Apollo 17 ascent module liftoff

[JayUtah]

I'd presume this is done in software by the PGNS or AGS, but there's a 400 ms timer in the CES, started by pushing the ABORT STAGE button, that arms the ascent engine if it isn't already armed...

400 ms is adequate to account for DPS thrust decay, for what it's worth.

[ka9q]

I'm still trying to find the checklist procedures for an abort late in descent, i.e., one that requires staging. They seem surprisingly hard to find.

[Bob B.]

Sorry for reviving this old thread.  But since my web article has been used by the conspiracy theorists in support of their craziness, I thought it was necessary to address it.  The following was added to my web page.  I'm posting it here only because I want to make sure that what I've said is correct.  If any one sees anything that is wrong, or that I could have said better, please let me know.  Thanks.

Launch Pad Acceleration (Added 25-March-2016)

Since publishing this article in 2009, some conspiracy theorists have used it to claim that the record video of Apollo 17's launch from the surface of the moon is not authentic. The argument made is that the lunar module is seen to rise off the launch pad with greater acceleration than suggested by the results of this simulation. That is entirely true, but instead of asking why it is true, the conspiracists jump immediately to the explanation they want to be true, i.e. the video is fake. A competent investigator would research other possible explanations for the observation that don't require extraordinary claims, but that's not what we get from conspiracy theorists. A conspiracist is not interested in seeking the truth, his objective is to find any evidence that seemingly supports his belief while ignoring or rejecting anything that contradicts it.

It should be stated that the objective of this simulation was not to precisely recreate the behavior of the lunar module at the moment of liftoff. The goal of the exercise was to see if the LM could attain lunar orbit given its reported mass and propellant load. Any momentary extra "push" given to the LM at liftoff would have inconsequential effects on the final outcome of the simulation; therefore, no attempt was made to simulate the exact liftoff conditions. The opening sequence of the simulation does not compare to the real-life launch in terms of velocity and altitude versus time, thus it is expressly unsuitable for the type of analysis performed by the conspiracy theorists. (It is telling that no conspiracy theorist has ever asked me about it.)

So why does the lunar module rise off its launch platform at a seemingly greater than expected acceleration? There are two main contributing factors that come into play.

First, when a rocket engine is fired, there is a brief period, called the ignition transient, during which extreme conditions can occur, such as high pressure and temperature peaks. For the LM's ascent engine, the ignition transient lasted for about 350 milliseconds, during which stronger than normal thrust was produced.

The second factor can be seen in the illustration to the right-bottom. Notice that the exit of the ascent engine nozzle sat tight against the upper deck of the descent stage. On start-up, the gas pressure at the nozzle exit rose to higher than normal values due to the constricted flow of exhaust gas. This produced a high degree of transient pressure thrust just at the moment of liftoff. Once the LM climbed high enough that the exhaust could flow from the nozzle unrestricted, the pressure and thrust fell to nominal levels.

These factors combined to give the ascent stage a brief but significant spike in thrust immediately after engine ignition. This extra "kick" caused the LM to jump off the launch platform, attaining greater altitude and speed within the first second of flight than otherwise possible, and producing the faster than expected initial climbout observed in the Apollo 17 video. As explained, this was only a transitory condition, after which the LM's acceleration was consistent with the steady-state operation of its ascent engine.

Also be advised that time measurements made from Internet-posted videos should be considered suspect. Most of these videos have gone through format and framerate conversions of unknown type and origin. These manipulations can change the playback speed, rendering the videos unreliable for making time and velocity measurements.

http://www.braeunig.us/apollo/LM-ascent.htm

[bknight]

Bob in your simulation you have the dry mass being reduced, what is effecting the dry mass?  Other than that the addition does a good job in describing the situation for normal rational people, I'm not sure that it would satisfy the hunger cravings of the hoaxers.

[Bob B.]

Bob in your simulation you have the dry mass being reduced, what is effecting the dry mass?

The mission reports I used gave the ascent stage's mass at launch, its mass at orbit insertion, and the mass of propellant used.  Subtracting the propellant mass from the launch mass, I found that the ascent stage's dry mass at launch was 9.8 lbm more than at orbit insertion.  I could not find an explanation for the difference, though I'm guessing it might be RCS propellant.  To account for it in the simulation, I just progressively lowered the dry mass during ascent.

Other than that the addition does a good job in describing the situation for normal rational people, I'm not sure that it would satisfy the hunger cravings of the hoaxers.

I know I'll never satisfy the conspiracy theorists, those guys are loons.  My goal is twofold: (1) supply an explanation for the rational people, and (2) get it on record that my simulation incorrectly models to first few seconds of flight.

[bknight]

FWIW, I didn't study the lift-off video to closely identify any anomalies, in vertical height.  More to what hunchbacked was trying to describe in his video, concerning the gyrations  caused by the RCS and camera angle  .

[ka9q]

The mission reports I used gave the ascent stage's mass at launch, its mass at orbit insertion, and the mass of propellant used.  Subtracting the propellant mass from the launch mass, I found that the ascent stage's dry mass at launch was 9.8 lbm more than at orbit insertion.  I could not find an explanation for the difference, though I'm guessing it might be RCS propellant.

RCS propellant should be separately accounted for.

Another possibility is cooling water exhausted through the sublimator. The LM is fully powered up during ascent, so its cooling system should be going full blast.