Author Topic: Basic Rocket Science Q & A  (Read 270776 times)

Offline clongton

  • Expert
  • Senior Member
  • *****
  • Posts: 10271
  • Connecticut
    • Direct Launcher
  • Liked: 2034
  • Likes Given: 623
Re: Basic Rocket Science Q & A
« Reply #40 on: 02/21/2009 09:09 PM »
OK, here's a stupid question:

For some launch vehicles with compartments, the air from these compartments is often replaced with nitrogen, to suppress any possible fires. Why don't they use helium, instead, which would make the launch vehicle lighter?


Helium is one of those elements that we have enough of - for now - but that will not always be the case. Mission planners and engine designers are already aware of the decreasing availability of helium. It's not a problem - yet - but it will be, especially if we use it like you suggest and it just gets vented. There just is not an over abundant supply of it on earth and we need to be aware of our usage and not waste it.
Chuck - DIRECT co-founder
I started my career on the Saturn-V F-1A engine

Offline AnalogMan

  • Member
  • Senior Member
  • *****
  • Posts: 2935
  • Cambridge, UK
  • Liked: 642
  • Likes Given: 20
Re: Basic Rocket Science Q & A
« Reply #41 on: 02/21/2009 11:29 PM »
OK, here's a stupid question:

For some launch vehicles with compartments, the air from these compartments is often replaced with nitrogen, to suppress any possible fires. Why don't they use helium, instead, which would make the launch vehicle lighter?


Helium is one of those elements that we have enough of - for now - but that will not always be the case. Mission planners and engine designers are already aware of the decreasing availability of helium. It's not a problem - yet - but it will be, especially if we use it like you suggest and it just gets vented. There just is not an over abundant supply of it on earth and we need to be aware of our usage and not waste it.

Twenty-five years ago I used to work in laboratories that used liquid helium for cooling a multitude of sensors on scientific experiments on a daily basis - even then we were very careful to capture and recover as much of the total boil-off as we could to minimise costs to our facility.

Offline madscientist197

  • Regular
  • Full Member
  • ****
  • Posts: 1014
  • Liked: 2
  • Likes Given: 0
Re: Basic Rocket Science Q & A
« Reply #42 on: 02/22/2009 07:51 AM »
I knew that Hydrogen embrittlement was a real issue, but I didn't realise that there was any issue with Helium.
John

Offline nomadd22

  • Full Member
  • *
  • Posts: 170
  • Liked: 0
  • Likes Given: 0
Re: Basic Rocket Science Q & A
« Reply #43 on: 02/22/2009 11:51 AM »
 The problem with helium is the places it gets into. It diffuses through things that are impermeable to air or nitrogen over time, and when the things are exposed to vacuum in just the few minutes it takes to launch you suddenly have pressurized helium in places you don't want it because it takes time to diffuse back out.
 It also makes transponders transmit in a high squeaky voice.

Offline Jim

  • Night Gator
  • Senior Member
  • *****
  • Posts: 31222
  • Cape Canaveral Spaceport
  • Liked: 9494
  • Likes Given: 298
Re: Basic Rocket Science Q & A
« Reply #44 on: 02/22/2009 04:28 PM »
Some materials and electronics are sensitive to helium too.
Sensitive to helium in what sense? It's supposed to be an inert gas. Diffusion problems?

Dunno exactly.  Helium can go through anything, apparently weakening some metals a tiny bit.  Only a problem in very low margin areas.  There are some spacecraft (guidance?) boxes that don't like helium.  I don't know the mechanism, just that the providers ask to make sure that their boxes aren't around any helium sources.

It was IMU with Hemispherical Resonant Gyro
http://www.es.northropgrumman.com/solutions/hrg/index.html

Helium changes the frequency* and mucks them up.

* not all of them operate in a vacuum

Offline mmeijeri

  • Senior Member
  • *****
  • Posts: 7442
  • Martijn Meijering
  • NL
  • Liked: 70
  • Likes Given: 162
Re: Basic Rocket Science Q & A
« Reply #45 on: 02/25/2009 05:01 AM »
I have a question about using regenerative cooling with rocket engines. I read somewhere that this works best with liquid hydrogen / liquid oxygen rockets. Apparently the thermodynamic qualities of H2 are very good, better than those of hydrocarbons, which have additional problems with coking as well. Oxygen is rarely used as a coolant because it's very reactive, especially when it's very hot.

What I don't understand is why you couldn't use a separate cooling fluid that does not have coking problems and doesn't react with the nozzle and use a heat exchanger. Then you could still use the oxygen as a heat sink. The Skylon people want to do something similar, but not for cooling the nozzle but for use in their precooler in air-breathing mode.

This sounds like a very simple concept, yet I have never read anything about it, so there's probably something wrong with it. Anyone know what?
« Last Edit: 02/25/2009 05:05 AM by mmeijeri »
We will be vic-toooooo-ri-ous!!!

Offline yinzer

  • Extreme Veteran
  • Full Member
  • ****
  • Posts: 1509
  • Liked: 0
  • Likes Given: 0
Re: Basic Rocket Science Q & A
« Reply #46 on: 02/25/2009 05:54 AM »
How does having a separate heat exchanger help with oxygen reactivity?  When you're done, you still have a bunch of hot, high pressure oxygen.

There is also the added weight of the second exchanger and the power required to move the coolant through it (which is much higher than you might imagine - thousands of horsepower for a mid-to-large rocket engine).
California 2008 - taking rights from people and giving rights to chickens.

Offline mmeijeri

  • Senior Member
  • *****
  • Posts: 7442
  • Martijn Meijering
  • NL
  • Liked: 70
  • Likes Given: 162
Re: Basic Rocket Science Q & A
« Reply #47 on: 02/25/2009 09:21 AM »
How does having a separate heat exchanger help with oxygen reactivity?  When you're done, you still have a bunch of hot, high pressure oxygen.

I was thinking that the heat capacity and conductivity of the oxygen tank would be so enormous that the oxygen wouldn't get very hot at all, even locally.

Quote
There is also the added weight of the second exchanger

Ah, I can see how that would be a problem. Would the heat exchanger be more massive than an ablative nozzle?

Quote
and the power required to move the coolant through it (which is much higher than you might imagine - thousands of horsepower for a mid-to-large rocket engine).

But is that any different from a normal regeneratively cooled engine?
We will be vic-toooooo-ri-ous!!!

Offline DMeader

  • Full Member
  • ****
  • Posts: 953
  • Liked: 98
  • Likes Given: 47
Re: Basic Rocket Science Q & A
« Reply #48 on: 02/25/2009 12:34 PM »
How about the weight of the fluid itself?

Oxidizer tank would have to be built stronger to handle the increase in pressure, as the oxidixer absorbs heat the pressure would begin to rise immediately.

Also adding complexity. Another fluid system, more pipes, more valves, tankage on the LV, infrastructure for servicing. Something else to fail.
« Last Edit: 02/25/2009 12:36 PM by DMeader »

Offline mmeijeri

  • Senior Member
  • *****
  • Posts: 7442
  • Martijn Meijering
  • NL
  • Liked: 70
  • Likes Given: 162
Re: Basic Rocket Science Q & A
« Reply #49 on: 02/25/2009 12:47 PM »
How about the weight of the fluid itself?

Oxidizer tank would have to be built stronger to handle the increase in pressure, as the oxidixer absorbs heat the pressure would begin to rise immediately.

Also adding complexity. Another fluid system, more pipes, more valves, tankage on the LV, infrastructure for servicing. Something else to fail.

I see, thanks guys.
We will be vic-toooooo-ri-ous!!!

Online Eerie

  • Member
  • Full Member
  • ****
  • Posts: 748
  • Liked: 104
  • Likes Given: 6
Re: Basic Rocket Science Q & A
« Reply #50 on: 02/25/2009 06:21 PM »
Can I find somewhere a total mass of everything artificial in orbit around Earth?

Offline yinzer

  • Extreme Veteran
  • Full Member
  • ****
  • Posts: 1509
  • Liked: 0
  • Likes Given: 0
Re: Basic Rocket Science Q & A
« Reply #51 on: 02/26/2009 02:05 AM »
How does having a separate heat exchanger help with oxygen reactivity?  When you're done, you still have a bunch of hot, high pressure oxygen.

I was thinking that the heat capacity and conductivity of the oxygen tank would be so enormous that the oxygen wouldn't get very hot at all, even locally.

The amount of oxygen available for cooling doesn't change.
Quote
Quote
There is also the added weight of the second exchanger
Ah, I can see how that would be a problem. Would the heat exchanger be more massive than an ablative nozzle?
Depends, but probably.
Quote
Quote
and the power required to move the coolant through it (which is much higher than you might imagine - thousands of horsepower for a mid-to-large rocket engine).
But is that any different from a normal regeneratively cooled engine?

In a normal regeneratively cooled engine, you have one heat exchanger operating at a large temperature differential.  With your scheme, you have two heat exchangers each operating at a smaller temperature differential, which means more pressure drop and thus power.
California 2008 - taking rights from people and giving rights to chickens.

Offline mmeijeri

  • Senior Member
  • *****
  • Posts: 7442
  • Martijn Meijering
  • NL
  • Liked: 70
  • Likes Given: 162
Re: Basic Rocket Science Q & A
« Reply #52 on: 02/26/2009 02:24 AM »
In a normal regeneratively cooled engine, you have one heat exchanger operating at a large temperature differential.  With your scheme, you have two heat exchangers each operating at a smaller temperature differential, which means more pressure drop and thus power.

Thanks! Continuing with my naive ideas, why couldn't you construct a rocket in such a way that either the fuel or oxygen tank surrounds the combustion chamber and nozzle for cooling? I'm not suggesting this is a good idea and maybe this would be an excellent way to build a bomb, I'm just trying to understand.
« Last Edit: 02/26/2009 02:36 AM by mmeijeri »
We will be vic-toooooo-ri-ous!!!

Offline usn_skwerl

  • Space Junkie
  • Full Member
  • **
  • Posts: 218
  • Ad Astra
  • South Bend, IN
  • Liked: 0
  • Likes Given: 0
Re: Basic Rocket Science Q & A
« Reply #53 on: 02/27/2009 05:42 AM »
Just an observation I had and wanted to pass along because I didn't quite hear it worded correctly and/or dumbed down until very recently.

The way I heard it described (for those of us unsure about specific impulse (measured in seconds)), it's defined as how long a rocket will fire using a pound of prop/oxidizer to provide a pound of thrust. If this is inaccurate, please correct me.
If we do not destroy ourselves, we will one day venture to the stars.

Offline Jim

  • Night Gator
  • Senior Member
  • *****
  • Posts: 31222
  • Cape Canaveral Spaceport
  • Liked: 9494
  • Likes Given: 298
Re: Basic Rocket Science Q & A
« Reply #54 on: 02/27/2009 12:15 PM »
Just an observation I had and wanted to pass along because I didn't quite hear it worded correctly and/or dumbed down until very recently.

The way I heard it described (for those of us unsure about specific impulse (measured in seconds)), it's defined as how long a rocket will fire using a pound of prop/oxidizer to provide a pound of thrust. If this is inaccurate, please correct me.

That is incorrect

It is thrust per propellant flow rate or  impulse (change in momentum) per unit of propellant.

Offline William Barton

  • Senior Member
  • *****
  • Posts: 3487
  • Liked: 2
  • Likes Given: 0
Re: Basic Rocket Science Q & A
« Reply #55 on: 02/27/2009 12:28 PM »
Just an observation I had and wanted to pass along because I didn't quite hear it worded correctly and/or dumbed down until very recently.

The way I heard it described (for those of us unsure about specific impulse (measured in seconds)), it's defined as how long a rocket will fire using a pound of prop/oxidizer to provide a pound of thrust. If this is inaccurate, please correct me.

That is incorrect

It is thrust per propellant flow rate or  impulse (change in momentum) per unit of propellant.

Isn't propellant flow rate a compound factor? Lb-thrust per lb-prop per second? (I will not be surprised if I got that wrong.)

Offline mmeijeri

  • Senior Member
  • *****
  • Posts: 7442
  • Martijn Meijering
  • NL
  • Liked: 70
  • Likes Given: 162
Re: Basic Rocket Science Q & A
« Reply #56 on: 02/28/2009 01:07 PM »
Occasionally I see people mention air-started SSME's. I get the impression that this is either hard or dangerous. Somehow the J2X is supposed to be better in this regard. Is there a simple explanation of what the problem is and how J2X is better?
We will be vic-toooooo-ri-ous!!!

Offline Jim

  • Night Gator
  • Senior Member
  • *****
  • Posts: 31222
  • Cape Canaveral Spaceport
  • Liked: 9494
  • Likes Given: 298
Re: Basic Rocket Science Q & A
« Reply #57 on: 02/28/2009 01:22 PM »
Occasionally I see people mention air-started SSME's. I get the impression that this is either hard or dangerous. Somehow the J2X is supposed to be better in this regard. Is there a simple explanation of what the problem is and how J2X is better?

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

Offline Antares

  • ABO^2
  • Senior Member
  • *****
  • Posts: 5191
  • Done arguing with amateurs
  • Liked: 342
  • Likes Given: 210
Re: Basic Rocket Science Q & A
« Reply #58 on: 03/02/2009 02:40 PM »
From the SpaceX thread:
Now i need to ask... I guess I'm missing a key point on how rocket engines are started..is there a summary some where on how they start up rocket engines..I didn't realize a charge is needed to start the engine..
jb

There's plenty of information on this.  Use Sutton Rocket Propulsion Elements or Huzel and Huang Modern Engineering for Design of Liquid-propellant Rocket Engines.  An older version can be found here for free.
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 kkattula

  • Member
  • Senior Member
  • *****
  • Posts: 2506
  • Melbourne, Australia
  • Liked: 10
  • Likes Given: 3
Re: Basic Rocket Science Q & A
« Reply #59 on: 03/02/2009 02:46 PM »
Occasionally I see people mention air-started SSME's. I get the impression that this is either hard or dangerous. Somehow the J2X is supposed to be better in this regard. Is there a simple explanation of what the problem is and how J2X is better?

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

There's non-ground GSE?   ;)