Author Topic: First Stage Mass For Landing Stage  (Read 5482 times)

Offline S.Paulissen

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First Stage Mass For Landing Stage
« on: 04/14/2015 10:41 pm »
There was a very simple question by a reporter from a Japanese newspaper that asked what the basic dimensions of the returning stage were as it landed.  The overall dimensions were not anything of note for us... except one.  He stated the weight of the landing stage was approximately 60000-70000lbs.  Placing it between 27272-31818kg.  I figured I'd point out this data point to people attempting to simulate this with at least somewhat verified accuracy.

http://www.space-multimedia.nl.eu.org/index.php?option=com_content&view=article&id=9365
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Offline Burninate

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Re: First Stage Mass For Landing Stage
« Reply #1 on: 04/15/2015 03:12 am »
Thanks so much for this, helps with calculations immensely.

Do you think we should take this as "Mass before boostback, mass immediately after separation", "Mass between boostback and terminal landing burn", or "Mass at end of terminal landing burn"?

EDIT:
They also said that when the first stage is landed, there is enough fuel margin for about 2 or 3 seconds of hover.
« Last Edit: 04/15/2015 03:40 am by Burninate »

Offline S.Paulissen

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Re: First Stage Mass For Landing Stage
« Reply #2 on: 04/16/2015 07:53 pm »
I interpreted it as the mass of the stage as it entered the landing burn, i.e. stage + pre-landing burn propellant.
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Offline matthewkantar

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Re: First Stage Mass For Landing Stage
« Reply #3 on: 04/17/2015 01:41 am »
If the stage weights in at 65,000 pounds or so, a 147,000 pound thrust Merlin engine at the much discussed 40 percent throttle setting (58,800#) would be able to hover. we have all heard over and over that it can't. What data point here is corrupt?

Matthew

Offline mvpel

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Re: First Stage Mass For Landing Stage
« Reply #4 on: 04/17/2015 02:13 am »
"40%" actually means 60-100%, as has been repeatedly posited, perhaps?
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Offline Kabloona

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Re: First Stage Mass For Landing Stage
« Reply #5 on: 04/17/2015 04:19 am »
"40%" actually means 60-100%, as has been repeatedly posited, perhaps?

Yes, this is a clever backdoor way of proving what Elon meant by his ambiguous statement: M1D cannot in fact throttle down to 40% thrust, but thrust can be reduced by 40%, ie to 60% thrust.

147,000 x .6 = 88,200 lbf, slightly more than the returning stage weight given by Hans, hence the need for a hoverslam maneuver.

Offline Jarnis

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Re: First Stage Mass For Landing Stage
« Reply #6 on: 04/17/2015 06:06 am »
"40%" actually means 60-100%, as has been repeatedly posited, perhaps?

This is still unclear. One guy did some fairly convincing math based on the videos and came into conclusion that either some variable given out by SpaceX is bogus, or the throttle setting at the very end, just before it is vertically stopped is, in fact, 40%.

http://www.reddit.com/r/spacex/comments/32w3tk/velocityacceleration_analysis_of_crs6_landing/

(For the 40% value, "21 tons at touchdown when velocity is 0" would be true. Change variables and you get different throttle value. For 60% or 70% throttle the stage would have to weight a lot more)
« Last Edit: 04/17/2015 06:09 am by Jarnis »

Offline Lars-J

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Re: First Stage Mass For Landing Stage
« Reply #7 on: 04/17/2015 05:04 pm »
"40%" actually means 60-100%, as has been repeatedly posited, perhaps?

This is still unclear. One guy did some fairly convincing math based on the videos and came into conclusion that either some variable given out by SpaceX is bogus, or the throttle setting at the very end, just before it is vertically stopped is, in fact, 40%.

http://www.reddit.com/r/spacex/comments/32w3tk/velocityacceleration_analysis_of_crs6_landing/

(For the 40% value, "21 tons at touchdown when velocity is 0" would be true. Change variables and you get different throttle value. For 60% or 70% throttle the stage would have to weight a lot more)

Yep. And it is already know that Merlin can throttle to 50% - at least M1D-Vac. So I would assume that M1D can do *at least* that, probably better.

Offline mvpel

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Re: First Stage Mass For Landing Stage
« Reply #8 on: 04/17/2015 05:41 pm »
Hans said ~30 tonnes though. 65k lbs is 29,483 kg, nearly 50% more than 21 tonnes. So it's probably not running at 40% throttle.
"Ugly programs are like ugly suspension bridges: they're much more liable to collapse than pretty ones, because the way humans (especially engineer-humans) perceive beauty is intimately related to our ability to process and understand complexity. A language that makes it hard to write elegant code makes it hard to write good code." - Eric S. Raymond

Offline Kabloona

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Re: First Stage Mass For Landing Stage
« Reply #9 on: 04/17/2015 06:17 pm »
"40%" actually means 60-100%, as has been repeatedly posited, perhaps?

This is still unclear. One guy did some fairly convincing math based on the videos and came into conclusion that either some variable given out by SpaceX is bogus, or the throttle setting at the very end, just before it is vertically stopped is, in fact, 40%.

http://www.reddit.com/r/spacex/comments/32w3tk/velocityacceleration_analysis_of_crs6_landing/

(For the 40% value, "21 tons at touchdown when velocity is 0" would be true. Change variables and you get different throttle value. For 60% or 70% throttle the stage would have to weight a lot more)

Actually his analysis seems to support 60% throttle: "For 60% or 70% throttle the stage would have to weigh a lot more (than 21T)..." Which we know it does: around 30T.

It certainly doesn't support 40% throttle, given what Hans said about stage mass.
« Last Edit: 04/17/2015 06:25 pm by Kabloona »

Offline Kabloona

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Re: First Stage Mass For Landing Stage
« Reply #10 on: 04/17/2015 09:27 pm »
Another useful data point. Wired Magazine did a frame-by frame video analysis of the stage position vs. time, which user acsawdey then further analyzed here:

http://forum.nasaspaceflight.com/index.php?topic=36527.msg1361261#msg1361261

He deduced a vertical deceleration of around 5 m/sec in the final few seconds before the stage touched down.

This implies a total vertical thrust producing about 1.5 g. For a stage weight of 65,000 lbf, that implies a thrust of 97,500 lbf, which is about 66% of the M1D's nominal 147,000 lbf.

Offline speedevil

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Re: First Stage Mass For Landing Stage
« Reply #11 on: 04/17/2015 10:00 pm »
This implies a total vertical thrust producing about 1.5 g. For a stage weight of 65,000 lbf, that implies a thrust of 97,500 lbf, which is about 66% of the M1D's nominal 147,000 lbf.
Adding aero drag, and the fuel used up until those last few seconds, that might be bang on 60%.

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