News piece: NASA supports SpaceX plan to fuel rockets with astronauts on board.

The move is criticized by many as unnecessarily dangerous.

From what I understand though, it shouldn't be necessary - Falcon 9 and Dragon 2 have enough payload capacity and delta-V to reach ISS with nominal payload, on non-supercooled oxygen. Dragon makes regular resupply runs to ISS with tons of payload, taking which would not be necessary on the manned missions. In fact most Dragon missions are volume limited and do not carry the maximum supported mass.

Are they trying to squeeze several extra kilograms of payload capacity out of the launch regardless? Are they going for 'tested and true' procedure of fueling up right before launch, resisting 'trying new things'? Or did they actually decide this is safer for the crew, to put them in a capsule equipped with LES when there's nothing that could explode yet, and wait through the entire procedure with a hand on the "abort" button, instead of risking the entire boarding procedure when the rocket is already fueled up and can explode? Something else yet?

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    $\begingroup$ Nice point about being safer to board when no fuel is there and stay with the hand on the button Vs entering after refuelling is done. In the former, you are at risk longer but you have a ready solution (unless everything explodes so fast that LES isn't enough?) $\endgroup$
    – BlueCoder
    Commented Aug 21, 2018 at 14:19
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    $\begingroup$ @BlueCoder Eh, if there is a potential explosion, the launch abort system should automatically kick in way faster than a human can react. I'm not sure what good hovering their finger over a button would do. $\endgroup$
    – TylerH
    Commented Aug 21, 2018 at 15:48
  • $\begingroup$ @TylerH: Creating cinematic tension. I discussed this on a different site, using the wording "Sitting comfortably, strapped in" and got criticized for "Comfort is not a primary concern for astronauts!" $\endgroup$
    – SF.
    Commented Aug 22, 2018 at 4:36

3 Answers 3


The Merlin-1D engines are now tuned to use the super cooled fuel and oxidizer. Thus you would be running the engines in an out of normal state, if not using it the same as all other launches with warmer propellant.

It would imply different software to handle the different performance levels.

Last thing you want to do is run things differently, if you can avoid it. Especially when people are on board.

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    $\begingroup$ The fuel/oxidizer tank size ratio is different too, isn’t it? $\endgroup$
    – jkavalik
    Commented Aug 21, 2018 at 15:14
  • $\begingroup$ @jkavalik I think my point about different software might cover that, as it would have to handle the different flow rates. $\endgroup$
    – geoffc
    Commented Aug 21, 2018 at 16:23
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    $\begingroup$ I would be really disappointed if converging guidance can't handle 20% difference in thrust. $\endgroup$
    – Joshua
    Commented Aug 21, 2018 at 16:31
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    $\begingroup$ @Joshua I am certain it can. But that is a major difference. One of the benefits of using an existing (not custom for this mission) booster is that it is common with previous usage of the booster. The more you change, the less it is common and the less the flight heritage means anything. $\endgroup$
    – geoffc
    Commented Aug 21, 2018 at 16:35
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    $\begingroup$ @jkavalik Non-optimal fuel/oxidizer mixes can actually increase thrust, at the cost of ISP. Consider the ideal gas law (imperfect but still useful) with a hydrolox engine; optimizing for P (to maximize exhaust velocity), you get P=nRT/V. R is constant, as is V for a given engine, so you get P proportional to nT. You can maximize T by optimizing ratios of H2 and O2, but you can increase n by flowing extra H2 (less efficient by mass and thus ISP). In practice, some engines intentionally run fuel-rich because the perfect ratio at the desired thrust makes them too hot. $\endgroup$
    – CBHacking
    Commented Aug 22, 2018 at 7:42

In addition to not wanting to change anything for crewed launches (as geoffc mentioned), adding crew to a fueled vehicle is a seriously dangerous move.

If you look at the AMOS-6 failure you'll see that the launch abort system probably would have saved any crew that was already on board. However, if the crew was actively being loaded, we would have lost not only the astronauts but also all of the pad crew as well - higher risk to more people.

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    $\begingroup$ The problem is not adding crew to a fueled vehicle but to a vehicle being fueled. If they didn't use supercooled fuel (which needs to be loaded as late as possible), they could simply fuel the rocket before and then load the crew afterwards. Loading the crew on an already fueled rocket is the NASA standard practice and so it was probably considered safer than what SpaceX is doing. See: spacenews.com/… $\endgroup$
    – BlueCoder
    Commented Aug 22, 2018 at 8:02
  • $\begingroup$ From the article: <<it has been the policy on all previous NASA crewed missions to fuel the rocket when the pad is cleared of personnel. “Only after the booster is fully fueled and stabilized are the few essential people allowed near it”>> I guess NASA trusted loading crew to a fully fueled and stabilized booster more than relying on LES on a booster being fueled :) $\endgroup$
    – BlueCoder
    Commented Aug 22, 2018 at 8:10
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    $\begingroup$ I like your point! Indeed having the austranauts, strapped onto the Crew Dragon and ready to abort, is probably a much safer option than having them walk into the capsule while the tank is being filled. $\endgroup$
    – pastullo
    Commented Aug 22, 2018 at 12:59

What is the primary reason for SpaceX motion to have astronauts board Dragon before fueling up the rocket?

From what I understand though, it shouldn't be necessary - Falcon 9 and Dragon 2 have enough payload capacity and delta-V to reach ISS with nominal payload, on non-supercooled oxygen.

For better performance they are switching to supercooled fuel. To maintain its temperature it needs to be loaded as late as possible while preflight checking requires the crew to be aboard well before launch. Rather than risking loading them without a safe means of escape they are loading the crew prior to oxidizer fueling. If there is a problem it is easier to escape from the top of the rocket than to escape the elevator or crew access arm. Installation photos at: "Crew Access Arm Installed for Starliner Missions.

Crew Access Arm

See: "Letter raises questions about SpaceX fueling plans and committee roles" (Nov 8 2016), by Jeff Foust:

"The break in precedent is required because of changes to the Falcon 9. The upgraded version of the rocket, introduced less than two weeks after Stafford’s original letter, uses “supercooled” liquid oxygen that is denser and provides greater performance. Maintaining those low temperatures requires fueling the rocket much closer to the scheduled launch time than is the case for other vehicles, which means that, for crewed missions, astronauts would have to board the rocket prior to fueling rather than after it is complete.

Tweet from Elon Musk and video showing a successful abort procedure.

Letter (.PDF) from Lt. Gen. Tom Stafford (Ret.) to Bill Gerstenmaier, who heads NASA’s human spaceflight operation:

December 9, 2015

Mr. William Gerstenmaier Associate Administrator for Human Exploration and Operations National Aeronautics 8nd Space Administration 300 E Street SW Washington, D.C. 20546

Dear Mr. Gerstenmaier,

We sincerely appreciated the briefing on the Commercial Crew Program from Kathy Lueders and Bill Jordan to our U.S. committee members. Thank you for making the briefing available to the committee. As is nornal when the committee begins reviewing a topic, the briefing raised about as many questions as it answered. I will not list all the topics we will continue to follow, but there is one major issue that I believe deserves your careful attention.

There is a unanimous, and strong, feeling by the committee that scheduling the crew to be on board the Dragon spacecraft prior to loading oxidizer into the rocket is contrary to booster safety criteria that has been in place for over 50 years, both in this country and internationally. Historically, neither the crew nor any other personnel have ever been allowed in or near the booster during fueling. Only after the booster is fully fueled and stabilized are the few essential people allowed near it.

Furthermore, in addition to the personnel risk, there is the risk of operating the engines outside their design input conditions. As an experienced "Prop" guy you know the problem here as well as anyone. Pump fed chemical engines require a sufficient and consistent input pressure to reduce the likelihood of cavitation or unsteady flow operations. We are concerned that there may be insufficient precooling of the tank and plumbing with the current planned oxidizer fill scenario, and without recirculatiOn there may be stratification of oxidizer temperature that will cause a variation in the input conditions to the oxidizer pump.

In summary, we are deeply concerned about introducing the practice of fueling with the crew onboard, and about the lack of even a recirculation pump for oxidizer conditioning on Falcon 9.

Sincerely, Thomas P. Stafford Lt. Gen., USAF (Ret.) Chairman NASA International Space Station Advisory Committee

  • $\begingroup$ Subcooled. "Supercooled" doesn't just mean "very cold", it means a liquid that's been cooled below its melting point without solidifying. SpaceX is subcooling their LOX to a temperature well below its boiling point, but still above its melting point. It's also not a change they're planning to make at some point: they've now been using subcooled LOX for almost 3 years, and about 2/3 of their launches so far have used it. $\endgroup$ Commented Aug 25, 2018 at 15:14

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