SpaceX Falcon 9 FT - ORBCOMM-2 - Dec. 21, 2015 (Return To Flight) DISCUSSION

[Proponent]

I wonder whether Falcon 9's sub-cooled RP-1 might be the thickest liquid propellant (excluding thixotropics) used in modern rocket history.  Per the attached paper, the kinematic viscosity of RP-1 at 20 ^oF is 3.3 cP, as opposed to 1.7 cP at 70 ^oF.  I wonder whether the chilling of the RP-1 is limited by viscosity.

EDIT:  Corrected viscosity figures (they were both slightly too small, as a result of an underlying error  in the calculation of density).

[Eagandale4114]

Is there a window for today's attempt?

[MarekCyzio]

Weather detoriated significantly today - we have an upcoming cold front. It rains hard at this moment.

[Chris Bergin]

Texas: Add dust.
Florida: Add salt.

don't forget the effects of Solar radiation ie sunshine

It's more than just ambient environmental conditions.

I know it's a totally different vehicle, but think back to the issues around the GUCP. http://www.nasaspaceflight.com/tag/gucp/

Could some of the problems be with the umbilical attachments to the vehicle at the launch site that are different than Texas? For a static fire in McGregor, you can bolt the fill lines directly to the stage while in Florida these lines have to be some sort of quick disconnect that detaches at launch.

The cooled LOX is above the melting point of nitrogen (it's actually right near the boiling point of nitrogen, so maybe they're using LN2 to cool it), so I don't think they're seeing solidification of air around the very cold parts like the GUCP, but if there is water condensing then freezing around areas where it didn't freeze before, you could run into problems keeping a good seal or maintaining electrical conductivity of the umbilicals - you might not see that in McGregor if the lines were bolted.

Oh the GUCP! That brings back "fun" memories!

[Michael.Kalenty]

Texas: Add dust.
Florida: Add salt.

don't forget the effects of Solar radiation ie sunshine

It's more than just ambient environmental conditions.

I know it's a totally different vehicle, but think back to the issues around the GUCP. http://www.nasaspaceflight.com/tag/gucp/

Could some of the problems be with the umbilical attachments to the vehicle at the launch site that are different than Texas? For a static fire in McGregor, you can bolt the fill lines directly to the stage while in Florida these lines have to be some sort of quick disconnect that detaches at launch.

The cooled LOX is above the melting point of nitrogen (it's actually right near the boiling point of nitrogen, so maybe they're using LN2 to cool it), so I don't think they're seeing solidification of air around the very cold parts like the GUCP, but if there is water condensing then freezing around areas where it didn't freeze before, you could run into problems keeping a good seal or maintaining electrical conductivity of the umbilicals - you might not see that in McGregor if the lines were bolted.

Oh the GUCP! That brings back "fun" memories!

Certainly brings back memories of STS-133 for me...

[Chris Bergin]

Weather detoriated significantly today - we have an upcoming cold front. It rains hard at this moment.

A few people noting that at me now. Wonder if it becomes an issue for the preps.

[WHAP]

I did not see this tweet posted here..

@elonmusk Did you guys get #Falcon9 O2 down to -340F on the test stand at #McGregor? Is this just an integration issue at LC40?

 Elon Musk ‏@elonmusk  6h6 hours ago
@craigcocca It worked in Texas

So.. it should not be a vehicle, but a stand issue?

It could be either.  Or both.

[WHAP]

https://twitter.com/elonmusk/status/677663227271118848

Elon Musk ‏@elonmusk  1m1 minute ago
@PaigeANjax -340 F in this case. Deep cryo increases density and amplifies rocket performance. First time anyone has gone this low for O2.

Here's an interesting response to Elon's tweets:

https://twitter.com/george_sowers/status/677724984165330944

George Sowers ‏@george_sowers 4h4 hours ago

.@elonmusk Thats why we don't bother. Lots of complexity for little gain.

https://twitter.com/george_sowers/status/677730399242457089

George Sowers ‏@george_sowers 4h4 hours ago

.@rocketrepreneur @elonmusk Tough to know the conditions of LOX at liftoff.

I guess Spacex could say the same about LH2.   It is just a matter of figuring the ROI.  LH2 requires more insulation everywhere and haz gas detection, subcooled LOX requires active cooling.

I'd venture that the use of LH2 (over RP1) is a whole lot more gain than the subcooling/densification that SpaceX is pursuing.  And most of the extra effort required to handle LH2 has been known for decades - but we still "learn" some interesting things every so often (stuff we should have known).

[notsorandom]

Can they do the test fire even when the Weather is red for launch? Certainly don't want the wind blowing the rocket around on take off. If the rocket it still held down what weather concerns would there be aside from maybe lighting?

[Lars-J]

Can they do the test fire even when the Weather is red for launch? Certainly don't want the wind blowing the rocket around on take off. If the rocket it still held down what weather concerns would there be aside from maybe lighting?

The rocket is only held down by 4 points at the base. There is a limit to how much wind it can handle when not held by the erector arms.

[abaddon]

It's hardly unprecedented for it to take a little extra time to shake down new equipment at a launch site.  I feel people are getting a little silly about "crazy LOX subcooling!!!".  To be fair, Elon was being a bit silly when he tweeted about how nobody has ever gone this cold before; while technically true, it feeds into this hyperbole.  Sowers' mild trolling doesn't help either.

And please, let's not start the LH2 vs LOX thing.  Both have advantages and drawbacks.

[LouScheffer]

https://twitter.com/elonmusk/status/677663227271118848

Elon Musk ‏@elonmusk  1m1 minute ago
@PaigeANjax -340 F in this case. Deep cryo increases density and amplifies rocket performance. First time anyone has gone this low for O2.

Here's an interesting response to Elon's tweets:

https://twitter.com/george_sowers/status/677724984165330944

George Sowers ‏@george_sowers 4h4 hours ago

.@elonmusk Thats why we don't bother. Lots of complexity for little gain.

I've wondered about this as well.  If you want 6-10% more fuel, why not make the rocket 6-10% longer?  There are several possibilities:

(a) The rocket is already long and thin.  Longer makes the bending moments and modes worse, maybe requiring other changes.
(b) Added mass of longer tanks.  This seems pretty small.
(c) There might be volume constraints to pumping through the engine.  So perhaps  you need the higher density to get higher thrust.
(d) Don't want to fiddle with the attachment points, transporter, umbilicals, etc. yet again
(e) Transportation might be harder (road transport, fitting under bridges, etc.)

At least the additional complexity is on the ground side, and not on the rocket itself.

[Proponent]

I guess Spacex could say the same about LH2.   It is just a matter of figuring the ROI.  LH2 requires more insulation everywhere and haz gas detection, subcooled LOX requires active cooling.

I'd venture that the use of LH2 (over RP1) is a whole lot more gain than the subcooling/densification that SpaceX is pursuing.  And most of the extra effort required to handle LH2 has been known for decades - but we still "learn" some interesting things every so often (stuff we should have known).

I think that's likely true, but I can see SpaceX's rationale: developing an LH2 engine would have been an expensive undertaking for a fledgling commercially-oriented rocket company.  ULA's hydrogen upper-stage engine, in contrast, was originally developed during the cost-plus era.  Given their respective origins, it makes sense that SpaceX densifies lox and RP-1 while ULA uses hydrogen.

[MarekCyzio]

It could be either.  Or both.

Or none. Maybe tweaking various parameters just takes more time than planned.

[RedLineTrain]

And please, let's not start the LH2 vs LOX thing.  Both have advantages and drawbacks.

At risk of bringing this up again, is there a temperature between -340F (F9 LOX) and -434F (hydrogen) where handling of liquids goes from relatively easy to relatively challenging?

[davey142]

And please, let's not start the LH2 vs LOX thing.  Both have advantages and drawbacks.

At risk of bringing this up again, is there a temperature between -340F (F9 LOX) and -434F (hydrogen) where handling of liquids goes from relatively easy to relatively challenging?

And the fact that Hydrogen atoms are small. It's the same thing that makes handling Helium challenging.

[Lars-J]

And please, let's not start the LH2 vs LOX thing.  Both have advantages and drawbacks.

At risk of bringing this up again, is there a temperature between -340F (F9 LOX) and -434F (hydrogen) where handling of liquids goes from relatively easy to relatively challenging?

And the fact that Hydrogen atoms are small. It's the same thing that makes handling Helium challenging.

Again, can we keep the LH2 vs KeroLox talk in a more appropriate thread?

[Lar]

Everything ties together of course, but I agree, let's try to stay focused. Stuff about why LH2 is hard to handle probably is not on topic for a RTF thread. Thanks!

[rkoenn]

LH2 engines are a big step up in difficulty to design and build.  Kerosene fueled engines are much easier to do but plenty difficult in their own right.  Also LH2 propellant is far less dense which means you need larger tankage.  LH2 fueled engines do give much higher performance but between the engine design and manufacturing difficulties and the larger tanks required kerosene is an easier fuel to work with.  If this new version Falcon has the tanks sized, which I assume it has, for the higher density LO2 then they have to put that in the rocket.  The propellant ratios would be sized for that and if they don't get the higher density during the engine burn they will run out of LO2 before kerosene which of course causes numerous problems primarily not getting to orbit.

[Kabloona]

LH2 and detailed discussions of deep cryolox are off topic. Please take those discussions to this new thread:

http://forum.nasaspaceflight.com/index.php?topic=39072.0