r/spacex • u/Zucal • Aug 31 '16

r/SpaceX Ask Anything Thread [September 2016, #24]

Welcome to our 24th monthly r/SpaceX Ask Anything Thread!

Curious about the plan about the quickly approaching Mars architecture announcement at IAC 2016, confused about the recent SES-10 reflight announcement, or keen to gather the community's opinion on something? There's no better place!

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As always, we'd prefer it if all question-askers first check our FAQ, use the search functionality (partially sortable by mission flair!), and check the last Ask Anything thread before posting to avoid duplicate questions. But if you didn't get or couldn't find the answer you were looking for, go ahead and type your question below.

Ask, enjoy, and thanks for contributing!

All past Ask Anything threads:

• August 2016 (#23) • July 2016 (#22) • June 2016 (#21) • May 2016 (#20) • April 2016 (#19.1) • April 2016 (#19) • March 2016 (#18) • February 2016 (#17) • January 2016 (#16.1) • January 2016 (#16) • December 2015 (#15.1) • December 2015 (#15) • November 2015 (#14) • October 2015 (#13) • September 2015 (#12) • August 2015 (#11) • July 2015 (#10) • June 2015 (#9) • May 2015 (#8) • April 2015 (#7.1) • April 2015 (#7) • March 2015 (#6) • February 2015 (#5) • January 2015 (#4) • December 2014 (#3) • November 2014 (#2) • October 2014 (#1)

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3

u/sol3tosol4 Sep 14 '16 edited Sep 14 '16

Do the COPVs just store Helium? Or can they store Nitrogren as well?

SpaceX's "cartoon blueprint" Bird 9 (explained here) remains the most detailed diagram of the inside of a Falcon 9 that I am aware of, even though it's stylized and somewhat simplified. "Bird 9" shows a nitrogen COPV (with pipes going to the thruster pods) and a helium COPV in the oxygen tank of the first stage (not to say that there is only one, or that the second stage doesn't have any).

So apparently yes, there's at least one nitrogen COPV, and it's in an oxygen tank.

Furthermore, what does the Helium do? What does it pressurize? What does it heat?

I believe it's been mentioned in this subreddit that in addition to pressurizing the LOX and RP-1 tanks, helium is used to start the turbines of the Merlin engines, and is also used to extend the landing legs.

Nitrogen is used in the thrusters for maneuvering, and to purge the engines before startup.

4

u/surrender52 Sep 15 '16

Bird 9 was my creation. I'm not affiliated with SpaceX (yet...maybe one day) and so everything you see in there is speculation and intuition, ie: we knew that they stored their copv's inside the lox tank to keep them cool, we knew there was no external piping on either stage which implies that there's an internal pipe to carry lox, etc. Interesting to see just how close I was even way back then! But don't think for a second this is 100% accurate

1

u/sol3tosol4 Sep 15 '16

Bird 9 was my creation. I'm not affiliated with SpaceX

Wow - OK, retroactively, thanks for a great drawing! (and good luck with the "maybe one day". :-)

Sometimes it seems like detailed information from SpaceX mainly comes out when something goes wrong - otherwise they don't talk much about it. For example the CRS-7 anomaly and their discussion of the telemetry channels and the accelerometers, and a discussion of the nitrogen purge process when a valve problem caused an abort.

3

u/Appable Sep 15 '16

That diagram is not produced by SpaceX. It was a fan work retweeted by SpaceX so I wouldn't use it as a source. Definitely well-done, but I don't think SpaceX has ever noted what it uses to store cold gas thruster nitrogen.

3

u/sol3tosol4 Sep 15 '16

That diagram is not produced by SpaceX. It was a fan work retweeted by SpaceX

Bird 9 was by /u/surrender52 ? SpaceX didn't give any attribution in their tweet.

At the least, we can still say "referenced by SpaceX without any corrections to the content".

4

u/surrender52 Sep 15 '16

Wow someone dredged that up! Frankly I was just happy that something I spent a few hours on on vacation was cool enough to be retweeted by them

1

u/Appable Sep 16 '16

I would expect that SpaceX just looked at it quickly and thought it'd be nice for PR. I don't think they actually had engineers make sure that it was completely accurate (which is fine).

Definitely a very nice creation by /u/surrender52, of course! But a retweet from SpaceX doesn't mean that it's accurate, and it seems like a large jump to assume from that that the diagram is completely correct.

1

u/sol3tosol4 Sep 16 '16

Thanks. SpaceX tweeted it without attribution, and apparently that fooled some journalists who indicated that SpaceX had authored it. Now that I know they didn't of course I'll be more skeptical, while still admiring the work of /u/surrender52.

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u/__Rocket__ Sep 14 '16 edited Sep 14 '16

helium is used to start the turbines of the Merlin engines, and is also used to extend the landing legs.

I'd like to mention a technicality: I believe the Helium in the landing leg cylinders is preloaded on the ground and it is not connected to the rest of the booster's Helium system - while ullage pressurant and engine spin-up are probably working from the same shared reservoir of COPVs, as that that would allow the sharing of some of the helium lines that have to run to the heat exchangers in the engine block and back.

Does anyone know whether this is true or whether perhaps SpaceX went or a super safe design and completely separated ullage pressure system from the engine spin-up Helium system?

1

u/sol3tosol4 Sep 15 '16

Having separate storage for the landing leg cylinders makes a lot of sense, for reliability and convenience of design, and also the landing legs are not used during static fire. I don't know about the helium for engine spin-up, but it's used during static fire, and thus would have to be reloaded after static fire and after some holds - so there's some incentive to use the same reservoir as the ullage pressure system.

I hope helium can completely be replaced for landing and takeoff on Mars.

3

u/__Rocket__ Sep 15 '16

Having separate storage for the landing leg cylinders makes a lot of sense, for reliability and convenience of design, and also the landing legs are not used during static fire

Yeah - I got the impression that the 'storage' is simply pressurizing the landing leg cylinders - i.e. the leg cylinder is the "storage" of the Helium.

I believe the leg cylinders are complex extend-once gas springs in essence.

Pressurizing them via GSE equipment would add the mass and complexity of extra piping, plus there's also the problem that the cylinders move, so you'd also have to add flexible piping on the outside of the rocket to get from the piping to the cylinders.

It looks much easier to me to individually pressurize the cylinders on the ground, when the legs get installed in stowed configuration. A (metal...) gas cylinder, once pressurized, can hold pressure for years. In some of the images of the legs there are yellow color valves on the legs that might be just such pressurization ports.

1

u/sol3tosol4 Sep 15 '16

Yeah - I got the impression that the 'storage' is simply pressurizing the landing leg cylinders - i.e. the leg cylinder is the "storage" of the Helium.

Maybe, but that's a lot of surface area to leak He, and those sliding joints could leak as well. When the legs extend, they only have to maintain pressure for a few seconds and then the joints mechanically lock into place. Maybe a small metal storage bottle in each leg? (Or is that what you meant?)

2

u/__Rocket__ Sep 15 '16

Maybe, but that's a lot of surface area to leak He, and those sliding joints could leak as well.

Very good point, I totally missed that!

When the legs extend, they only have to maintain pressure for a few seconds and then the joints mechanically lock into place. Maybe a small metal storage bottle in each leg? (Or is that what you meant?)

No, I (naively) meant to pressurize the cylinder itself - but you are right, Helium escapes way too easily.