Author Topic: Basic Rocket Science Q & A  (Read 282418 times)

Offline ugordan

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Re: Basic Rocket Science Q & A
« Reply #320 on: 08/13/2009 01:50 PM »
Yes, you could assume that if your burn delivering that deltaV is short enough so it doesn't incur additional gravity losses of you flying significantly uphill while accelerating. In that case it would be dV > Ve, though if you already start in orbit and for a high thrust vehicle (chemical propulsion) the gravity losses would not be high.

Offline Danny Dot

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Re: Basic Rocket Science Q & A
« Reply #321 on: 08/13/2009 02:08 PM »
How do you calculate the escape velocity of a rocket and how is it dependent on the density of or choice of propellant?

Escape velocity or exhaust velocity?


Escape velocity - exhaust velocity is C = 9.8*Isp in the basic rocket equation relating deltaV, dry mass and propellant mass.  I'm familiar with this [ dV = C*ln((M+P)/M) ] but not the relationship between the deltaV required of a vehicle depending on its... whats the word density impulse?  How do you lower the dV required when C, exhaust velocity is fixed, or how do you compare the benefits of a propellant choice with slightly different C and grossly different density of P?

I always thought is was 32.2 times ISP   ::)

Danny Deger
« Last Edit: 08/13/2009 05:01 PM by Danny Dot »
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Offline engstudent

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Re: Basic Rocket Science Q & A
« Reply #322 on: 08/13/2009 02:23 PM »
How do you calculate the escape velocity of a rocket and how is it dependent on the density of or choice of propellant?

Escape velocity or exhaust velocity?

I always thought is was 32.2 times ISP   ::)

Danny Deger

Escape velocity - exhaust velocity is C = 9.8*Isp in the basic rocket equation relating deltaV, dry mass and propellant mass.  I'm familiar with this [ dV = C*ln((M+P)/M) ] but not the relationship between the deltaV required of a vehicle depending on its... whats the word density impulse?  How do you lower the dV required when C, exhaust velocity is fixed, or how do you compare the benefits of a propellant choice with slightly different C and grossly different density of P?

could be - I'd just have to change almost everything else before I try to get a number, kg - lbs, etc

I was curious about how the choice of propellant effects a theoretical SSTO rocket in terms of density and energy of the propellants and I'm not sure this helps me get to the effect of propellant choice on a launch vehicle - maybe escape velocity was the wrong concederation.  But Ive seen it used in a paper comparing the theoretical performance of SSTOs, I suppose you cant talk about a launch vehicles performance without concedering, drag, resistance, gravity loss - but I don't know how to do that either yet. {maybe I should start with a simpler question}  Maybe how is orbital velocity related to escape velocity?  :P

 
« Last Edit: 08/13/2009 02:31 PM by engstudent »
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Offline Antares

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Re: Basic Rocket Science Q & A
« Reply #323 on: 08/13/2009 05:32 PM »
Well, circular orbital velocity is the same formula with the 2 taken out.  So dVe is (sqrt(2)-1)*sqrt(GM/r)
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Offline Tnarg

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Re: Basic Rocket Science Q & A
« Reply #324 on: 08/24/2009 08:36 AM »
I'm looking for a program to work out orbits and delta-v budgets for moving around the solar system.  The main questions I have is 'if I increase my delta-v budget my this much how long would it take to get into a stable orbit around mars.

How do people work this out and is there any software I can use to help me work it out.

Thanks.

PS another question I've asked before but never got an answer but I want to know so I'm going to keep asking is:  If I had a 25 tonnes fuel tank that is made to lift into orbit and picked up by my space ship how much would be tank and how much would be fuel?

Offline Mr Natural

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Re: Basic Rocket Science Q & A
« Reply #325 on: 08/24/2009 09:24 AM »
If I had a 25 tonnes fuel tank that is made to lift into orbit and picked up by my space ship how much would be tank and how much would be fuel?

 The rocket Equation I use is as follows:
m_prop = m_pay*{[e^(delta_v/I_sp*g)]-1}*(1-m_inert)/{1-[f_inert*e^(delta_v/I_sp*g)]}
 where
 m_prop = propellant mass
 m_pay  = payload mass
 m_inert = empty mass, including propulsion system
 delta_v = the velocity requirements of the tank
 I_sp     = Specific Impulse of the propellant
 g         = gravity constant of the body in question (Earth = 9.80665)

This is a very difficult question to answer, as there are many more data elements needed to calculate that.For Example,
1/ What is the delta_v trajectory requirements for the tank
2/ What type of propellant are you using - IE what is the Specific Impulse
3/ A tank cannot by itself get into orbit, it requires a propulsion system
...So the mass of the propulsion system is needed to be known as well, is this included in the 25 tonnes or is it 'extra' mass.

There are many vatiables that need to be known before a constructive answer can be tabulated. 

Offline Tnarg

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Re: Basic Rocket Science Q & A
« Reply #326 on: 08/24/2009 10:01 AM »
"m_inert = empty mass, including propulsion system"

This is my unknown.  I'm putting together this ship in LEO and I can lift 25 tonnes at a time and If I know every thing else including the empty mass of my space ship and want to work out how many fuel tanks I need, I need to know the 'm_inert mass' of my fuel tanks.

Offline Antares

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Re: Basic Rocket Science Q & A
« Reply #327 on: 08/26/2009 05:55 PM »
Anyone have a good reference for rocket plume electrical (and RF?) characteristics?
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Offline mmeijeri

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Re: Basic Rocket Science Q & A
« Reply #328 on: 08/26/2009 09:45 PM »
Just set SCE to Aux and you'll be fine. ;)
We will be vic-toooooo-ri-ous!!!

Offline C5C6

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Re: Basic Rocket Science Q & A
« Reply #329 on: 08/27/2009 11:59 PM »
Would a capsule in a retrograde orbit need extra shielding for a safe reentry??

Offline Danny Dot

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Re: Basic Rocket Science Q & A
« Reply #330 on: 08/28/2009 12:10 AM »
Would a capsule in a retrograde orbit need extra shielding for a safe reentry??

Does retrograde mean going West?  If it does the answer is yes.  Probably a 10% increase in mass to the heat shield.

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Offline ginahoy

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Re: Basic Rocket Science Q & A
« Reply #331 on: 09/09/2009 06:13 AM »
When flying in a supersonic jet (or the shuttle), does the sound level in the cockpit (or crew module) change after the vehicle goes supersonic? I would guess that the answer is no since the crew is moving at the same speed as the source of the noise.

Offline Danny Dot

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Re: Basic Rocket Science Q & A
« Reply #332 on: 09/09/2009 02:31 PM »
When flying in a supersonic jet (or the shuttle), does the sound level in the cockpit (or crew module) change after the vehicle goes supersonic? I would guess that the answer is no since the crew is moving at the same speed as the source of the noise.

I the F-4, if my back seater was talking too much, I would just go supersonic so I couldn't hear him anymore  ;D

Not really.  I don't remember any difference at all in the sound in the cockpit.  I have a good description of a "fun" ride I had over the Kissimee river on what it is like to go supersonic.  Look in www.lulu.com/dannydeger for a free copy. 

Danny Deger
« Last Edit: 09/09/2009 02:32 PM by Danny Dot »
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Offline ANTIcarrot

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Re: Basic Rocket Science Q & A
« Reply #333 on: 10/07/2009 11:25 PM »
As a matter of special effects...

1) In space no one can hear you scream. But what if you stood in the exhaust of a rocket. Ignoring material considerations, what would you be likely to hear as you moved 'upsteam' along the exhaust closer and closer to the engine? Dull roar? High pitched whistle?

2) If you were looking sideways through a hydrogen rocket exhaust in vacuum (at close range, and moving in the same direction) and there were no other bright objects around, would you still see the same faint blue glow and shock cones you see inside the atmosphere? Would sub-optimal altitude compensation produce any interesting effects? What would be the length of the vissible plume?

Any true or suffficiently interesting answers appreciated.

Offline Antares

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Re: Basic Rocket Science Q & A
« Reply #334 on: 10/08/2009 04:22 PM »
1) In space no one can hear you scream. But what if you stood in the exhaust of a rocket. Ignoring material considerations, what would you be likely to hear as you moved 'upsteam' along the exhaust closer and closer to the engine? Dull roar? High pitched whistle?

2) If you were looking sideways through a hydrogen rocket exhaust in vacuum (at close range, and moving in the same direction) and there were no other bright objects around,
a) would you still see the same faint blue glow and shock cones you see inside the atmosphere? Would sub-optimal altitude compensation produce any interesting effects?
b) What would be the length of the vissible plume?

1) Sound is oscillating gas pressure, sensed on a diaphragm of some sort (ear drum, microphone).  So in a textbook engine, there would be none.  In real life, however, I can't imagine it being anything other than a dull roar.  The turbulence in the flow would be broadband in frequency (chaos theory applies IIRC).

2a) By definition, no shock cones on a plume into vacuum.
2a&b) This is fairly easily solvable using known emission lines of combustion products (which depend on propellant) and equations of black body radiation and flow expansion.
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Offline Danny Dot

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Re: Basic Rocket Science Q & A
« Reply #335 on: 10/08/2009 04:33 PM »
snip
2a) By definition, no shock cones on a plume into vacuum.
snip

This is certainly true after the gasses have dispersed into the vacuum, but close to the nozzle there might be enough gas in the exhaust to form a visible shock wave.  Next time I am next to a nozzle in the vacuum of space, I will try and get a picture for us.

Danny Deger
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Offline ugordan

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Re: Basic Rocket Science Q & A
« Reply #336 on: 10/08/2009 06:16 PM »
snip
2a) By definition, no shock cones on a plume into vacuum.
snip

This is certainly true after the gasses have dispersed into the vacuum, but close to the nozzle there might be enough gas in the exhaust to form a visible shock wave.

I don't get it. If there is no back pressure on the nozzle exit, what would the (supersonic) exhaust interact with? Indeed, there would have to be something with respect to what the flow would be supersonic in the first place, no?

Offline Danny Dot

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Re: Basic Rocket Science Q & A
« Reply #337 on: 10/08/2009 06:19 PM »
snip
2a) By definition, no shock cones on a plume into vacuum.
snip

This is certainly true after the gasses have dispersed into the vacuum, but close to the nozzle there might be enough gas in the exhaust to form a visible shock wave.

I don't get it. If there is no back pressure on the nozzle exit, what would the (supersonic) exhaust interact with? Indeed, there would have to be something with respect to what the flow would be supersonic in the first place, no?

Any time you turn a gas flowing at faster than the speed of sound it generates a shock wave.  The exhaust is a super sonic flow of a gas. 

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Offline Antares

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Re: Basic Rocket Science Q & A
« Reply #338 on: 10/08/2009 07:10 PM »
Indeed, there would have to be something with respect to what the flow would be supersonic in the first place, no?

The sonic speed of a flow is determined by sqrt(gamma*R*T), all of the flow itself.
gamma = Cp/Cv ratio of specific heats
R = universal gas constant
T = temperature of the flow
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Offline strangequark

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Re: Basic Rocket Science Q & A
« Reply #339 on: 10/08/2009 11:42 PM »
Minor nitpick on the gas constant. For a speed of sound calculation, it is the gas constant for the gas, not the universal constant. For instance:

R.air=287 J/kg-K
R.universal=8314 J/kmol-K