Author Topic: Basic Rocket Science Q & A  (Read 257231 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.

Online nicp

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Re: Basic Rocket Science Q & A
« Reply #1102 on: 06/23/2017 06:16 PM »
An old but vague memory suddenly popped up. Which may very well be utterly wrong.

Somewhere amongst my hundred or so books on rocketry and spaceflight I'm sure there was a reference somewhere (probably dating to events between 1945 and 1970) describing cryogenic (oxygen I think) lines making a lot of scary noise during fueling operations. This might even be X-1 or X-15 territory.

If memory serves the noise was sort of continuous, not sudden bangs or anything and genuinely worrying to those not used to it.

But now I come to think about it I don't remember any launch coverage with audio containing anything much, other than the noise (before launch) of venting gases.

Is my memory incorrect? I've done a quick search and come up blank.
It's entirely possible I'm fooling myself and my memory is wrong...
Where's my Guinness?