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I was browsing through Wikipedia when I came across this infographic on the Rocketdyne F-1. It gives the engine dry weight as 18,500 pounds, and the engine burnout weight as 20,180 pounds, meaning the engine somehow gains 1,680 pounds in-flight.

enter image description here

Can anyone confirm what causes the weight discrepancy? 1,680 pounds of soot stuck to the engine bell seems excessive.

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    $\begingroup$ Could it be fluid trapped in the piping? I.e. Dry, no fuel/oxidizer. Burnout, residual fuel? 1680 lbs of fuel in something the size of teh F1, especially with the cooling rings on the main bell that might be believable. (Assume density of water, 1680 lbs=763 kilos, which is 763 litres of water. Which is 214 gallons. Not that much really. $\endgroup$ – geoffc Jun 12 '15 at 14:27
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    $\begingroup$ I make it about 798 liters of perfect LOX/RP-1 combustion reactants, so it looks like it works. It's a bit disappointing, I was hoping for some amazing insight and it's just full plumbing. I'll give it a few days and if no sparklingly amazing answers come I'll take the question down. $\endgroup$ – ForgeMonkey Jun 12 '15 at 15:00
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    $\begingroup$ Residual props are usually the answer... $\endgroup$ – Deer Hunter Jun 12 '15 at 17:31
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    $\begingroup$ I would bet this is correct. Shuttle had lots of trapped prop after MECO which was subsequently dumped. $\endgroup$ – Organic Marble Jun 12 '15 at 19:28
  • $\begingroup$ I had heard (a probably false) rumor that it was possible to reignite the shuttle engines briefly in orbit using the trapped fuel. $\endgroup$ – Joshua Feb 15 '16 at 21:10
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The difference in the engine weights is due to trapped propellant. Referring to the Saturn V Flight Manual for SA-503, Figure 2-21, we can see that the residual propellant in the 5 engines is 2160 lbs LOX and 6585 lbs RP-1. Dividing the sum of these numbers (8745) by 5 we get a per-engine trapped prop mass of 1749 which is probably close enough to the 1680 lbs/engine shown in the graphic above.

I have reproduced the appropriate table for your convenience below.

enter image description here

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The weight difference is due to residual propellants in the engine. The engine cannot run dry, mainly for two reasons.

The first reason is the bearings in the turbopumps. They need to be constantly cooled by the pumped fluid. Because this is a LOX-Kerosene Engine, the cooling is done with Kerosene, as can be seen on page 11 (p.25 of the pdf) of this document:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19750012398.pdf

The barrier seal is clearly marked and located on the oxygen side of the bearings. In Kerosene, an overheating bearing can cause all sorts of headaches, such as thermal stress and as a result little spalls that could cog something downstream.

In hydrogen-oxygen engines on the other hand, the oxygen and hydrogen pumps are on separate shafts, because the lower density of hydrogen means that a higher rotational speed is needed. In that case the LOx pump bearings are cooled by liquid oxygen and running them dry is disastrous. The pump will ignite very quickly. This happened on a test stand for the SSME bearings once. Notice the scary font on page 25 of this pdf:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100023061.pdf

The second big reason that comes to my mind is the regenerative cooling. Whatever propellants and pressure you use, the combustion temperature is by far larger than the survivable temperature of your chamber wall. In the F1, the chamber is cooled by the fuel and the nozzle extension is cooled by a film of dumped turbine exhaust.

When the engine is shut down by closing valves directly downstream of the pumps, the fuel is left stagnant in the cooling tubes because there is nothing to push it onwards.

It is possible to purge the channels using an inert gas, but then the tubes would be filled with gas, while the engine still burns, possibly causing an unclean shutdown. Probably not an explosion, since the cooling channels are filled with inert gas now, so them burning through will not be so much of a problem. However, the additional complexity is most likely not worth it, especially for a first stage.

As for the nozzle extension, the cooling requirement is not quite as high here, and it is cooled by gas to begin with. When the main valves close, the gas generator is quickly drained of propellants and the cooling of the nozzle extension stops. But since the combustion chamber is extinguished a the same time, this is not likely to cause any trouble.

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