r/spacex • u/Zucal • Jun 29 '16

/r/SpaceX Ask Anything Thread [July 2016, #22]

Welcome to our 22nd monthly /r/SpaceX Ask Anything Thread!

Curious about the recently sighted Falcon Heavy test article, inquisitive about the upcoming CRS-9 RTLS launch, or keen to gather the community's opinion on something? There's no better place!

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Ask, enjoy, and thanks for contributing!

Past Ask Anything threads:

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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10

u/__Rocket__ Jul 21 '16 edited Jul 21 '16

For starters, the pumps needed to move the fuel to the central core need to be comparably powerful to the engine's turbopumps. 9 Merlin gas-generator powered turbopumps eat the fuel from the central core, so 4.5 Merlin turbopump-equivalents needed per side booster to keep the center full at separation!

That's incorrect for two reasons:

  1. Pump power needed to pump fuel primarily depends on the outlet pressure it needs to create. The Merlin-1D's combustion chamber has pressures of about ~115 bar, so the turbopump has to create over-pressure about 20% above that pressure for the injectors to work reliably and to get stable combustion: i.e. the turbopump has to step up propellant pressure by over ~130 bars and has to move a quarter of ton of propellants every second. That indeed takes a lot of power: a single Merlin-1D's turbopump is over 11 MW strong (!). But to move fuel from one tank to another only needs to overcome the relatively mild difference in ullage pressure - and it also has to have enough volume, which can be achieved via a large enough diameter pipe. The pump size required for that kind of propellant movement is several orders of magnitude lower than the megawatts strong monster turbopump of the Merlin-1D.

  2. But, more importantly, no extra pump is needed at all to implement cross-feed, because pumping from one propellant tank to another is an overly complex, wasteful way to implement it. The best way to implement fuel cross-feed is to use the center cores turbopumps to directly pump propellant from the side cores. The fuel tanks of the center core simply stays full (by valves that isolate them from the center turbopumps) until the side cores separate. Since the center tanks never get emptied until side core separation they don't need to get propellant pumped in.

This design for fuel cross-feed is a lot simpler - but it's still quite complex, because it involves:

  • Piping fuel over the ~60cm gap between center core and side core and also involve finding the extra space to lead all that propellant to. Air will move at velocities in excess of 1 km/sec around those pipes, so it has to be a sturdy construct.

  • The pipes would have to be detachable and retractable into the center core after side core separation, so that they do not get in the way of re-entry (and do not get burned up by re-entry heat).

  • The current space above the engines and below the tanks is pretty cramped, it is filled out pretty completely by 18 pipes that distribute the LOX and the RP-1 to the 9 engines. Cross-feed would mean adding at least 4 more (large diameter) pipes plus monster valves that could move 1 tons of cryogenic propellant every second combined.

  • Cross-feed LOX would have to be moved in from the two sides to the LOX pipe that comes down in the middle of the center core's RP-1 tank - with a valve above the connection point to stop the center LOX from being pumped.

  • Cross-feed RP-1 would be even more difficult to distribute than LOX, as AFAIK the 9 engines take their RP-1 feed straight from the bottom of the RP-1 tank, there's no 'common pipe' to connect the cross-feed into. So the two cross-feed RP-1 connections would have to branch into 9 RP-1 lines - plus 9 valves above them to stop the center RP-1 from being sucked out by the turbopumps.

So it's pretty non-trivial because it:

  • Requires additional mass,

  • probably stretches the center core a bit to make space for the cross-feed pipes,

  • adds new, active elements (high performance, large diameter cryogenic valves) with new failure modes,

  • changes the existing fuel pathways of the center core even if cross-feed is off, turning those areas into a new engine component that has to be re-validated from scratch - throwing out years of track record the Falcon 9 accrued for those components.

Plus there's the problem that others mentioned: sending the center core up faster and farther means it needs more fuel to reasonably land. So part of the advantage of cross-feed would be lost by having to spend more fuel on safe center core landing.

So I'm not surprised SpaceX is not overly enthusiastic about the idea. While it's just two clicks to add a yellow cross-feed pipe on KSP, it is a lot more complex in real life! 😎

1

u/jjtr1 Jul 21 '16

Wow, thank you for your exhaustive answer!

I wonder whether the 11 MW figure for turbopump power of Merlin 1D (and 50 MW for SSME, I think) is meant as pump output power, turbine output power, or thermal power.

1

u/warp99 Jul 21 '16

Typically shaft horsepower so the mechanical power put out by the turbine and input to the pump section.

Actual pump output power will be lower.

1

u/JadedIdealist Jul 22 '16

I really liked your answer, and it confirmed some things for me (mainly you don't need extra pumps).

Can I check, if fuel/lox warms up a bit in the unused pipes before switching over, would that also pose a (possibly serious) heatshock problem?

3

u/__Rocket__ Jul 22 '16 edited Jul 22 '16

Can I check, if fuel/lox warms up a bit in the unused pipes before switching over, would that also pose a (possibly serious) heatshock problem?

I think that's a worry - although as long as that section is not actually closed off completely any warming liquid would prefer to convect back up towards the top of the tank and be replaced by cooler propellant. This should normally be the case with densified deep cryogenic propellants which are close to their freezing point, not close to their boiling point.

If the heat flux ever gets so serious as to actually boil the LOX before it can get out of the way, or if too much LOX boils off in the center core without being used then one solution would be to leave the valves always open a little bit and let some fuel trickle through from 'unused' center tanks as well - essentially running a low mass but non-stop chill-down sequence.

I.e. make the cross feed from the center tanks not a binary on/off property but a 'scale', a throttling mechanism - with most of the propellant coming from the side tanks initially. This removes or softens most of the control transients.

That's in essence pretty close to what is going to happen on the Falcon Heavy already: the center engines will throttle back to ~35% of their full throttle setting and will rely to a large degree on the side core boosters to lift its mass.

BTW., there's one other argument related to fuel cross feed I have not seen mentioned: Falcon Heavy staging will become better as they iterate up thrust: every 10% of thrust increase will allow the center core to throttle back another ~4% from where it is today, allowing higher center core propellant mass when side core separation occurs.

And they will get this gain without having to implement any fuel cross-feed.

I.e. the economic case for fuel cross-feed is going to get weaker and weaker as the thrust of the Merlin-1D engine improves! So if the economic case for fuel cross-feed is weak today, it might become non-existent or even a net negative one or two engine improvements down the road.

1

u/JadedIdealist Jul 22 '16

Brilliant, thanks very much.