Apollo 17 ascent module liftoff

[HeadLikeARock]

According to the calculations on Bob Braeunig's site, the Apollo 17 ascent module should have risen 0.7 metres after the first second. If you look at the the video, a rough guesstimate is more like 2-3 metres. 

Anyone care to offer an explanation for this? My own wild guess is an initial "reverse pressure" due to the engine bell being so close to the descent stage, but I've been unable to find anything in the literature about this.

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

[JayUtah]

You get stronger-than-normal thrust during the ignition transient which, for the APS is about the first 350 milliseconds after ignition.  That can account for greater performance.  Also, don't trust your estimates too much, especially from that TV.

[HeadLikeARock]

You get stronger-than-normal thrust during the ignition transient which, for the APS is about the first 350 milliseconds after ignition.  That can account for greater performance.

Thanks for that. Done some reading up on lift-off ignition overpressure, makes perfect sense.

Also, don't trust your estimates too much, especially from that TV.

Generally I agree. If I was doing a more accurate study I've have figured my measurements more accurately: in this case, it was pretty clear that the height after 1 second was more than 0.7 metres.

Regardless, lift-off ignition overpressure seems the likely culprit! Of course, this will now enter the hoax canon as a "Clavius term"...

[ka9q]

It seems to me that the precise time of ignition may be hard to fix. There's a noticeable "clack" in the downlink at liftoff. I can't tell for sure what that is, probably the firing of the separation pyros. And I don't know when the helium pyros pressurizing the propellant tanks were fired, or if the ascent engine propellant valves were opened before, after or precisely at the same instant as the separation pyros. Some helium might be trapped in the propellant lines. And when the propellants do reach the engine, it takes a short time for ignition to occur and chamber pressure to increase.

Also, I wouldn't assume perfect synchronization between the audio and video. The video is coming from the LRV, the audio from the LM, and they take very different paths both in signal processing and in earthbound transmission from the tracking site to Houston. You probably wouldn't notice any lipsynch problems with astronauts whose faces are usually obscured by visors.

[Allan F]

If I understand the sequence correctly, the valves pressurizing the ascent fuel tanks were opened some time before liftoff, and not AT liftoff. Early enough so the crew could read the pressure indicators before liftoff.

[ka9q]

I think you're right. I looked at the A17 flight plan and the CDR is scheduled to perform APS pressurization within 15 minutes of ascent. And the transcript has Cernan firing the helium squibs at 07:17:11:33 and at 07:17:12:06. (I do wish they'd be consistent in using either day:hr:min:sec or hr:min:sec!) Then they open a second set of valves that actually pressurize the tanks.

At T-10 seconds, Cernan hits the ABORT STAGE button. During descent this would separate the two stages and ignite the ascent engine, so I wonder if this fired the separation pyros at this point in the countdown or if the button is then used in a different way. It was common on Apollo to reuse controls and indicators in ad-hoc ways at different points in the mission.

If this did immediately separate the stages, I wonder what kept the ascent stage from sliding off the descent stage on an incline...

[Allan F]

Looking at pictures of the ascent stage, the bottom was not "clean" like a sliced off part. The ascent engine bell protuded quite a bit, and the fuel/oxidizer tanks also had significant portions of their bulk beneath the separation plane. I don't think the ascent stage was in any danger of sliding off even if the guillotines were fired before ascent engine ignition.

[Chew]

I recall seeing a drawing of the LM with guide pins between the stages, presumably to keep the Ascent Module in place after the stages were separated.

[onebigmonkey]

I had a look in the Apollo 17 mission report and there is no mention of anything unusual or unexpected in there concerning the ascent module take off.

Whatever happened didn't surprise them. I'm guessing the 'dynamic physiological effect' lift off produced is the old stomach to the floor sensation

[Peter B]

Just a thought, but given the Saturn V was held down and not released until full thrust had been registered, is it possible a similar process would have been used for the LM? That is, (1) register that the engine had fired and was producing full thrust before (2) cutting the umbilicals and firing the pyros?

Or given the lack of space beneath the APS engine bell would that have been too dangerous?

[ka9q]

Just a thought, but given the Saturn V was held down and not released until full thrust had been registered, is it possible a similar process would have been used for the LM?

Possible, but a relatively small pressure-fed hypergolic engine like the APS builds thrust far more quickly than the huge F1 with its many valves that have to be properly sequenced and its turbopump that must be brought up to speed. The five engines were not started simultaneously, which is easy to see in the slow-motion films. Also, RP-1 and LOX do not spontaneously ignite on contact so a hypergol starter cartridge was used to get the party started.

[JayUtah]

The small pressure-fed engines have ignition transients lasting only fractions of a second.  The large pump-fed engines' transients are 6-8 seconds.

[Bob B.]

Thanks for that. Done some reading up on lift-off ignition overpressure, makes perfect

Are you talking about overpressure in just the engine?  Because it should also be noted that the for a brief period after ignition, the exhaust was trapped between the descent and ascent stages.  This would have exerted an additional pressure on the ascent stage, causing it to accelerate faster than expected for that first second.

[JayUtah]

Bob, we've considered that but I think there's some question how best to quantify it.  It becomes a constricted flow problem combined with a leak-rate problem, and I think it's tractable but I would have to derive the aperture form factor in terms of a circumferential opening.  But in general, yes you'd get a certain high degree of transient pressure thrust just at the initial climbout.

The other side of the coin is that ABORT STAGE was also meant to be used during flight to abort the landing and return to orbit, as in Apollo 10.  You really don't want to ignite the APS under that rule with the descent stage still bolted on.  Somewhere I have a reference for the exact LM staging sequence, right down to deadface timings, etc.  I will go find it.

[HeadLikeARock]

Thanks for that. Done some reading up on lift-off ignition overpressure, makes perfect

Are you talking about overpressure in just the engine?  Because it should also be noted that the for a brief period after ignition, the exhaust was trapped between the descent and ascent stages.  This would have exerted an additional pressure on the ascent stage, causing it to accelerate faster than expected for that first second.

Bob

My initial thought was exactly what you've described. Jay has also mentioned the ignition transient causing on over-pressure in the engine bell. Presumably a combination of these 2 led to the "anomalous" initial acceleration, compared to the calculated value for height.

It's little details like this that add so much to the authenticity of the Apollo record. Who'da thunked to consider faking that? What initially looks slightly odd turns out to be exactly what you'd expect once you look at all the minutiae.