Apollo 11 Lunar Surface Journal

Post-Landing Fuel-Line Blockage

Commentary Copyright © 2003 by Eric M. Jones.
All rights reserved.
Last revised 1 January 2008.


103:20:52 Duke: Tranquility, Houston. We have an indication that we've frozen up the descent-fuel helium heat exchanger - and with some fuel trapped in the line between there and the valves...And the pressure we're looking at is increasing there. Over.

103:21:10 Armstrong: Roger. Understand. (Long Pause)

[Later, the NASA Public Affairs commentator tells the press that there is a small amount of fluid trapped in a line and, if the pressure had continued to increase, the worst that could have happened would have been a small leak which would then have relieved the pressure.]

[The following post-mission analysis was taken from the Apollo 11 Mission Report: "During simultaneous venting of the descent propellant and supercritical helium tanks, fuel in the fuel/helium heat exchanger was frozen by the helium flowing through the heat exchanger. Subsequent heat soakback from the descent engine caused expansion of the fuel trapped in the section of the line between the heat exchanger and the engine shutoff valve ( fig. 16-10). The result was a pressure rise in this section of line. The highest pressure in the line was probably in the range of 700 to 800 psia (interface pressure transducer range is 0 to 300 psia). The weak point in the system is the bellows links, which yield above 650 psia and fail at approximately 800 to 900 psia. Failure of the links would allow the bellows to expand and relieve the pressure without external leakage. The heat exchanger, which is located in the engine compartment, thawed within about 1/2 hour and allowed the pressure in the line to decay. On future missions, the solenoid valve (fig 16-10) will be closed prior to fuel venting and opened some time prior to lift-off. This will prevent freezing of fuel in the heat exchanger and will allow the super-critical helium tank to vent later. The helium pressure rise rate after landing is approximately 3 to 4 psi/hr and constitutes no constraint to presently planned missions. Appropriate changes to operational procedures will be made." There were no similar problems on the later flights.]

[During my conversations with Neil and Buzz in 1991, I got the impression that they did not consider this to be a serious problem. Because I couldn't see how an anomaly in the descent stage could be of concern, I didn't ask them about it. However, late in 1995 I raised the issue with Neil and he provided the following comment: "After landing, we vented both fuel and oxidizer tanks as planned. The pressure subsequently rose, probably due to evaporation of residual propellant in the tank as a consequence of the high surface temperature. Then we vented again. The ground was getting a different reading than we were, due to a different transducer location - I think theirs was in a trapped line. The worst that could happen would be a line or tank split. As we would no longer be using the descent stage, It was a less than serious problem. In summary, I wasn't worried about it."]

[My question to Neil was triggered by Journal Contributor Tom Frieling, who called my attention to a discussion in the book Chariots for Apollo by Stoff and Pelligrino, pages 166-168, which indicates that the Flight Controllers and Grumman engineers considered this to be a hazardous situation in that, if the line burst and fuel was sprayed on the still-hot descent engine, an explosion might occur. Fortunately, while the problem was being discussed, it cleared up on its own. See, also, Murray and Cox, Apollo: A Race to the Moon, pages 363-364. In an early 1996 telephone conversation, Gene Kranz confirmed that the situation was of concern, primarily because it was unexpected.]

[During the 19 September 2001 interview by Ambrose, Brinkley, and members of the Oral History Project Team, Neil was asked about the fuel line.]

[Rusnak: "Well, then I had a few specific questions. One of the gentlemen we had in here some months ago was Bob [Robert L.] Carlton, who was a flight controller, who was on the LM Control console. He was recalling the lunar landing and the events shortly thereafter where he and his colleagues were having quite a few moments of tension because there was a problem with the LM descent engine. There was a piece of ice stuck in one of the fuel lines. So they were thinking some real problems were going on. I was wondering if you could give us your perspective, what you knew sitting there in the LM, and what you knew of what was going on on the ground."]

[Armstrong: "Well, we were spring loaded to the suspicion position at that point. We recognized that right after landing, where you had to do thermal conditioning surrounding the craft, that it hadn't seen before with all that hot surface underneath it and cold on top, that there were going to be all kinds of conceivable difficulties with plumbing and valves and pressure systems, relief valves and so on. So we were ready to leave if we had to, and we were listening carefully to their instructions. But I can't remember the details of what we were thinking at that point in time."]

[Finally, in his excellent 2001 book Moon Lander, Grumman's Thomas J. Kelly discusses the situation in some detail. Briefly, the temperature of the fuel trapped in the line was increasing because of the heat of the nearby engine. "The rocket fuel was a mixture of two forms of hydrazine and, at temperatures above four hundred degrees (Fahrenheit, about 200 Celsius), it became unstable. The temperature of the trapped fuel had climbed above three hundred degrees and would reach four hundred in just a few minutes. "We all felt that the consequences of an explosion, even of the relatively small amount of fuel remaining in that short section of line, was unpredicatable and unacceptable." Discussions between Grumman, NASA, and others resulted in a decision to have the crew to "burp" the descent engine by commanding it to fire and immediately shut down by performing a "momentary flick of the manual firing button". That would briefly open the valve on the engine side of the blocked section of line and relieve the building temperature and pressure. CapCom Charlie Duke was about to pass that instruction along to the crew when the fuel frozen in the heat exchanger thawed and the problem disappeared. Charlie informs Neil and Buzz of this fact at 104:46:11.]


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