This answer to the question Apollo-11 delta-v “due to residual pressure in the docking tunnel pushing the spacecraft apart” detected as it happened? Was it corrected? quotes from the Apollo 11 Mission Report; MSC-00171. Here is a longer quote from Section 7.8 LUNAR ORBIT NAVIGATION:

The coordinates obtained from the landmark tracking during revolution 12 deviated from the best preflight estimate of the center of the landing site ellipse by 0.097 degree north , 0.0147 degree east , and 0.038 mile below . These errors are attributed to the R2 potential model deficiencies. The large difference in latitude resulted from an error in the spacecraft state vector estimate of the orbit plane ; these were the data used to generate the sighting angles. The difference in longitude could also have been caused by an error in the estimated state vector or from tracking the wrong landmark .

The third problem area was the large number of trajectory perturbation in revolutions 11 through 13 because of uncoupled attitude maneuvers, such as hot firing tests of the lunar module thrusters, undocking impulse, station-keeping activity, sublimator operation and possibly tunnel and cabin venting. The net effect of these perturbations was a sizeable downrange miss.

A comparison between the lunar landing point coordinates generated from various data sources is presented in table 5-IV. The difference, or miss distance , was 0.0444 degree south and 0.2199 degree east, or approximately 4440 and 21 990 feet, respectively. The miss in latitude was caused by neglecting the two-revolution orbit plane propagation error , and the miss in longitude resulted from the trajectory perturbations during revolutions 11 through 13.

Question: How much could "sublimator operation" (second paragraph) have possibly caused a trajectory perturbation large enough to even be considered as a contributing factor to Apollo 11's trajectory error and "sizable downrange miss"?

Sublimation of water at least, is the conversion of solid ice to gas in a low vapor pressure environment such as space or a home freezer. It removes heat and is used to cool space suit coolant; see How have space suits dissipated the heat removed from astronauts? Roughly how much thrust and delta-v could this have possibly produced? Is it anywhere near large enough to have made a difference?

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    $\begingroup$ Note that the operation of the LM sublimator did put Apollo 13 off course during the trans-Earth return phase, requiring midcourse corrections to keep the spacecraft in the proper reentry corridor and apparently puzzling controllers on the ground -- the LM, of course, wasn't normally present for trans-Earth flight. $\endgroup$ Jul 22, 2019 at 19:32
  • $\begingroup$ @RussellBorogove if there is a source with some numbers or a calculation, that might make for a good answer. (I thought it was just the missing moon rocks, but all I know is what I learned in the movie) $\endgroup$
    – uhoh
    Jul 22, 2019 at 23:43
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    $\begingroup$ I think "missing moon rocks" was easier to explain than sublimator thrust. They also changed an amp-hour constraint to an amp constraint, Shepard's out-of-practice-ness due to being off flight status for years while struggling with Menière's disease to an ear infection, etc., etc. $\endgroup$ Jul 22, 2019 at 23:48
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    $\begingroup$ At just 1 m/s difference prograde, a lunar orbit is already lagging 12 seconds more behind every orbit. At one and a half km/s, that adds up to quite a lot of distance after a short while. $\endgroup$ Jan 28, 2020 at 8:58
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    $\begingroup$ When the ISS was a lot smaller, thrust force from the sublimators on the Russian EVA suits would cause its attitude control system to lose control of the station. So it's not completely crazy. ieeexplore.ieee.org/document/1656153 $\endgroup$ Jan 28, 2020 at 16:02

1 Answer 1


Frank O'Brien in his book "The Apollo Guidance Computer: Architecture and Operation", on p. 229 concludes that effect from sublimator vapour thrusting could've been measurable:

A quite real source [of unaccounted in state vector translation] arises with the exhaust from the LM sublimator, used to cool the vehicle's electronics. Although producing the faintest amount of thrust, it also operates for hours at a time and can build up a measurable error - as evident in Apollo 11's descend trajectory.

and refers to a memo "Vent Bent, Descent Lament", written by NASA engineer who coordinated mission techniques during the Apollo program, Bill Tindall.

One of the forum.nasaspaceflight.com users kindly shared this document (along with 3 other memos related to the subject, see links at the bottom) on this webpage.

Tindall identifies the sublimator (he calls it "water boiler") venting as one of "unacceptable" factors:

Until now, this characteristic [unwanted maneuvers of the spacecraft] has been primarily an annoyance, but when it comes to attempting pin point landing on the lunar surface it can really tear us up! I would like to recommend that your CCB initiate action with both spacecraft [presumably CSM and LM] contractors to identify all vent and dump sources and initiate well engineered fixes to the unacceptable ones to make them non-propulsive even if it costs some money. This request particularly applies to the LM water boiler.

In another memo ("A lengthy status report on lunar point landing") he is elaborating on changes to be implemented after Apollo-11 mission:

...undocking earlier eliminates all the rest of the known ones [perturbations to the LM trajectory] except that darned LM water boiler venting (we must leave fixing this to the CCB)...

And, finally, in memo "Apollo 12 descent - Final comments, Rev A" Tindall mentions that a dedicated test was performed to measure the thrust from the sublimator and provides a ballpark figure for the effect it would take on LM trajectory:

For one reason or another, GAC has made a precise measurement of the LM water boiler thrust level. According to Ron Kubicki, the results of their tests will be added to the data book. The preliminary estimate of the effect on the PDI state vector, if the venting is ignored in the RTCC orbit determination and integration programs, is an error in the order of 4,000 ft. in an uprange direction (i.e., short).

Unfortunately it is not clear what document the results of that test should have been reported in, and I couldn't find on internet any further information on mentioned results of this test.

In his following memo "VENTS", Tindall draws attention to the fact that effect of these propulsive vents had been underestimated until the Apollo-11 lunar landing and claims the sublimator vents to be among the major error sources:

It is evident that we did not benefit from our Gemini experience in the design of Apollo spacecraft when it came to vents... ...the water boilers in the CSM and LM now show up among the major error sources affecting our LM point-landing capability...

In the past we didn't even know about the damned things [unaccounted in state vector propulsive vents] til we started searching for the perturbative force that screwed up the flight! [most definitely meaning the Apollo-11 flight]

All these memos are dated between Apollo-11 and Apollo-12 missions.

It is interesting to note that Apollo Operations Handbook: Lunar Module, Volume I - MIT (dated 01 April, 1971, i.e. between Apollo-14 and Apollo-15 missions), on p.2.6-18 mentions presence of a thrust deflector above the sublimator vapour duct exit port:

The sublimator decreases the temperature of the coolant by rejecting heat to space through the sublimation of water, followed by venting of the generated steam through an overboard duct. A thrust deflector located above the duct exit port diffuses the axhaust [sic] steam, thereby decreasing the thrust effect on the vehicle.

On a separate note, it would be interesting to find out whether the thrust deflector had been mentioned in any of LM documentation issued prior to Apollo-11 flight. If not, this might mean that an upgrade was performed to remedy the anomaly findings mentioned in Bill Tindall's notes.

Links to the memos:

  1. "Vent Bent, Descent Lament", 28 August 1969
  2. "A lengthy status report on lunar point landing", 29 August 1969
  3. "Apollo 12 descent - Final comments, Rev A", 04 November 1969
  4. "VENTS", 06 November 1969
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    $\begingroup$ (If you don't know what Tindallgrams are, see this question. They're a great little piece of NASA lore.) $\endgroup$
    – DylanSp
    Jan 29, 2020 at 13:00
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    $\begingroup$ CCB = Configuration Control Board, the group with final responsibility over spacecraft changes. GAC = Grumman Aerospace Corporation, makers of the LM. $\endgroup$
    – hobbs
    Feb 1, 2020 at 4:07

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