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Forum:	Mercury - Gemini - Apollo
Topic:	Force of gravity during Apollo lunar liftoffs

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Your Reply: HTML is ON UBB Code is ON Smilies Legend	[QUOTE]Originally posted by Blackarrow: [B] [QUOTE]Originally posted by moorouge: [b] Which is basically what the Webber International University web site said. [/b][/QUOTE] I'll take that as a compliment![/B][/QUOTE]
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<table border=0 cellpadding=0 cellspacing=0 width="600"><TR><td bgcolor="#000000"><TABLE BORDER=0 cellpadding=4 border=0 cellspacing=1 WIDTH="100%"><TR bgcolor="#660000"><TD COLSPAN=2><CENTER>T O P I C     R E V I E W</CENTER></TD></TR><TR bgcolor="#494949"><TD valign=top>Duke Of URL</TD><TD>Does anybody know what the G forces of the liftoffs from the Moon were?</TD></TR><TR bgcolor="#333333"><TD valign=top>ejectr</TD><TD>Murdoc Online: <A HREF="http://www.murdoconline.net/archives/870.html" TARGET=_blank>May the (G) Force be with You</A> <BLOCKQUOTE>This is a simple graph of the G forces experienced during the trip from the launch pad to low earth orbit in a Saturn V.</BLOCKQUOTE></TD></TR><TR bgcolor="#494949"><TD valign=top>Duke Of URL</TD><TD>To be clear, I was interested about the G forces to which astronauts were subjected during a liftoff from the lunar surface. In other words, what G force did they get while leaving the lunar surface. Once again for clarity, when the astronauts blasted off the surface of the Moon. (and only the surface of the moon, please) how many g's were they pulling?Sorry for any confusion.</TD></TR><TR bgcolor="#333333"><TD valign=top>Robert Pearlman</TD><TD>From the Apollo 15 Flight Journal, citing comments by David Scott: <BLOCKQUOTE>"It was also very low g. The pictures show the thing popping off the ground. Pop? I think we went from 1/6th g to maybe a half or whatever. You weren't being pushed hard. We were standing up and you would think, boy, all those g's standing up. Not really. You could hardly tell."</BLOCKQUOTE></TD></TR><TR bgcolor="#494949"><TD valign=top>Headshot</TD><TD>This is a very good question.About a half of a g is what I recall Pete Conrad estimating during an interview with Paris-Match back in late '69 or early '70.I reviewed the Apollo XII Mission Report, but could not find an acceleration graph of the lunar liftoff, nor did I find any comments from Conrad or Bean in their Tech Debrief.</TD></TR><TR bgcolor="#333333"><TD valign=top>moorouge</TD><TD>You may find <A HREF="http://www.webber.edu/wunker/PhysicalScience/LEM.htm" TARGET=_blank>this</A> provides your answer.</TD></TR><TR bgcolor="#494949"><TD valign=top>Duke Of URL</TD><TD> <BLOCKQUOTE>quote:<HR>Originally posted by moorouge: You may find this provides your answer.<HR></BLOCKQUOTE> (Dopey joke alert!) It gives values as Newtons but doesn't say how many single Newtons equal a Fig Newton.</TD></TR><TR bgcolor="#333333"><TD valign=top>Guillaume</TD><TD>I remember Alan Bean saying that it felt like an elevator ride.</TD></TR><TR bgcolor="#494949"><TD valign=top>Blackarrow</TD><TD>If the g-force on lunar liftoff is about 0.5g, that would be three times normal lunar gravity, the equivalent of 3g acceleration after liftoff from Earth. However, I suspect that what matters is the difference rather than the multiplier. The difference is 2g lifting off from Earth, but from the moon it's a pretty modest .33g increase. I hope that makes sense. At least I didn't use Newtons (a sure way to lose most of your audience).</TD></TR><TR bgcolor="#333333"><TD valign=top>moorouge</TD><TD> <BLOCKQUOTE>quote:<HR>Originally posted by Blackarrow: The difference is 2g lifting off from Earth, but from the moon it's a pretty modest .33g increase. I hope that makes sense. <HR></BLOCKQUOTE>Which is basically what the Webber International University web site said. </TD></TR><TR bgcolor="#494949"><TD valign=top>Duke Of URL</TD><TD> <BLOCKQUOTE>quote:<HR>Originally posted by moorouge: Which is basically what the Webber International University web site said. <HR></BLOCKQUOTE> It did, but only for people that knew the math. A dolt like me who is easily confused is more comfortable with elevator analogies.</TD></TR><TR bgcolor="#333333"><TD valign=top>moorouge</TD><TD> <IMG SRC="http://www.collectspace.com/ubb/smile.gif"></TD></TR><TR bgcolor="#494949"><TD valign=top>Blackarrow</TD><TD> <BLOCKQUOTE>quote:<HR>Originally posted by moorouge: Which is basically what the Webber International University web site said. <HR></BLOCKQUOTE> I'll take that as a compliment!</TD></TR><TR bgcolor="#333333"><TD valign=top>moonguyron</TD><TD>Here is some simple math that will answer all your questions about "g" forces during any powered phase of flight. Stay with me a bit here and you will see it is simple high school math. For any burn you need the change in velocity in feet/sec. For example the lunar liftoff went from zero to 6047 ft/sec. ( I get this from an Apollo 17 Flight Plan I have.) Now divide this change in velocity by the time in seconds of the burn. In this case 7 minutes 14 seconds equals 434 seconds. This yields 13.935 ft/sec squared. (That's the average acceleration during the burn). Now all we have to do is divide this 13.935 by 32 ft/sec squared (the acceleration of one "earth g') and you get .435 "g" average coming off the moon. Another example: TLI: Change in velocity was 10375 ft/sec. Time of burn was 344 seconds. 10375 divided by 344 is 30.16 ft/sec squared average during the burn. Divide this by 32 ft/sec squared and average "g" is .94 for the burn. I have done this for all burns for a flight to the moon and back just to get a feel for the "g" load they experienced. Except for launch all accelerations were quite benign. Launch on the other hand was quite different and was not nearly so linear as great gobs of propellant were used which changed dramatically the vehicle weight during boost which changed acceleration as it burned off. For that I would refer you to the Saturn V Manual where graphs show the "g" load during each stage to orbit.Hope this helps.</TD></TR><TR bgcolor="#494949"><TD valign=top>Duke Of URL</TD><TD> <BLOCKQUOTE>quote:<HR>Originally posted by moonguyron: Here is some simple math that will answer all your questions about "g" forces during any powered phase of flight. Stay with me a bit here and you will see it is simple high school math.<HR></BLOCKQUOTE>Great. I slept through high school math.</TD></TR><TR bgcolor="#333333"><TD valign=top>xlsteve</TD><TD>As a practical description, I've heard Alan Bean describe it as riding in one of those glass elevators in a mall or hotel. </TD></TR><TR bgcolor="#494949"><TD valign=top>moorouge</TD><TD>Now we've settled the 'g' at lift-off on the moon, what would have been the 'g' experienced by Stafford and Cernan when they fired up the ascent stage in lunar orbit? The same or more or less?</TD></TR><TR bgcolor="#333333"><TD valign=top>moonguyron</TD><TD>Since I do not have access to the velocity change or time of acceleration we can not apply the above math. However we do know that all ascent engines are essentially the same in thrust, and,although the AP-10 LM was somewhat lighter they would experience very close to the same acceleration ("g" load) as all subsequent departing LM's, just not for as long a duration. </TD></TR><TR bgcolor="#494949"><TD valign=top>LM-12</TD><TD>The Apollo 12 splashdown was a hard landing in choppy seas. Pete Conrad described the splashdown as "a tremendous impact". The post-flight Mission Report estimated that the impact acceleration was about 15g. </TD></TR><TR bgcolor="#333333"><TD valign=top>John Charles</TD><TD> <BLOCKQUOTE>quote:<HR>Originally posted by moonguyron: That's the average acceleration during the burn...<HR></BLOCKQUOTE> Good information, thanks! During lunar launch, the perceived g level was lower than the average and reached its maximum value at orbit insertion because the mass of the ascent stage decreased as propellants were used but the thrust of the engine remained constant. Have you looked at the final g level at burn out by dividing the empty weight of the ascent stage by the engine's thrust? (I haven't but it would be interesting.)</TD></TR><TR bgcolor="#494949"><TD valign=top>moonguyron</TD><TD>John, I don't have access to the numbers (ascent stage weight and engine thrust) but if you can get them, let's do the math. I did mention that these are approx. numbers based on an average. And as mentioned, as in launch when vast quantities of propellant dramatically change the vehicle weight these accelerations change correspondingly. </TD></TR></TABLE></td></tr></table>

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