's a risk of the pressurant (air) simply blowing a tunnel through the jelly, not touching most of the propellant. There's also the issue of properly mixing the fuel and oxidizer, given that the fuel only has a very limited surface area exposed to oxidizer flow, unlike a common hybrid rocket.
  11. @sevenperforce I'd say that having a catalyst bed directly exposed to the combustion chamber is generally a bad idea; silver won't stay solid at the temperatures that cat bed is going to be exposed in. Much better to have a separate preburner containing the catalyst bed, into which a small amount of HTP would be decomposed, that exhausts into the combustion chamber joined with the rest of the HTP. Once combustion is achieved, the catalyst/preburner chamber can either be left on or closed off; undecomposed HTP can sustain combustion after it has been started.
  12. Going off on a slight tangent here. What if the cat pack only decomposed a limited amount of HTP, enough to maybe light up an ignition system or drive a turbopump? The rest of the HTP goes in as is, the decomposed HTP from earlier (along with some fuel) acting as a starting slug for the rocket.
  13. @sevenperforce I'm thinking of using a reverse-bladder to solve the gas-tunneling issue; rather than encase the napalm in a bladder and have the pressurant gas compress that, we make the pressurant gas inflate a bladder which then pushes on the propellant.
  14. Which is why. Self pressurization of HTP tank would mean either in-tank decomposition or spraying decomposed HTP (hot steam + oxygen gas) into HTP tank. Both are risky business. Peroxide rate of decomposition increases with rising temperature. It's best to keep it away from anything hot, IMO. That, or use it immediately after being heated in, say, a regeneratively-cooled chamber.
  15. This is what I was trying to say. Even if the individual igniters are reliable, in serially staged rockets in which each individual engines have their own igniters, the probability of one engine failing to fire, resulting in a jeopardized launch, rise considerably. Consider an igniter that has a failure rate of 1 in 10 (exaggerated to simplify calculations). If a two-stage rocket uses that igniter, one in each stage, and a single ignition failure means failure to reach orbit, that means the rocket would have a failure rate of about 1 in 5. Which is why I don't recommend any propellant combination that isn't hypergolic, at least for S2 and above; if we need the performance, we can use a non-hypergolic S1 and ignite it externally. Clarke's text was talking about tank pressurization. I think we'll run into the same problem unless we use bladders of some sort. I don't see why we shouldn't go with both ablative chambers and nozzles in one rocket. If the heat load on each part is different, we can vary the ablative material thickness to match cooling requirements. This is where a turbopump would be needed. I know it's complicated machinery, but it's the only way to transfer pressure between liquid without mixing them together. I was about to say 'use bladders', but then we're using decomposed HTP (=hot GOX) as pressurant. It'd eat through the bladder material anyway.