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  1. @p1t1o Of course. Avoiding detection by radar means, in addition to radio-absorbent coating, needing ugly angular shapes like the ol' F-117. Even so, there are tactical advantages to being detectable at closer ranges (say, Earth-Moon distance) as opposed to being detectable at half a solar system away. You can arrange for multiple stealth ships to attack from different approaches without defenders not knowing where they'll come from until the attack has already begun, for instance. Covert transport of 'important individuals' is also possible, as well as covert mine-laying and other similar 'hush-hush' activities, as long as distance is kept.
  2. I think you missed the point. MatterBeam's ship design constantly boils off liquid hydrogen to keep the ship's hull at very cold temperatures (22 K, 4 K if liquid helium is used instead of hydrogen). This makes the ship very difficult to detect. The ship's propulsion system also releases its exhaust at the same temperature (post-expansion in a large nozzle bell), so the ship can get itself into orbit nearly undetected. The only way to detect cold-running ships like this is to have the sensors/telescopes be cooled down to the same temperature. This works out in the stealth ships' favor - keeping a sensor cold for a long time requires either enormous stores of liquid hydrogen/helium on that sensor itself or a power-hungry heat pump, which probably needs a fission reactor to power it. This would make cold sensors expensive to build and run, limiting their numbers and their up times. In contrast, a cold-stealth ship only needs enough hydrogen stores to last its mission, can choose to launch when most of the cold sensors are in their down times, avoid known cold sensor positions and scout ships, etc - they have better tactical choices than their opponents.
  3. @p1t1o Atomic Rockets already have a section on MatterBeam's design (see my 1st post on this thread). That's where I got the references on that design.
  4. @MatterBeam's stealth ship isn't supposed to crash into the planet - and it doesn't need to. It can covertly insert itself into orbit, which can then drop the spies via MOOSE pods.
  5. 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.
  6. 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.
  7. The staged NERVA design @DDE referred to is the Boeing IMIS.
  8. @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.
  9. 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.
  10. 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.
  11. 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.
  12. 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.
  13. @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.
  14. @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.
  15. @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.