Author Topic: Basic Rocket Science Q & A  (Read 272129 times)

Offline fatjohn1408

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Re: Basic Rocket Science Q & A
« Reply #740 on: 09/02/2012 07:08 PM »
Where do you see it as a constraint?  As free molecular heating?
For Pegasus, it is not just the fairing.  The wings char.
Also, there is interference heating. 

Well I just had one study that claimed that heat flux is an important constraint when simulating launcher trajectories. However, I cannot find a second source, so perhaps my assumption is wrong?

Offline Moe Grills

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Re: Basic Rocket Science Q & A
« Reply #741 on: 12/04/2012 08:18 PM »
 Ignoring clear differences in thrust forces, specific impulses
and chemical combinations (combustion) absent in "plasma" rocket engines); doesn't the fact that combustion temperatures in
"chemical" rocket engines, which reach intensities of 3000-4000 Kelvin
in certain cases, generate a plasma?

  I find it difficult to accept the idea that at 3000-4000 Kelvin superheated
CO2, NO2 and or H20 do not behave like a plasma. I would think that highly agitated electrons and nuclei would not tend to recombine immediately inside rocket motor combustion chambers; but I could be wrong.
 

Offline beb

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Re: Basic Rocket Science Q & A
« Reply #742 on: 12/04/2012 08:38 PM »
And your question is... what?

The chief difference between chemical engines and plasma engines us that chemical engines do not interact with the exhaust gases. All its thrust comes from the expansion of gases. Plasma engines use electric and magnetic fields to accelerate the plasma exhaust gases.

Offline Robotbeat

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Re: Basic Rocket Science Q & A
« Reply #743 on: 12/04/2012 08:38 PM »
The energy source is different. Plasma rockets use electricity, chemical uses the chemical energy.

Flame is, of course, a weak plasma.
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Offline Moe Grills

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Re: Basic Rocket Science Q & A
« Reply #744 on: 12/05/2012 10:42 PM »
And your question is... what?

The chief difference between chemical engines and plasma engines us that chemical engines do not interact with the exhaust gases. All its thrust comes from the expansion of gases. Plasma engines use electric and magnetic fields to accelerate the plasma exhaust gases.

  I think we're not on the same page; but that's my fault.
My question was sort of ambivalent...my regrets.
But I got the answer I was looking for below.

Offline Moe Grills

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Re: Basic Rocket Science Q & A
« Reply #745 on: 12/05/2012 10:46 PM »
The energy source is different. Plasma rockets use electricity, chemical uses the chemical energy.

Flame is, of course, a weak plasma.

Weak plasma or strong.
A combustion generated plasma rather than an electrically generated one
is what I would call it.
Thanks for your answer. Cheers.
BTW...a 'weak' combustion generated plasma still presents unique possibilities if one thinks outside the box....But I'll take that topic to
"advanced concepts".   ;)

Offline Proponent

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Re: Basic Rocket Science Q & A
« Reply #746 on: 12/06/2012 07:24 AM »
The energy source is different. Plasma rockets use electricity, chemical uses the chemical energy.

Flame is, of course, a weak plasma.

And JP Aerospace suggests a plasma engine in which the plasma is the exhaust of a chemical engine, which is then accelerated by electromagnetic means.  The advantage of this weird hybrid approach is that it's not necessary to generate a large amount of electricity to create the plasma in the first place.

Offline cambrianera

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Re: Basic Rocket Science Q & A
« Reply #747 on: 12/06/2012 08:46 AM »
FYI, a low temperature plasma is normally used to heat treat components in Plasma nitriding; the combination of low pressure and DC high voltage generates a Hydrogen- Nitrogen plasma acceleration layer (the glowing layer in pic).
Concerning rockets engines, beb's explanation is the best, plasma sensibility to EM fields is used to accelerate it (MHD generators do the inverse job, picking energy from the plasma content of a very high temperature combustion flame).
Oh to be young again. . .

Offline BobCarver

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Re: Basic Rocket Science Q & A
« Reply #748 on: 12/06/2012 06:29 PM »
For an in-depth discussion of this topic, see AIAA 95-4079 Rocket-Induced Magnetohydrodynamic Ejector A Single-Stage-to-Orbit Advanced Propulsion Concept, by J. Cole, J. Campbell, and A. Robertson NASA Marshall Space Flight Center Huntsville, AL at http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19960021025_1996043871.pdf


Offline Warren Platts

Re: Basic Rocket Science Q & A
« Reply #749 on: 12/20/2012 06:05 PM »
Is static electricity buildup ever a problem on ordinary spacecraft operations? If so, are there any engineering solutions that work in the vacuum of space that are commonly applied?
"When once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for there you have been, and there you will always long to return."--Leonardo Da Vinci

Offline deepseaskydiver

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Re: Basic Rocket Science Q & A
« Reply #750 on: 12/20/2012 06:50 PM »
Where do you see it as a constraint?  As free molecular heating?
For Pegasus, it is not just the fairing.  The wings char.
Also, there is interference heating. 

Well I just had one study that claimed that heat flux is an important constraint when simulating launcher trajectories. However, I cannot find a second source, so perhaps my assumption is wrong?

A second source would be the Atlas V User Guide:
"For Atlas V 500 series missions, the PLF is jettisoned during the booster phase of flight. Before PLF jettison,
the RD-180 engine is throttled down to maintain 2.5 g acceleration. Typically, the PLF is jettisoned when the
3-sigma free molecular heat flux falls below 1,135 W/m2 (360 Btu/ft2-hr). For sensitive SC, PLF jettison can
be delayed to reduce the heat flux with minor performance loss. After PLF jettison, the RD-180 is throttled
up."
So it can be a constraint that really originates from the payload and not necessarily from the launch vehicle.

Offline Jim

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Re: Basic Rocket Science Q & A
« Reply #751 on: 12/20/2012 07:05 PM »
Where do you see it as a constraint?  As free molecular heating?
For Pegasus, it is not just the fairing.  The wings char.
Also, there is interference heating. 

Well I just had one study that claimed that heat flux is an important constraint when simulating launcher trajectories. However, I cannot find a second source, so perhaps my assumption is wrong?

A second source would be the Atlas V User Guide:
"For Atlas V 500 series missions, the PLF is jettisoned during the booster phase of flight. Before PLF jettison,
the RD-180 engine is throttled down to maintain 2.5 g acceleration. Typically, the PLF is jettisoned when the
3-sigma free molecular heat flux falls below 1,135 W/m2 (360 Btu/ft2-hr). For sensitive SC, PLF jettison can
be delayed to reduce the heat flux with minor performance loss. After PLF jettison, the RD-180 is throttled
up."
So it can be a constraint that really originates from the payload and not necessarily from the launch vehicle.

The assumption is wrong and therefore that isn't the second source.
It only affects fairing jettison time.  The trajectory  isn't really affected. 

Offline deepseaskydiver

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Re: Basic Rocket Science Q & A
« Reply #752 on: 12/20/2012 08:19 PM »
Where do you see it as a constraint?  As free molecular heating?
For Pegasus, it is not just the fairing.  The wings char.
Also, there is interference heating. 

Well I just had one study that claimed that heat flux is an important constraint when simulating launcher trajectories. However, I cannot find a second source, so perhaps my assumption is wrong?

A second source would be the Atlas V User Guide:
"For Atlas V 500 series missions, the PLF is jettisoned during the booster phase of flight. Before PLF jettison,
the RD-180 engine is throttled down to maintain 2.5 g acceleration. Typically, the PLF is jettisoned when the
3-sigma free molecular heat flux falls below 1,135 W/m2 (360 Btu/ft2-hr). For sensitive SC, PLF jettison can
be delayed to reduce the heat flux with minor performance loss. After PLF jettison, the RD-180 is throttled
up."
So it can be a constraint that really originates from the payload and not necessarily from the launch vehicle.

The assumption is wrong and therefore that isn't the second source.
It only affects fairing jettison time.  The trajectory  isn't really affected. 

Jettison time controls when the throttle down to 2.5 G occurs, which is a pretty significant event during booster flight. I guess you can split hairs about what the conversation is about and what is meant by "constraint", but I thought it would be helpful to point out a case where significant events in a trajectory are based off of heating. After all, it seemed to me the question was looking for more sources as to why "heat flux is an important constraint when simulating launcher trajectories".

Offline Jim

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Re: Basic Rocket Science Q & A
« Reply #753 on: 12/20/2012 09:10 PM »
Where do you see it as a constraint?  As free molecular heating?
For Pegasus, it is not just the fairing.  The wings char.
Also, there is interference heating. 

Well I just had one study that claimed that heat flux is an important constraint when simulating launcher trajectories. However, I cannot find a second source, so perhaps my assumption is wrong?

A second source would be the Atlas V User Guide:
"For Atlas V 500 series missions, the PLF is jettisoned during the booster phase of flight. Before PLF jettison,
the RD-180 engine is throttled down to maintain 2.5 g acceleration. Typically, the PLF is jettisoned when the
3-sigma free molecular heat flux falls below 1,135 W/m2 (360 Btu/ft2-hr). For sensitive SC, PLF jettison can
be delayed to reduce the heat flux with minor performance loss. After PLF jettison, the RD-180 is throttled
up."
So it can be a constraint that really originates from the payload and not necessarily from the launch vehicle.

The assumption is wrong and therefore that isn't the second source.
It only affects fairing jettison time.  The trajectory  isn't really affected. 

Jettison time controls when the throttle down to 2.5 G occurs, which is a pretty significant event during booster flight. I guess you can split hairs about what the conversation is about and what is meant by "constraint", but I thought it would be helpful to point out a case where significant events in a trajectory are based off of heating. After all, it seemed to me the question was looking for more sources as to why "heat flux is an important constraint when simulating launcher trajectories".

That only applies to the 5m version of Atlas.  The 4m fairing is jettison during second stage flight and there is no throttling of the second stage engine.  Same applies for the Delta IV and II.   

Offline Warren Platts

Re: Basic Rocket Science Q & A
« Reply #754 on: 12/26/2012 07:29 PM »
1. Is static electricity buildup ever a problem on ordinary spacecraft operations?

2. If so, are there any engineering solutions that work in the vacuum of space that are commonly applied?

1. Electric propulsion that relies on accelerating ions carries away a positive charge, thus charging the spacecraft with a negative charge that must be dissipated.

2. There are "spacecraft neutralizers" (e.g., the LISA probes have them) that consist of field emitter arrays--arrays of tiny nanospikes that use field emission of electrons to emit electrons directly into the vacuum to neutralize the spacecraft.

http://www.sciencedirect.com/science/article/pii/S0094576508003664

http://erps.spacegrant.org/uploads/images/images/iepc_articledownload_1988-2007/2011index/IEPC-2011-083.pdf



"When once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for there you have been, and there you will always long to return."--Leonardo Da Vinci

Offline strangequark

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Re: Basic Rocket Science Q & A
« Reply #755 on: 12/28/2012 03:14 PM »
1. Electric propulsion that relies on accelerating ions carries away a positive charge, thus charging the spacecraft with a negative charge that must be dissipated.

The exhaust beam of electric thrusters is neutralized, otherwise the positive ions would eventually halt, reverse course, and start accelerating back toward the spacecraft.
Don't flippantly discount the old rules of this industry. Behind each one lies a painful lesson learned from broken, twisted hardware. Learn those lessons, and respect the knowledge gained from them. Only then, see if you can write new rules that will meet those challenges.

Offline Warren Platts

Re: Basic Rocket Science Q & A
« Reply #756 on: 12/28/2012 10:30 PM »
1. Electric propulsion that relies on accelerating ions carries away a positive charge, thus charging the spacecraft with a negative charge that must be dissipated.

The exhaust beam of electric thrusters is neutralized, otherwise the positive ions would eventually halt, reverse course, and start accelerating back toward the spacecraft.

The solar wind plasma serves as a ground; thus, I would guess that any positive ions would be neutralized by solar wind electrons, leaving the spacecraft with a net negative charge--hence the need for spacecraft neutralizers. However, I'm not an expert, which is the reason I asked the question in the first place.
"When once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for there you have been, and there you will always long to return."--Leonardo Da Vinci

Offline strangequark

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Re: Basic Rocket Science Q & A
« Reply #757 on: 01/05/2013 06:25 AM »
1. Electric propulsion that relies on accelerating ions carries away a positive charge, thus charging the spacecraft with a negative charge that must be dissipated.

The exhaust beam of electric thrusters is neutralized, otherwise the positive ions would eventually halt, reverse course, and start accelerating back toward the spacecraft.

The solar wind plasma serves as a ground; thus, I would guess that any positive ions would be neutralized by solar wind electrons, leaving the spacecraft with a net negative charge--hence the need for spacecraft neutralizers. However, I'm not an expert, which is the reason I asked the question in the first place.

I should have been more clear. I was making a statement of fact, based on previous work at a company which designs and manufactures gridded ion and Hall effect thrusters. A hot cathode, integral to the thruster, serves as an electron source, neutralizes the ion beam, and completes the virtual circuit. There is no reliance on in-space electron sources, and this is why the thrusters generate thrust within a ground-based vacuum chamber.

The cathodes are like the Hollow Cathodes depicted here. These can be seen in the Hall Thrusters here, at the top of the devices (often dual-redundant in the Russian designs, as shown).
« Last Edit: 01/05/2013 06:26 AM by strangequark »
Don't flippantly discount the old rules of this industry. Behind each one lies a painful lesson learned from broken, twisted hardware. Learn those lessons, and respect the knowledge gained from them. Only then, see if you can write new rules that will meet those challenges.

Offline baldusi

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Re: Basic Rocket Science Q & A
« Reply #758 on: 01/05/2013 08:55 PM »
I've been wondering about a full stage CH4/LOX engine. I always thought that on a FRSC version, you could use an expander cycle to get as many starts as you'd want. But that made me think, can you make a FSC with a single shaft turbopump? Can the turbopump have two turbines on the same shaft?
So you can start it up on the expander cycle and then transition to the power of both gas generators. Is doing two separated turbopumps more efficient?
I understand that in the H2/LOX case it might get different given the widely different volume, density and specific heat, right?

Offline Christian La Fleur

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Re: Basic Rocket Science Q & A
« Reply #759 on: 03/04/2013 05:40 AM »
Our planet rotates on itself and revolves around the sun.  If we follow the idea that our galaxy is also in motion, are we not in reality moving at tremendous amounts of speed?.......If we were to create a model of our galaxy and present it in a classroom (for the naked eye)...and we were to move the model over even a fraction of a sliver every minute or hour, this movement would represent millions of kilometers per minute or per hour.......

Thumbs up or Thumbs down........