Author Topic: Basic Rocket Science Q & A  (Read 271351 times)

Offline mmeijeri

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Re: Basic Rocket Science Q & A
« Reply #60 on: 03/02/2009 02:59 PM »
it is only hard for the SSME since it wasn't designed for it.   It is head started which means the start box is small, the initial conditions are very critical.  Also it depends on ground GSE for start and purges

Thanks!
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Offline Spacenick

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Re: Basic Rocket Science Q & A
« Reply #61 on: 03/02/2009 05:17 PM »
How much energy/m^2 does a tile on the Space Shuttle have to dissipate?
Wouldn't it be possible to construct a metallic heat shield cooled by some material say wax being molten during reentry? I mean a phase transition should take a lot of energy off the metallic heat shield.

Offline LegendCJS

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Re: Basic Rocket Science Q & A
« Reply #62 on: 03/02/2009 06:50 PM »
How much energy/m^2 does a tile on the Space Shuttle have to dissipate?
Wouldn't it be possible to construct a metallic heat shield cooled by some material say wax being molten during reentry? I mean a phase transition should take a lot of energy off the metallic heat shield.

You can do back of the envelope calculations for things like this by assuming that all the kenetic energy in an orbiting vehicle is disipated as heat.  Thus:

1/2 *mass*orbital_velocity^2 = dissipated_energy_per_m^2 * area_of_vehicle

Where mass is the vehicles mass,
orbital velocity is 175000 mph or close to that,
dissipated_energy_per_m^2 is the quantity you want to know about
area_of_vehicle is the cross-sectional area profile of the vehicle during re-entry.

Now you need the mass of wax able to undergo a phase change and take up the same amount of energy- if this is any significant amount of the vehicle mass then you have to account for the added mass in the first equation.

However, I'm not aware of the phase of wax in zero pressure at orbital ambient temperatures, it might already want to liquefy on you every time the vehicle is in sunlight...
Remember: if we want this whole space thing to work out we have to optimize for cost!

Offline renclod

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Re: Basic Rocket Science Q & A
« Reply #63 on: 03/02/2009 07:01 PM »
How much energy/m^2 does a tile on the Space Shuttle have to dissipate?

7 giga joules per m^2, give or take


Offline kneecaps

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Re: Basic Rocket Science Q & A
« Reply #64 on: 03/02/2009 07:16 PM »
Wouldn't it be possible to construct a metallic heat shield cooled by some material say wax being molten during reentry? I mean a phase transition should take a lot of energy off the metallic heat shield.

You are really just talking about an ablative heat shield using a novel material :) The key point with the Shuttles TPS is that it is not an ablator and you can dust it off and fly it again (on paper!). It is simply unfortunate that such a great heatshield is so susceptible to phsyical damage.
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Offline mmeijeri

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Re: Basic Rocket Science Q & A
« Reply #65 on: 03/02/2009 07:23 PM »
I'm trying to understand how a rocket nozzle works and I have some questions. I only have a very limited understanding of thermodynamics and fluid flow. I worked on the software design of a multiphase flow simulator with experts on multiphase flow and thermodynamics for a couple of years and picked up a tiny little bit.

So here are my questions and my own possibly highly incorrect guesses what the answers are:

- How essential is choked flow through the throat? Would a de Laval nozzle still have an effect if the flow were not choked? My guess is yes, but it would be less effective.

- How essential is it that the fluid is compressible? Would it work with a liquid? My guess is absolutely essential and no, it wouldn't work.

- How essential is it that the gas is hot? My guess is very important, though I wonder what the reference point is.

- What if you left off the nozzle? My guess is you would still have thrust just less and your gas would be very hot.

- What if you left off both the throat and the nozzle? My guess is you'd get a nice flame but very little thrust.

- What if you kept the nozzle but left off the throat? My guess is that this is the same as my first question, you wouldn't get choked flow.
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Offline Nascent Ascent

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Re: Basic Rocket Science Q & A
« Reply #66 on: 03/02/2009 07:29 PM »
Quote
How essential is choked flow through the throat? Would a de Laval nozzle still have an effect if the flow were not choked? My guess is yes, but it would be less effective.

It wouldn't BE a deLaval nozzle if it weren't "choked".  The convergent-divergent aspect of the deLaval nozzle is the basis of the design.

It is also essential that the working medium is compressible (i.e. a gas).
« Last Edit: 03/02/2009 07:30 PM by Nascent Ascent »
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Offline kneecaps

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Re: Basic Rocket Science Q & A
« Reply #67 on: 03/02/2009 07:30 PM »
http://www.grc.nasa.gov/WWW/K-12/airplane/ienzl.html

Have a play with this applet, put it in 'nozzle' mode, it starts as a turbine otherwise, see if you can answer some of your questions by experimentation, always more educational :)
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Offline LegendCJS

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Re: Basic Rocket Science Q & A
« Reply #68 on: 03/02/2009 07:31 PM »
Wouldn't it be possible to construct a metallic heat shield cooled by some material say wax being molten during reentry? I mean a phase transition should take a lot of energy off the metallic heat shield.

You are really just talking about an ablative heat shield using a novel material :) The key point with the Shuttles TPS is that it is not an ablator and you can dust it off and fly it again (on paper!). It is simply unfortunate that such a great heatshield is so susceptible to phsyical damage.

I'm pretty sure he was talking about a system where the metal was on the "outside" and the interior of the metal shell was packed with wax.  The wax would melt during re-entry, but a couple of hours later on the ground the wax would have re-solidified, and thus his idea was for a re-usable heat shield. 

Not to say it has any chance of being practical:  at a latent heat of fusion of 200 J per gram, you would need 35,000kg of parafin to melt in order to absorb the quoted 7 giga-joules per meter squared for every square meter of heat shield.  Plus wax expands in volume quite a bit when it melts.
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Offline kneecaps

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Re: Basic Rocket Science Q & A
« Reply #69 on: 03/02/2009 07:37 PM »
Wouldn't it be possible to construct a metallic heat shield cooled by some material say wax being molten during reentry? I mean a phase transition should take a lot of energy off the metallic heat shield.

You are really just talking about an ablative heat shield using a novel material :) The key point with the Shuttles TPS is that it is not an ablator and you can dust it off and fly it again (on paper!). It is simply unfortunate that such a great heatshield is so susceptible to phsyical damage.

I'm pretty sure he was talking about a system where the metal was on the "outside" and the interior of the metal shell was packed with wax.  The wax would melt during re-entry, but a couple of hours later on the ground the wax would have re-solidified, and thus his idea was for a re-usable heat shield. 

Not to say it has any chance of being practical:  at a latent heat of fusion of 200 J per gram, you would need 35,000kg of parafin to melt in order to absorb the quoted 7 giga-joules per meter squared for every square meter of heat shield.  Plus wax expands in volume quite a bit when it melts.

Ah...you saw it the other way around to me :) I see what was intended now :)
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Offline Spacenick

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Re: Basic Rocket Science Q & A
« Reply #70 on: 03/02/2009 07:55 PM »
@LegendCJS: Yeah, that was kind of the idea, seems like most materials will be molten too easily.
 Maybe someone could expand how the real concepts for metallic actively cooled heat shields looked like?

Offline mmeijeri

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Re: Basic Rocket Science Q & A
« Reply #71 on: 03/02/2009 07:58 PM »
Thanks!

It wouldn't BE a deLaval nozzle if it weren't "choked".

So does that mean it stops being a de Laval nozzle when it's turned off? ;-)
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Offline Antares

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Re: Basic Rocket Science Q & A
« Reply #72 on: 03/03/2009 05:45 AM »
Quote
- How essential is choked flow through the throat?

Choked means Mach 1.  If it doesn't get to Mach 1, it won't be supersonic in the divergent section.  Low thrust, very low Isp.

Quote
How essential is it that the fluid is compressible? Would it work with a liquid?

Force is rate of change of momentum, right?  M-dot v in this case.  So for high thrust you need to maximize each one.  A liquid is going to be denser and more viscous, requiring more energy to get both a higher m-dot and higher v.  Low thrust, very low Isp.

Quote
- How essential is it that the gas is hot? My guess is very important, though I wonder what the reference point is.

It needs to be energetic - so it can become speedy.  When it's nearly static near the injector face or unburned solid propellant, it's REALLY hot.  Adiabatically, in the nozzle, it's also REALLY hot - if you say stick your finger in the flow.  If, instead, you put your finger on the edge of the flow in a big enough nozzle, it might FEEL cold because so much of the static enthalpy has become velocity (this is not wind chill factor).

Quote
What if you left off the nozzle? My guess is you would still have thrust just less and your gas would be very hot.

You would have an uncontrolled expansion from the throat, full of shocks too.  Lots of entropy and other losses.  Temperature would vary depending on where in the shock-expansion field you were.

Quote
What if you left off both the throat and the nozzle?

Remember the scene in Hot Shots where the aircraft tech is roasting weinies or marshmallows at the nozzle of the fighter?

Quote
What if you kept the nozzle but left off the throat?

No, it would still choke at the throat, unless the mass flow were so low that it didn't need to accelerate to sonic to get out of the combustion chamber.  Flow in the nozzle would be indeterminate (subsonic or supersonic) without more information like chamber pressure and ambient pressure.

Huzel & Huang; Sutton; J.D. Anderson are your friends.
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Offline renclod

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Re: Basic Rocket Science Q & A
« Reply #73 on: 03/03/2009 07:20 AM »
Maybe someone could expand how the real concepts for metallic actively cooled heat shields looked like?

I would suggest you read this piece online at spacedaily under "Rocket Science" :
" Cult spacecraft Part One: The Little Spaceplane That Couldn't "
Dec.04, 2008
by Jeffrey F. Bell


Offline William Barton

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Re: Basic Rocket Science Q & A
« Reply #74 on: 03/03/2009 08:06 AM »
Am I correcting in thinking the big problem with active TPS is heat rejection? In one of my old stories, I had a liquid-metal (sodium) cooled TPS that used the collected heat to drive a laser to dump the energy overboard. It being a story, I kind of glossed over the conversion efficiency issues, "in the future" someone would figure it out.

Offline mmeijeri

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Re: Basic Rocket Science Q & A
« Reply #75 on: 03/03/2009 08:27 AM »
Hi Antares,

Thanks for the lengthy reply. And thanks 'kneecaps' for the link to that applet. Based on your replies I have some gedanken experiments I'd like to run by you if that's ok.

Quote
- How essential is choked flow through the throat?
Choked means Mach 1.  If it doesn't get to Mach 1, it won't be supersonic in the divergent section.  Low thrust, very low Isp.

And compared to the situation without the nozzle? Would it still convert some (possibly tiny) amount of heat to macroscopic kinetic energy?

Quote
Quote
How essential is it that the fluid is compressible? Would it work with a liquid?
Force is rate of change of momentum, right?  M-dot v in this case.  So for high thrust you need to maximize each one.  A liquid is going to be denser and more viscous, requiring more energy to get both a higher m-dot and higher v.  Low thrust, very low Isp.

I'm imagining attaching a garden hose to a de Laval nozzle and turning on the tap. I'd expect water to come out at a higher velocity than without the nozzle. I'd also be worried about my hose or nozzle bursting. I'd expect the same thrust as without the nozzle, just with a meaner jet. I wouldn't expect a hot water tap to produce better results than a cold water tap.

Quote
Quote
- How essential is it that the gas is hot? My guess is very important, though I wonder what the reference point is.
It needs to be energetic - so it can become speedy.  When it's nearly static near the injector face or unburned solid propellant, it's REALLY hot.  Adiabatically, in the nozzle, it's also REALLY hot - if you say stick your finger in the flow.  If, instead, you put your finger on the edge of the flow in a big enough nozzle, it might FEEL cold because so much of the static enthalpy has become velocity (this is not wind chill factor).

So a leaf blower would produce the same 'thrust' with a nozzle as without it? And a hair dryer slightly more with a nozzle than without it?

Quote
Quote
What if you left off both the throat and the nozzle?
Remember the scene in Hot Shots where the aircraft tech is roasting weinies or marshmallows at the nozzle of the fighter?

Lol!

Quote
Quote
What if you kept the nozzle but left off the throat?
No, it would still choke at the throat, unless the mass flow were so low that it didn't need to accelerate to sonic to get out of the combustion chamber.  Flow in the nozzle would be indeterminate (subsonic or supersonic) without more information like chamber pressure and ambient pressure.

Let me see if I understand.

Suppose we have a fixed pressure at the inlet and a (lower) fixed pressure at the outlet. Is it the case that for every positive pressure differential the flow through a de Laval nozzle would choke in steady state?

And the only way it could not choke would be if instead you specified mass flow at the inlet and that mass flow were sufficiently low to prevent choking at the throat? And even then it would depend on the other conditions.

Quote
Huzel & Huang; Sutton; J.D. Anderson are your friends.

And having some experts around doesn't hurt either. Thanks again guys!
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Offline Antares

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Re: Basic Rocket Science Q & A
« Reply #76 on: 03/03/2009 05:48 PM »
Quote
Quote
Quote
How essential is it that the fluid is compressible? Would it work with a liquid?
Force is rate of change of momentum, right?  M-dot v in this case.  So for high thrust you need to maximize each one.  A liquid is going to be denser and more viscous, requiring more energy to get both a higher m-dot and higher v.  Low thrust, very low Isp.
I'm imagining attaching a garden hose to a de Laval nozzle and turning on the tap. I'd expect water to come out at a higher velocity than without the nozzle. I'd also be worried about my hose or nozzle bursting. I'd expect the same thrust as without the nozzle, just with a meaner jet. I wouldn't expect a hot water tap to produce better results than a cold water tap.

No, the water would come out slower because it did not reach Mach 1 at the throat.  In all of these cases, m-dot is constant and m-dot equals density times area times velocity.  So if your area goes up in an incompressible flow or a compressible, subsonic flow, your velocity goes down.

Quote
Quote
Quote
- How essential is it that the gas is hot? My guess is very important, though I wonder what the reference point is.
It needs to be energetic - so it can become speedy.  When it's nearly static near the injector face or unburned solid propellant, it's REALLY hot.  Adiabatically, in the nozzle, it's also REALLY hot - if you say stick your finger in the flow.  If, instead, you put your finger on the edge of the flow in a big enough nozzle, it might FEEL cold because so much of the static enthalpy has become velocity (this is not wind chill factor).
So a leaf blower would produce the same 'thrust' with a nozzle as without it? And a hair dryer slightly more with a nozzle than without it?

Both of those would be less thrust because the flow is subsonic and the nozzle would decelerate the flow.

Quote
Quote
Quote
What if you kept the nozzle but left off the throat?
No, it would still choke at the throat, unless the mass flow were so low that it didn't need to accelerate to sonic to get out of the combustion chamber.  Flow in the nozzle would be indeterminate (subsonic or supersonic) without more information like chamber pressure and ambient pressure.
Let me see if I understand.

1) Suppose we have a fixed pressure at the inlet and a (lower) fixed pressure at the outlet. Is it the case that for every positive pressure differential the flow through a de Laval nozzle would choke in steady state?

2) And the only way it could not choke would be if instead you specified mass flow at the inlet and that mass flow were sufficiently low to prevent choking at the throat? And even then it would depend on the other conditions.

1) No.  There's a diagram in Anderson's Fundamentals of Aerodynamics that you really need to look at showing the varieties of nozzle flow depending on pressure ratio.  Area ratio is the other factor obviously.
2) "It would depend on the other conditions." Yes.  If the pressure or area ratio were low, the flow wouldn't accelerate sufficiently to be sonic at the throat.

Read Anderson.  Ignore the equations if you're not good at math.  The explanations and pictures are really good.
If I like something on NSF, it's probably because I know it to be accurate.  Every once in a while, it's just something I agree with.  Facts generally receive the former.

Offline mmeijeri

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Re: Basic Rocket Science Q & A
« Reply #77 on: 03/03/2009 05:54 PM »
Read Anderson. 

I will. Thanks again.

Quote
Ignore the equations if you're not good at math.  The explanations and pictures are really good.

I studied maths, I should be OK :-)
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Offline Spacenick

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Re: Basic Rocket Science Q & A
« Reply #78 on: 03/03/2009 06:40 PM »
I thought about this equation again:
"1/2 *mass*orbital_velocity^2 = dissipated_energy_per_m^2 * area_of_vehicle"

and it seems to me that it can't be correct. It' uses the correct equation for the energy dissipated over the whole reentry, but because this energy is is not only converted into space craft heating, but also in kinetic energy of air molecules (as in pushing the air out of the way), heating and compressing of air, creation of plasma from air and electric discharges.
I'm pretty sure that only a tiny percentage of the orbital energy is actually converted into heating up of the spacecraft itself, especially since the blunt shape of reentry vehicles keeps the shock wave off the vehicle hull and therefor reduces energy transfer extremely.

I foudn the wikipedia article for atmospheric reentry to be very imformative
http://forum.nasaspaceflight.com/index.php?topic=13543.75

Offline vt_hokie

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Re: Basic Rocket Science Q & A
« Reply #79 on: 03/03/2009 11:45 PM »
I found that what you find at 75ºE is the major Indic Ocean perturbation, but it's a negative perturbation, meaning (and I checked this several times through different sources) it exerts less gravitational pull.

Yes, this is one of the gravity wells, the other being near 105 deg W longitude.  A geostationary satellite that fails on-orbit will oscillate around the nearest gravity well with an amplitude roughly equal to the original distance from that well.  So, the further from a well a failed satellite is, the more of a problem it becomes for other satellites when it fails!

Of course, such sats will be above GEO when drifting west and below when drifting east.  (And meanwhile they'll have higher inclinations, up to ~15 deg depending on where they are in the inc cycle.)  But they have to be watched for potential intercepts with operational spacecraft nonetheless!

« Last Edit: 03/04/2009 05:07 AM by vt_hokie »