Author Topic: SpaceX Deep Cryo LOX Discussion  (Read 88158 times)

Offline Kabloona

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SpaceX Deep Cryo LOX Discussion
« on: 12/18/2015 05:30 pm »
This is the place to discuss Spacex Falcon 9's use of deep cryo LOX and the associated difficulties and tradeoffs.

Offline RedLineTrain

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Re: SpaceX Deep Cryo LOX Discussion
« Reply #1 on: 12/18/2015 05:36 pm »
My question is:  is there a temperature between -340F (F9 LOX) and -434F (hydrogen) where handling of liquids goes from relatively easy to relatively challenging?

I know that there's the challenge of molecule size inherent in hydrogen, but am curious just about the cryogenics piece.

Offline leaflion

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Re: SpaceX Deep Cryo LOX Discussion
« Reply #2 on: 12/18/2015 05:49 pm »
My question is:  is there a temperature between -340F (F9 LOX) and -434F (hydrogen) where handling of liquids goes from relatively easy to relatively challenging?

I know that there's the challenge of molecule size inherent in hydrogen, but am curious just about the cryogenics piece.

Yes.  At LH2 temperatures the only way you can test your components is with LH2 or LH2 temp helium.  That stuff is really expensive, so you don't get to test things as much.  With LOX (even subcooled LOX), you can test with LN2 and you get the right thermal environment, and about the right sized gas molecules.  LN2 is cheap as dirt compared to LH2 or Helium.

Furthermore, LH2 rapidly becomes 2 phase LH2/really cold GH2, while subcooled LOX stays a liquid.

Offline RoboGoofers

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Re: SpaceX Deep Cryo LOX Discussion
« Reply #3 on: 12/18/2015 05:56 pm »
My question is:  is there a temperature between -340F (F9 LOX) and -434F (hydrogen) where handling of liquids goes from relatively easy to relatively challenging?

I know that there's the challenge of molecule size inherent in hydrogen, but am curious just about the cryogenics piece.

The gases in Air start to condense and freeze on un-insulated LH2 piping and fittings.

Another issue is, I would assume, that the lower temp would stress the tank structure more, reducing the number of tanking/detanking cycles.
« Last Edit: 12/18/2015 06:09 pm by RoboGoofers »

Offline Kabloona

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Re: SpaceX Deep Cryo LOX Discussion
« Reply #4 on: 12/18/2015 06:56 pm »
Cross post from Steve Pietrobon re the propellant gain from densification, for future reference:

Quote
Re: SpaceX Falcon 9 FT - ORBCOMM-2 - RTF (Return To Flight) DISCUSSION THREAD
Reply #1036 on: Today at 08:13 AM
LikeQuote
Quote from: Dante80 on Today at 01:29 AM
More importantly, given the temperatures listed above, could someone make an educated guess about the amount of fuel and oxidizer carried?

That's not an easy question! For the RP-1 density I used two sources:

http://kinetics.nist.gov/RealFuels/macccr/macccr2008/Bruno2.pdf
http://atlasbases.homestead.com/Analysis_of_RP-1_Fuel_Density_-_SAWE0323.pdf

The first report gives a density range from 0.8236 to 0.8436 kg/L at -6.7 C. The second report gives a range from 0.8176 to 0.8265 kg/L. At 21.1 C, the range for the second report is 0.7968 to 0.8056 kg/L. The 0.8056 kg/L density is close to other values I have seen, so I believe the second report is more valid with the value of 0.8256 kg/L being very close to the low end of the first report. Thus, I will assume the following values

dfl = 0.8056 kg/L (low fuel density at 21.1 C)
dfh = 0.8265 kg/L (high fuel density at -6.7 C)

For the LOX density, I used the equation given in Section 3.3 of

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

I get the following densities (pascal program attached).

dol = 1.1420 kg/L (LOX low density at -183.0 C)
doh = 1.2539 kg/L (LOX high density at -206.7 C)

The Merlin 1C mixture ratio is 2.2.

http://iacse.commercial-space.net/wp-content/uploads/2008/10/iac-08d213.pdf

malu5531 worked out a ratio of 2.36 for the Merlin 1D+. I couldn't find a source for the actual ratio.

https://forum.nasaspaceflight.com/index.php?topic=32983.0

Assuming a ratio of 2.2 we get

dl = 1.0102 kg/L (low density)
dh = 1.0795 kg/L (high density)

For a ratio of 2.36 we get

dl = 1.0158 kg/L (low density)
dh = 1.0867 kg/L (high density)

That gives increases of 6.86% and 6.98%, respectively. According to

http://spaceflight101.com/spacerockets/falcon-9-v1-1-f9r/

the v1.1 first stage propellant mass is 395.7 t. Thus, the increase in propellant mass could range from 27.1 to 27.6 t.

The second stage was already reported to have a 10% increase in propellant volume.

http://www.spacelaunchreport.com/falcon9ft.html

The second stage v1.1 has a propellant mass of 92.67 t. Thus, the increase would be from 16.25 to 16.38 t.
« Last Edit: 12/18/2015 06:59 pm by Kabloona »

Offline CyndyC

Re: SpaceX Deep Cryo LOX Discussion
« Reply #5 on: 12/18/2015 07:43 pm »
Everyone has said a lot about the technical aspects, but what about the dangers involved? What kind of dangers are there to densifying LOX and flying with it, or even just testing it?
"Either lead, follow, or get out of the way." -- quote of debatable origin tweeted by Ted Turner and previously seen on his desk

Offline Jim

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Re: SpaceX Deep Cryo LOX Discussion
« Reply #6 on: 12/18/2015 07:46 pm »
No different than LOX itself, it is just a little colder

Offline jongoff

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Re: SpaceX Deep Cryo LOX Discussion
« Reply #7 on: 12/18/2015 09:23 pm »
My question is:  is there a temperature between -340F (F9 LOX) and -434F (hydrogen) where handling of liquids goes from relatively easy to relatively challenging?

I know that there's the challenge of molecule size inherent in hydrogen, but am curious just about the cryogenics piece.

-340F is cold enough to liquify both Nitrogen and Oxygen in the atmosphere. I thought that was one of the big headaches of LH2 was that you had to deal with Liquid oxygen forming on the tanks and such.

~Jon

Offline jongoff

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Re: SpaceX Deep Cryo LOX Discussion
« Reply #8 on: 12/18/2015 09:35 pm »
No different than LOX itself, it is just a little colder

When subcooled LOX warms it expands instead of vaporizes. For normal LOX, you just have a vent valve to let off excess pressure as it builds up. But with subcooled LOX you either end up venting liquid or could get an overpressure. Though I'm sure they've thought of this and designed and built the stage to be able to safely deal with issues like that.

I do wonder though if in the end they'll find out that in chasing performance they've made their vehicle a lot more finicky, and less likely to launch on time. We'll see as time goes on--these issues they're working through today may be easy ones to solve and then they're back to business as usual (my hope), or they may learn the hard way that the propellant densification trick is more hassle than it's worth... Good thing SpaceX is good at adapting!

~Jon

Offline Steven Pietrobon

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Re: SpaceX Deep Cryo LOX Discussion
« Reply #9 on: 12/19/2015 12:32 am »
I've updated my LOX program to also calculate RP-1 density with temperature and calculate average propellant density given the mixture ratio. Some results below.

LOX temperature in Centigrade: -183
LOX Density = 1.1422 kg/L
RP-1 temperature in Centigrade: 21.1
RP-1 Density = 0.8057 kg/L
LOX to RP-1 mixture ratio: 2.2
Propellant Density = 1.0103 kg/L
LOX to RP-1 mixture ratio: 2.36
Propellant Density = 1.0159 kg/L

LOX temperature in Centigrade: -206.7
LOX Density = 1.2541 kg/L
RP-1 temperature in Centigrade: -6.7
RP-1 Density = 0.8265 kg/L
LOX to RP-1 mixture ratio: 2.2
Propellant Density = 1.0796 kg/L
LOX to RP-1 mixture ratio: 2.36
Propellant Density = 1.0868 kg/L

The NK-33 engine also uses subcooled propellants. From

http://lpre.de/resources/articles/AIAA-1998-3361.pdf

we have

LOX temperature in Centigrade: -190
LOX Density = 1.1764 kg/L
RP-1 temperature in Centigrade: -34.4
RP-1 Density = 0.8473 kg/L
LOX to RP-1 mixture ratio: 2.59
Propellant Density = 1.0615 kg/L
« Last Edit: 12/19/2015 12:36 am by Steven Pietrobon »
Akin's Laws of Spacecraft Design #1:  Engineering is done with numbers.  Analysis without numbers is only an opinion.

Offline Robotbeat

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Re: SpaceX Deep Cryo LOX Discussion
« Reply #10 on: 12/19/2015 12:41 am »
The propellant densification is pretty darned important. Just for the first stage, the prop densification roughly provides enough performance to pay for the weight penalty of the legs.

"Amateurs obsess over Isp. Professionals obsess over mass fraction."
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Offline jongoff

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Re: SpaceX Deep Cryo LOX Discussion
« Reply #11 on: 12/19/2015 12:52 am »
The propellant densification is pretty darned important. Just for the first stage, the prop densification roughly provides enough performance to pay for the weight penalty of the legs.

"Amateurs obsess over Isp. Professionals obsess over mass fraction."

I think that obsessing over either type of performance if it ends up making operations a ton more challenging may be foolish. Hopefully I'm wrong about how much of a headache it is.

~Jon

Offline abaddon

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Re: SpaceX Deep Cryo LOX Discussion
« Reply #12 on: 12/19/2015 01:13 am »
Someone seems obsessed here alright.

Offline Lee Jay

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Re: SpaceX Deep Cryo LOX Discussion
« Reply #13 on: 12/19/2015 01:39 am »
Someone help me.

I remember reading that, in some cases, subcooled LOX can make things easier because some engine components (I'm remembering bearings but not sure) have a tough time with boiling LOX.  Normally, I think this is dealt with by pressurizing the LOX to keep it from boiling but subcooling adds margin against boiling.

Offline Mike_1179

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Re: SpaceX Deep Cryo LOX Discussion
« Reply #14 on: 12/19/2015 01:49 am »
Someone help me.

I remember reading that, in some cases, subcooled LOX can make things easier because some engine components (I'm remembering bearings but not sure) have a tough time with boiling LOX.  Normally, I think this is dealt with by pressurizing the LOX to keep it from boiling but subcooling adds margin against boiling.

A turbopump would have a pretty bad day if it was running LOX and GOX came through all of a sudden. GOX is a lot easier to pump, so the turbine would spin up pretty quickly and destroy itself (along with stuff around it). But if you're getting GOX into your turbopump, you're doing something really wrong.

Now to be fair, since sub-cooled LOX has a higher viscosity, the turbopump would run at a slower speed than if it was warmer. Slower speed means it's easier on the bearings or longer life, which a benefit if you want to re-use parts.

One other benefit is less LOX is wasted chilling down the second stage engine; first stage chilldown LOX can be replenished, but second stage LOX can't since chilldown happens during ascent. Colder LOX means less is needed to get the engine down to temperature (unless it needs to get to a lower temperature to run subcooled LOX)
« Last Edit: 12/19/2015 01:52 am by Mike_1179 »

Offline Robotbeat

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Re: SpaceX Deep Cryo LOX Discussion
« Reply #15 on: 12/19/2015 02:19 am »
The propellant densification is pretty darned important. Just for the first stage, the prop densification roughly provides enough performance to pay for the weight penalty of the legs.

"Amateurs obsess over Isp. Professionals obsess over mass fraction."

I think that obsessing over either type of performance if it ends up making operations a ton more challenging may be foolish. Hopefully I'm wrong about how much of a headache it is.

~Jon
It's true that you don't want to destroy your operations. But mass fraction is one of the few areas that you can still dramatically improve on a launch vehicle given modern materials and manufacturing techniques as well as a little Gary Hudson "cleverness."
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Online LouScheffer

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Re: SpaceX Deep Cryo LOX Discussion
« Reply #16 on: 12/19/2015 04:03 am »

I think that obsessing over either type of performance if it ends up making operations a ton more challenging may be foolish.
I don't see why this should be a lot of hassle in the long term.  You keep a tank of cold LOX and a tank of cold RP-1 near the pad.  The LOX tank is already vacuum insulated and the RP-1 tank is only -8 C, which requires no exotic insulation.  Keeping the tanks cold is well developed industrial technology, and with no weight limit it can be redundant.  So it's hard to see launches scrubbed or delayed because of insufficient cooled fuels.

Filling the rocket should be little different once you recalibrate and adjust your pumps/valves/pre-chill/etc for the lower temperatures.  Keeping it cold during a long launch hold might involve pumping the warm liquid from the top of the tank and refilling with cold liquid from the bottom.  As long as you have a big enough supply of cold liquid this does not seem particularly difficult (presuming the booster is plumbed for this, which I'd assume it is).

You now have to worry about water/ice condensing on your RP-1 handling equipment.  But this has always been true of LOX, so I suspect this is stock technology.

So what kind of long-term hassles do people foresee?




Offline woods170

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Re: SpaceX Deep Cryo LOX Discussion
« Reply #17 on: 12/19/2015 10:29 am »
Going back to the subject of the thread title: valve stiction becomes a bigger issue using deep cryo LOX. One of Elon's tweets provides a clue in that direction.

Offline Jim

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Re: SpaceX Deep Cryo LOX Discussion
« Reply #18 on: 12/19/2015 11:06 am »

  Keeping it cold during a long launch hold might involve pumping the warm liquid from the top of the tank and refilling with cold liquid from the bottom.


There are no umbilical/fill and drain valves at the top of the tanks.

Offline Semmel

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Re: SpaceX Deep Cryo LOX Discussion
« Reply #19 on: 12/19/2015 11:10 am »

  Keeping it cold during a long launch hold might involve pumping the warm liquid from the top of the tank and refilling with cold liquid from the bottom.


There are no umbilical/fill and drain valves at the top of the tanks.

Ok, I am at a loss now. How do they keep the LOX cold then? They must replace LOX to keep it below boiling temperature, venting GOX is not enough for that. Or maybe I have that part wrong in which case, please enlighten me.

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