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The Isp of thermal rockets is based, more or less completely off of:
A) temperature of propellant
B) mass of propellant per molecule
right?
What is the formula? If anyone says exhaust velocity/9.8, I'll... do something.
-Josh
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If anyone says exhaust velocity/9.8, I'll... do something.
Actually, it is exhaust velocity/g
So what you really want to know is how to calculate exhaust velocity ...
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I don't like all of their assumtions, but this is golden:
Ve2 = kRgasTc [1 - (pe/pc)(k-1)/k] / (k-1)
where
k = ratio of specific heats, cp/cv
pe = nozzle exit pressure
pc = combustion chamber pressure
Tc = combustion chamber temperature
Rgas = exhaust flow specific gas constant RR/MM
RR = universal gas constant
MM = exhaust gas molecular weight
so, using a thermal rocket fueled by water, with an ideal, and perfect de laval nozzle in a vacuum,
Ve^2=29(Tc)
Is this correct?
-Josh
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so, for my delayed K+ rocket (http://www.newmars.com/forums/viewtopic.php?t=5740, at the very bottom), Ve would be...
Ve^2=29*1 885 000 000
so
Ve^2= 54 665 000 000
so Ve=234,000 (234,000 whats? km/s?)
Is any of this correct?
-Josh
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I'm interested in the idea of using beamed-power thermal rockets to reach orbit. Coupled with an Airship Solar-power station with a beam on, it could probably reach orbit, or get near, but I'm not sure. The rocket engine would consist of mirrors to bounce the laser light back, superheating a gas (air?) which would be expelled out of the back for thrust. An extremely good targeting system would be needed to track the craft with the laser and avoid vaporising passing aircraft.
Use what is abundant and build to last
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Well, the usual list of problems:
Power, the beam generator would require multi-gigawatt-scale output for a few minutes to reach high Isp and thrust, perhaps more given the short firing time available. This pretty much makes it impractical for the airship to power the beam generator. Transmitting energy up to the airship would also prove problematic.
Acceleration, that in order to reach nearly orbital velocities while the "target vehicle" is within the horizon of the beam generator, the acceleration it would be subjected to would be very high. While not as high as a railgun, it would be too severe for people most likely and impose some nasty restraints on payload.
Thrust, since the target vehicle would fly from horizon to horizon in at most several minutes, the vehicle must have thrust similar or likely more than most launch vehicles, making the engine difficult to construct.
Angle, that the thermal engine on the target vehicle must be facing the beam generator, which will further limit the "time on target" that the beam can illuminate the vehicle to produce thrust, making the above items even more problematic.
Atmospheric scattering, if you go the laser route, it will be hard to hit a target with a focused beam while it is in the lower atmosphere since density gradients and other atmospheric effects will alter the beam's path, reducing the power-on-target and further compounding the power requirements of the beam generator.
Beam generator mass & volume,whatever energy directing device you use, it will be gigantic in order to generate and direct a beam of that magnitude. A beam in the megawatt scale, likely around 10,000 times smaller than the beam you would need, takes up a good chunk of a cargo planes' payload.
Missing, that if you miss the vehicle or the beam otherwise fails, then the target vehicle is going to have to perform a super/hypersonic abort, which is not a happy thing anybody ever wants to do. Furthermore, the vehicle itself would have to carry jet engines or something in order to abort, since the point thrust is lost and speed drops is unknown unlike Shuttle.
Hitting stuff on the ground, that you obviously couldn't base this anywhere within many, many miles of people or property; the best place would obviously be over the ocean, where you have sufficient room with little chance of hitting anything, right? If you are way out in the middle of the ocean, what happens when the target vehicle has to abort? Where would it land? How would you recover it?
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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