Post by vegemeister on Jan 21, 2017 20:14:02 GMT
So you may have noticed that it's fairly easy to make resistojet rockets that appear to violate conservation of energy. Let's take a look at this one (note that the exhaust velocity in the title is incorrect; I copied from another engine):
There are a couple problems in this image. Most obviously, the claimed thrust power is more than 66 times the electrical power input. IIRC, qswitched answered another complaint about this by referencing the fuel pressure. So maybe we're getting pump power for free? I tried to figure the energy balance with rudimentary Wikipedia chemistry knowledge.
So we're getting 42 MW free from the pump. That ain't nothing, but it still isn't even enough to dissociate the fuel, much less develop the claimed thrust.
Second, the thrust is too small for the claimed exhaust velocity and mass flow rate. Thrust should equal v_exhaust * mass_flow, which for this engine is 2.59 MN, but the thrust shown is only 2.09 MN. The thrust power also seems to be affected. As calculate from mass flow and exhaust velocity, it should be 8.22 GW.
Interestingly, if we increase the chamber contraction ratio and wall thickness, the mass flow/exhaust velocity relations for thrust and thrust power become correct:
Engine is still 8300% efficient, though.
I'm slightly suspicious that this violation-of-thermodynamics issue may also affect NTRs. Since they don't show the thermal power of the reactor, it's hard to tell. The missing-thrust problem does affect NTRs, but it rarely comes up because NTR's usually have larger chamber contraction ratios to make room for the reactor.
There are a couple problems in this image. Most obviously, the claimed thrust power is more than 66 times the electrical power input. IIRC, qswitched answered another complaint about this by referencing the fuel pressure. So maybe we're getting pump power for free? I tried to figure the energy balance with rudimentary Wikipedia chemistry knowledge.
Electrical Input
100 MW
Pump power
74.9 MPa * 409 kg/s / 730 kg/m^3 = 42 MW
Thrust power output
-6.63 GW
Dissociation power (100%)
409 kg/s / 140 g/mol * 300 kJ/mol decane enthalpy of formation = -876 MWSo we're getting 42 MW free from the pump. That ain't nothing, but it still isn't even enough to dissociate the fuel, much less develop the claimed thrust.
Second, the thrust is too small for the claimed exhaust velocity and mass flow rate. Thrust should equal v_exhaust * mass_flow, which for this engine is 2.59 MN, but the thrust shown is only 2.09 MN. The thrust power also seems to be affected. As calculate from mass flow and exhaust velocity, it should be 8.22 GW.
Interestingly, if we increase the chamber contraction ratio and wall thickness, the mass flow/exhaust velocity relations for thrust and thrust power become correct:
Engine is still 8300% efficient, though.
I'm slightly suspicious that this violation-of-thermodynamics issue may also affect NTRs. Since they don't show the thermal power of the reactor, it's hard to tell. The missing-thrust problem does affect NTRs, but it rarely comes up because NTR's usually have larger chamber contraction ratios to make room for the reactor.
