Author Topic: Basic Rocket Science Q & A  (Read 270860 times)

Offline ScepticMatt

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Re: Basic Rocket Science Q & A
« Reply #1020 on: 11/02/2015 07:24 AM »
Math question regarding convergent-divergent nozzles:

I understand that  area expansion during subsonic will slow the exhaust gas while it will increase it in the supersonic region.

i.e. -( 1-M^2 )dV/V = dA/A

But how does it break the sound barrier? During sonic chocked flow (M=1), how and why will changing the nozzle area affect exhaust gas velocity?

Offline SWGlassPit

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Re: Basic Rocket Science Q & A
« Reply #1021 on: 11/02/2015 04:19 PM »
Changing the nozzle area changes the combustion chamber pressure.  If the pressure upstream is too low, a shock wave will form at some point in the nozzle resulting in subsonic exit flow (and probably causing a fair amount of harm to the nozzle itself).  A wider nozzle throat results in lower upstream pressure.

Offline Dante80

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Re: Basic Rocket Science Q & A
« Reply #1022 on: 11/02/2015 05:08 PM »
Theoretically speaking, a H2/Be mixture with LOX oxidizer is shown here to give abnormally large Isp figures (something like 540 Isp vac possible).

Has something like this been tried in the past? Does it make sense for LV usage?

Another one shown is H2/Li with Fluorine as the oxidizer. Again, does it make sense?

« Last Edit: 11/02/2015 05:11 PM by Dante80 »

Offline Jim

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Re: Basic Rocket Science Q & A
« Reply #1023 on: 11/02/2015 05:19 PM »
Theoretically speaking, a H2/Be mixture with LOX oxidizer is shown here to give abnormally large Isp figures (something like 540 Isp vac possible).

Has something like this been tried in the past? Does it make sense for LV usage?

Another one shown is H2/Li with Fluorine as the oxidizer. Again, does it make sense?


There is a thread on fluorine and it is not a good idea to use it.

Offline ScepticMatt

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Re: Basic Rocket Science Q & A
« Reply #1024 on: 11/02/2015 08:21 PM »
If the pressure upstream is too low, a shock wave will form at some point in the nozzle resulting in subsonic exit flow.
Interesting. Would you mind going a bit more into the detail as for the conditions needed for this to happen (or to avoid it)?

Offline Dante80

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Re: Basic Rocket Science Q & A
« Reply #1025 on: 11/03/2015 03:36 AM »
There is a thread on fluorine and it is not a good idea to use it.

Thanks for the hint. It got me to this, and I'm really enjoying reading it. Cheers..C:

a random excerpt..
Quote
(....)  But then Pino, in 1949, made a discovery that can fairly be described as revolting. He discovered that butyl mercaptan was very rapidly hypergolic with mixed acid. This naturally delighted Standard of California, whose crudes contained large quantities of mercaptans and sulfides which had to be removed in order to make their gasoline socially acceptable. So they had drums and drums of mixed butyl mercaptans, and no use for it. If they could only sell it for rocket fuel life would indeed be beautiful.

Well, it had two virtues, or maybe three. It was hypergolic with mixed acid, and it had a rather high density for a fuel. And it wasn't corrosive. But its performance was below that of a straight hydrocarbon, and its odor ! Well, its odor was something to consider.

Intense, pervasive and penetrating, and resembling the stink of an enraged skunk, but surpassing, by far, the best efforts of the most vigorous specimen of Mephitis mephitis. It also clings to the clothes and the skin. But rocketeers are a hardy breed, and the stuff was duly and successfully fired, although it is rumored that certain rocket mechanics were excluded from their car pools and had to run behind. Ten years after it was fired at the Naval Air Rocket Test Station NARTS the odor was still noticeable around the test areas.

California Research had an extremely posh laboratory at Richmond, on San Francisco Bay, and that was where Pino started his investigations. But when he started working on the mercaptans, he and his accomplices were exiled to a wooden shack out in the boondocks at least two hundred yards from the main building.

Undeterred and unrepentant, he continued his noisome endeavors, but it is very much worth noting that their emphasis had changed. His next candidates were not petroleum by-products, nor were they chemicals which were commercially available. They were synthesized by his own crew, specifically for fuels. Here, at the very beginning of the 50's, the chemists started taking over from the engineers, synthesizing nc:w propellants (which were frequently entirely new compounds) to order, instead of being content with items off the shelf.

Anyhow, he came up with the ethyl mercaptal of acetaldehyde and the ethyl mercaptol of acetone, with the skeleton structures:



respectively. The odor of these was not so much skunk-like as garlicky, the epitome and concentrate of all the back doors of all the bad Greek restaurants in all the world.

And finally he surpassed himself with something that had a dimethylamino group attached to a mercaptan sulfur, and whose odor can't, with all the resources of the English language, even be described. It also drew flies. This was too much, even for Pino and his unregenerate crew, and they banished it to a hole in the ground another two hundred yards farther out into the tule marshes.

Some months later, in the dead of night, they surreptitiously consigned it to the bottom of San Francisco Bay...
« Last Edit: 11/03/2015 03:53 AM by Dante80 »

Offline Proponent

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Re: Basic Rocket Science Q & A
« Reply #1026 on: 11/03/2015 10:05 AM »
If the pressure upstream is too low, a shock wave will form at some point in the nozzle resulting in subsonic exit flow.
Interesting. Would you mind going a bit more into the detail as for the conditions needed for this to happen (or to avoid it)?

In the case of an ideal gas flowing through an ideal nozzle, the pressure drops by a factor of [(k + 1) / 2]k/(k - 1) from the nozzle entrance to the throat, k being the ratio of the specific heat at constant pressure to that at constant volume (see a textbook like Sutton's Rocket Propulsion Elements).  If the ambient pressure is higher than the pressure at the throat (the nozzle exit in a converging-only nozzle), you won't get supersonic flow.  A representative value of k is 1.2, so the chamber pressure needs to be at least 1.8 times ambient.  In a real nozzle with a non-ideal gas, the pressure would need to be a little higher.

EDIT: Clarified pressure condition, prompted by deltaV's comment, below]
« Last Edit: 11/05/2015 11:30 AM by Proponent »

Offline deltaV

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Re: Basic Rocket Science Q & A
« Reply #1027 on: 11/05/2015 05:10 AM »
If the ambient pressure is higher than the nozzle exit pressure, you won't get supersonic flow.

Sshhhhhhh. If SSME hears you it'll stop working at sea level (its nozzle exit pressure is much below atmospheric) and SLS will have to find a new engine.
« Last Edit: 11/05/2015 05:19 AM by deltaV »

Offline Proponent

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Re: Basic Rocket Science Q & A
« Reply #1028 on: 11/05/2015 11:17 AM »
My reference to nozzle exit pressure was in the context of a converging-only nozzle, which makes sense in the context of ScepticMatt's question.  I absolutely guarantee that the pressure at minimum cross-section of the SSME (ie, the throat) is much greater than atmospheric.

Offline Christian La Fleur

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Re: Basic Rocket Science Q & A
« Reply #1029 on: 11/17/2015 03:57 PM »
Is it possible to use centrifugal force/energy, to subdue, separate, control and expell atoms in a desired direction/path?

Offline deltaV

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Re: Basic Rocket Science Q & A
« Reply #1030 on: 11/19/2015 04:41 AM »
Is it possible to use centrifugal force/energy, to subdue, separate, control and expell atoms in a desired direction/path?
Your question is a bit vague, but a centrifugal pump (https://en.wikipedia.org/wiki/Centrifugal_pump) connected to a nozzle sort of fits that description.
« Last Edit: 11/19/2015 04:42 AM by deltaV »

Offline Proponent

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Re: Basic Rocket Science Q & A
« Reply #1031 on: 11/25/2015 08:36 PM »
It's been mentioned that SpaceX subcools first stage propellants on the Falcon 9 and that Antares subcools (or subcooled -- I don't know whether the new version will do the same) lox from 94 K to 78 K.

Do we know how much SpaceX subcools either propellant?  Or how much subcooling is used on any other launch vehicles?

Offline baldusi

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Re: Basic Rocket Science Q & A
« Reply #1032 on: 11/26/2015 07:44 PM »
Antares 100 cores (the ones with the NK-33) used subcooled LOX because it was required by the NK-33. Dr. Elias has stated here that he desired to do away with it, but given that the 200 cores are re-engined 100 cores, if they didn't subcool they will loss propellant mass. Given that the 300 cores (the ones designed for the RD-181) have increased propellant load, one would assume that they will keep using the subcooled LOX until the 300 cores.

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Re: Basic Rocket Science Q & A
« Reply #1033 on: 12/07/2015 09:48 PM »
Bumbling about the internet today, I saw someone say that the Antares and the Soyuz 2-1v used LOX at different temperatures (even though they use the same base NK-33 rocket engine), which made me wonder how much of a density difference the engine can handle.

Soyuz 2-1v (-192C)
Antares (196C)

However, I could not find a density chart for liquid oxygen at different temperatures. I had to dig up an online calculator to work this out and I'm not certain of its accuracy. Can anyone help me out with this?

LOX at -192 C = 736.62 kg/m3
LOX at -196 C = 979.81 kg/m3

Is this correct? It seems like an enormous leap in density.
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Offline Burninate

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Re: Basic Rocket Science Q & A
« Reply #1034 on: 12/08/2015 01:02 AM »
First reference found:

http://www.engineeringtoolbox.com/cubical-expansion-coefficients-d_1262.html

Does not list oxygen, but the highest value listed for a liquid would offer about a 1% volume increase from a 4K rise in temperature.  The lowest, water, is nearly negligible.

I'm going to bet that your references are based on differing assumptions.
« Last Edit: 12/08/2015 01:03 AM by Burninate »

Offline Burninate

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Re: Basic Rocket Science Q & A
« Reply #1035 on: 12/08/2015 01:06 AM »
http://www.apithailand.com/data.html

lists

"Coefficient of thermal Expansion of Liquid @ 25 C   0.00954 1/C"

which would indicate just under 4% expansion from 4K increase.

Liquid oxygen at 25C is under extreme high pressure and may not be representative of cryogenic liquid oxygen: coefficient of thermal expansion in real substances is not perfectly fixed over all temperatures.

Offline Burninate

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Re: Basic Rocket Science Q & A
« Reply #1036 on: 12/08/2015 01:15 AM »
http://encyclopedia.airliquide.com/encyclopedia.asp?LanguageID=11&GasID=48

lists, just to muddy the waters

Liquid density (1.013 bar at boiling point) : 1141.2 kg/m3

Offline Burninate

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Re: Basic Rocket Science Q & A
« Reply #1037 on: 12/08/2015 01:37 AM »
An exhaustive look from probably the best reference you'll find:

http://www.nist.gov/srd/upload/jpcrd423.pdf

gives densities in mol/dm^3 for a variety of pressures.

You'll probably want to pay attention to the 0.2Mpa or 0.3MPa isobar, or 30 to 45 psi, which seems to be the typical range for a launch vehicle tank.  1 atmosphere ~= 0.1MPa.

-192C is 81.15K
-196C is 77.15K
« Last Edit: 12/08/2015 03:59 AM by Burninate »

Offline strangequark

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Re: Basic Rocket Science Q & A
« Reply #1038 on: 12/08/2015 01:48 AM »
The NIST Webbook database is the best public resource I've found. Like Burninate indicates, anything NIST is pretty much gold, this is just a little more interactive.

NIST Webbook

1204 to 1185 kg/m3 for that shift, or a 1.6% difference. There's some small pressure dependence, but it's negligible at these temperatures and realistic tank pressures (~.1 kg/m3 difference for 30 vs 50 psia).
« Last Edit: 12/08/2015 01:49 AM by strangequark »
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Re: Basic Rocket Science Q & A
« Reply #1039 on: 12/08/2015 03:09 PM »
Thank you both! I'm pretty sure now that I was not using the online calculator I had found correctly.

strangequark, that page is exactly what I really needed, thank you!

A 1.6% difference in density makes a lot more sense and that must be well within the tolerances of the NK-33 engine.

For future reference, anyone discussing the merits of LOX densification, etc.

LOX at -184 C = 1141.7 kg/m3 (just below boiling point)
LOX at -192 C = 1185.4 kg/m3
LOX at -196 C = 1204.3 kg/m3
LOX at -210 C = 1268.4 kg/m3
LOX at -217 C = 1299.0 kg/m3 (just above freezing point)
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