Author Topic: Basic Rocket Science Q & A  (Read 247368 times)

Offline Proponent

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Re: Basic Rocket Science Q & A
« Reply #1100 on: 05/01/2017 03:50 PM »
So, if the exhaust pressure is below the ambient pressure, you end up with a negative term in your thrust equation.  i.e. (Pe - P0)*Ae < 0.  And ergo, a reduction in thrust. 

Yes: if the nozzle is over-expanded, then it is actually sucking the rocket backward to some extent.  If this is hard to imagine, it's because our intuition about fluids is based entirely on subsonic flows.

Offline Proponent

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Re: Basic Rocket Science Q & A
« Reply #1101 on: 05/01/2017 03:56 PM »
Intrigued by the LANTR-concept, I tried to replicate the different Isps for different mixture ratios by using the formula Ve^2=2k/(k-1)*R*T/M, which I've found here and simplified a bit by removing the part with different pressures, as that factor would most likely be rather close to 1.

However, I always get results that are way off, e.g., if I go for pure hydrogen & T=2900, according to the 1st link
I should get an Isp of around 940, so a Ve of around 9000. But what I actually get is: Ve=sqrt(2*1.666/0.666*8.314*2900/1)=347.311m/s, which is off by a factor of 30(!).

Could anybody tell me what exactly I'm missing here?

If you're doing the calculation in SI units, than the molecular weight of atomic hydrogen is 0.001 kg/mol.  At 2900 K, though, I would think most of the hydrogen would be molecular, so 0.002 kg/mol might be more accurate, and the specific heats would need to be adjusted too.

It's a relatively minor point, but ideal-gas specific heats may not be very accurate at high temperatures.  As the temperature rises, molecules behave less and less like the rigid rotors, because vibrations and bending become significant.

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