general questions for anyone who knows....

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general questions for anyone who knows....

Postby wookie » Sat Aug 11, 2012 3:09 pm

i've been following the nprize periodically and have a few laymans questions about a few things..i don't know very much about this subject and i was wondering if anyone here knew anything about these kinds of things..

programmes such as "rocksim" are able to predict the flightpath of rockets being launched from the ground but if you were launching from a high altitude balloon how do you predict the flightpath/ height reached then? are there programmes you could use or basic rule of thumb equations you could use based on impulse, mass, drag that you could use (as a purely hypothetical excercise)? does basic software exist to at least help to predict flight under the conditions of high altitude/ speed/ low temps???

as i understand it the shape of an exit cone on a rockets exhaust is quite important. the idea being that the pressure of the exhaust at the edge of the exhaust cone should be the same as the ambient pressure. as i've understood it the idea is that if you have a higher or lower pressure exhaust gas than ambient you get problems. a difference in pressure means a loss of efficiency. if you have a high pressure difference it means that the energy of the exhaust is in pressure rather than velocity of the exhaust mass. you want exhaust velocity rather than pressure. a low pressure at ground level means that ambient pressure is trying to force air into the chamber meanign bad efficiency.


the narrowing of the cone at the throat is important because the narrower it is the faster the gas at lower altitudes the throat should be smaller rather than wider and at higher altitudes wider rather than narrower? - correct? or am i just making this stuff up?!


the narrower the throat of the combustion chamber the higher the pressure being built up in the combustion chamber meaning a thicker chamber wall to contain these pressures. a narrower throat the faster gases travel BUT a higher ablation rate - that is the gases start tearing layers of the throat at a faster rate? a combustion chmaber construction should factor in the pressure in the chamber, the velocity of the gases over the throat and the speed/pressure of the gases at the edge of the exhaust cone (rule of thumb exit cone being 30 deg in total)????

i have seen some combustion chambers said to be made from carbon fibre. carbonfibre is resilient enough to take rocket exhaust apparently - true?? what would be a good material to build a combustion chamber from so that ablation doesn't become a problem and it isn't too weighty?? i'm talking solid fuels though.

high altitude launch from a balloon: in amateur rocketry fins are used to give a passive guidance to the rocket, the rocket is slightly nose heavy and there is a balance between centre of pressure and centre of gravity and when correct the rocket flies straight and true. at higher altitudes with the atmosphere being thinner, this must mean that the rocket being launched must fly off the guide rail very quickly or have larger fins than at lower altitudes or have some kind of other guidance on board to control the rocket. true???

firing solid fuel at high altitude is hard because the lower temps make it harder for the fuel to be lit???? solid fuel typically uses an electrical system using 12V being fed through a specialised ignitor that is pushed into the end of the motor. the lower temps mean that the energy being created by the ignitor is absorbed into the much colder solid fuel meaning that "activation energy" is harder to achieve- true? you cover the end of the exhaust of a rocket on a balloon so that heat is retained in the propellent material???

solid fuel doesn't suffer as much from resonance in the combustion chamber because the length of the "empty" tube left by the burnt solid fuel varies quickly enough to mean that even if a resonant frequency was achieved it disappears as the fuel burns. rocket motors with fixed volumes can suffer from resonance and end up destroying the chamber?? true???

mach 1 at ground level is a much higher speed than at high altitude? less molecules at high altitude mean a less dense medium meaning that the molecules at high altitude have less "warning" that something like a rocket nose is coming their way? as i understand it below the speed of sound molecules ahead of a moving object start moving away from a nose cone before it hits them. at mach 1 and above the air molecules in the path of a rocket has no warnign of a moving nose cone and are struck violently out of the way creating a shock wave. how does a fast moving object react with air at high altitude given the much less dense medium it passes through. does the nose of the rocket get hot at higher altitudes even though there are much less molecules available to strike???

oh yes if a rocket is being launched from 30km up would a balloon launched rocket still need to have a really pointy end or could it have a much blunter shape instead given theless dense medium its flying through???
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Re: general questions for anyone who knows....

Postby jacob.chancery » Wed Apr 30, 2014 6:24 pm

Hey Wookie, I wonder if you still visit this forum.

You seem to have a good grasp of rocketry fundamentals. The answer to most of your questions is yes, you are correct. Although not every rocket uses ablative cooling. Regeneratively cooled liquid engines don't ablate.
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