SpaceX has announced an upgrade to their Falcon 9 rocket, achieving greater thrusts with, as far as I can tell, the same rocket motors (at least, they apparently still call the motors "Merlin 1D"). See this question, which had no conclusive answer on engine changes. The change takes the Merlin 1D from 147K lb thrust at sea level to 170K lb, an increase of 15.6%.

The increased thrust is apparently due to switching to using extremely chilled fuel (RP-1 kerosene) and oxygen. Not just liquid, but barely above the temperature at which it would freeze solid. This increases the density of the propellants, presumably allowing more mass of fuel to be contained in the tanks and increasing the flow rate (mass per unit time) through the turbopumps and rocket nozzles.

My question is, why are they only doing this now? 15% more thrust from the same engines, and increased fuel capacity (even though it also increases launch weight), seem like highly desirable changes. What are the risks of this approach that led to it not being used previously?

A few guesses:

  • Is the change in propellant temperature sufficient to significantly increase the risk of failures in components that must handle cryogenic fluids?
  • Is it significantly harder to build spacecraft-usable tanks that can hold near-frozen O2 (and RP-1), instead of "merely" liquid O2?
  • Does increasing the mass/time flow rate through the motors (assuming that's actually what's happening) pose a greater risk to the motors? If so, does the fact that the motors can take it imply they were overbuilt for the old propellant density? (Merlin 1D already had the highest T/WR of any production liquid-fueled rocket motor.)
  • Are there other changes that are required to support this upgrade, and was engineering those changes difficult or very costly?

While I'm obviously hoping to get as full a picture as possible, feel free to post partial answers (i.e. one thing you know of that poses a significant challenge) as answers rather than comments.

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    $\begingroup$ I'm no expert on the matter but I've read in several documents discussing propellant density that preventing slush blocking the flow at near freezing point is quite challenging. RP-1 is basically liquid paraffin so I can see how that would be really difficult to do and that you'd need certain level of confidence in the system and that it can handle it before you're willing to give it a go. Maybe it's like producing fine wine and such levels of optimization simply need their time to mature? $\endgroup$
    – TildalWave
    Dec 11, 2015 at 1:37
  • $\begingroup$ Oh yeah, I can see how it would be a huge problem if there was a risk of the propellants solidifying at any point in the system. Preventing that (or possibly handling a certain amount of it?) would be important, though I don't know how hard it would be to control the temperature and pressure precisely enough to avoid that risk. $\endgroup$
    – CBHacking
    Dec 11, 2015 at 1:45
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    $\begingroup$ Don't have the full picture, so posting as a comment...This paper implies that just producing the stuff is significantly more difficult (required new facilities) than the usual propellants. However this is mostly focused on LH2. ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20000033847.pdf $\endgroup$ Dec 11, 2015 at 1:48

5 Answers 5


The Merlin 1D's are claimed to have been able to run at the Falcon 9 1.1 Full Thurst level all along. However to get them into initial use on the Falcon 9 1.1 earlier, they were initially certified for only 85% of the design maximum.

Thus this evolution is just to run them at full settings. If I understand it correctly, the densified propellant and oxidizer is not to achieve the extra thrust but rather to take advantage of it, with minimal tank stretches.

The current first stage is reported to be at the very limits of simple trucking on the US Interstate system. They need to be able to legally and safely drive it from Hawthorne, CA to McGregor, TX, and from either to Cape Caneveral, FL. Based on those roads, they are loathe to stretch the first stage much further to avoid hitting those limits.

So if you increase the thrust, you increase the flow through the engine, which means you use the fuel/oxidizer up faster, and run through the tank quicker, which means you cannot thrust as long. Densifying helps but packing more into the same sized place.

There are probably many issues with cooling the propellant, but at least one is that you have less time on the pad from fueling to launch before the fuel and oxidizer warm up too much to be able to get the needed performance out of the vehicle.

There is significantly expensive hardware needed, pretty near the launch site (Thus needing to be hardened to survive launches) to chill the fuel and oxidizer. (Running insulated pipes is hard, but doable, as NASA discovered with the LH2 lines for Saturn V).

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    $\begingroup$ Your statement about the thrust increase not being caused by the densified prop but rather changes to engine settings matches what a SpaceX fanboi friend told me. And it also makes sense to me. But my friend couldn't come up with any references for this claim, do you have any ? (Not challenging, would just like to read them). $\endgroup$ Dec 13, 2015 at 15:04

We saw one of the major drawbacks of using the condensed propellant tonight. First, the LOX and RP-1 are not loaded until last minute (-10:00) so that they heat up at a minimum. This led to tonight's scrub, I imagine. (My guess is they were ready to pull the umbilical and the LOX wasn't fully loaded.) Another big drawback we are seeing is that once the LOX/RP-1 are loaded if there is any hold to the launch, they must be unloaded and reloaded before another attempt, which takes too much time for most launch windows. This is why, when there's a halt at -1:40, they scrub the whole launch, which we saw tonight.


It is interesting that, according to SES-9 launch webcast page on Livestream, they scrubbed yesterday's launch because the propellants were not cold enough:

Launch update (25 Feb. 2016):

The Falcon 9 remains healthy in advance of SpaceX and SES’s mission to deliver the SES-9 satellite to Geostationary Transfer Orbit. Out of an abundance of caution, the team opted to hold launch for today to ensure liquid oxygen temperatures are as cold as possible in an effort to maximize performance of the vehicle. SpaceX is now targeting tomorrow, Thursday Feb. 25, at 6:46pm ET for launch of SES-9.

Probably not a serious concern in many cases, but sometimes scrubs can be a big problem for schedule and cost.

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    $\begingroup$ Thank you for the edit, and now they have scrubbed two times. Since SpaceX is trying to keep costs down, that has to be a pretty large hit if it keeps up. However, they are still pretty early in the learning curve so hopefully they will work out the bugs. Nevertheless, loading and launching in 30 minutes is very sporty. And are they going to load the astronauts first, and then the propellants? Seems kind of risky. $\endgroup$ Feb 26, 2016 at 15:05

A reason for increasing the fuel density may also be to maximize the acceleration in the early part of the flight. The time a rocket spends at low speed is very wasteful of fuel and worsens the payload to rocket weight ratio. For this reason, heavy rockets have two or more additional boosters which improve the acceleration in the first phase of the flight. Improving the power by using densified fuel helps and is mostly useful at the beginning, while the rocket is most heavy.


You don't want to carry more fuel fuel than what you need because that's just extra weight.

Landing on land however requires extra fuel (the stage has to fly back) and thus they had to make chill the fuel further so they can fit more of it inside the tank.

As for the thrust, I think geoffc is right and they are simply running the engine at higher setting. They probably had to do that to be able to carry the extra fuel weight.


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