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Post by Hypersonic on Nov 8, 2007 14:39:43 GMT -4
IMO, I would think that the Mission Reports would have more reliable information. I'm sure you're right. It may be a case of "this is the weight on this day before we did x."
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Post by Hypersonic on Nov 8, 2007 18:11:32 GMT -4
From The Apollo Spacecraft Chronology, Volume IV
"Because of propellant limitations in the ascent stage, landing and subsequent liftoff from the moon would be impossible."
I'm looking for a slice of humble pie...
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Post by PhantomWolf on Nov 8, 2007 23:23:53 GMT -4
Nah, a lot of interesting information come out in the discussion.
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Post by cos on May 6, 2008 10:16:32 GMT -4
So we know Snoopy could have landed but wasn't carrying enough ascent stage propellant to reach lunar orbit. However, I read in Michael Collins 'Carrying the Fire', that they practiced simulations of taking the CM down to 50000 ft to 'rescue' the LEM and this was something that they were prepared to do in a live mission. Could Snoopy have landed and reached sufficient altitude for Charlie Brown to have rescued it?
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Bob B.
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Post by Bob B. on May 6, 2008 10:26:39 GMT -4
Could Snoopy have landed and reached sufficient altitude for Charlie Brown to have rescued it? Snoopy would have had to at least reach some type of lunar orbit to have any chance for rescue. To my knowledge, Snoopy did not have enough propellant to do this. Simply attaining a suborbital arc wouldn't have been enough for the CSM to rendezvous with it.
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Post by cos on May 7, 2008 10:31:27 GMT -4
D'oh, of course. Thanks for your reply to my stupid question. Back to rocketry 101 for me...
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Post by echnaton on May 7, 2008 10:47:15 GMT -4
Could Snoopy have landed and reached sufficient altitude for Charlie Brown to have rescued it? Snoopy would have had to at least reach some type of lunar orbit to have any chance for rescue. To my knowledge, Snoopy did not have enough propellant to do this. Simply attaining a suborbital arc wouldn't have been enough for the CSM to rendezvous with it. Which leads to a interesting hypothetical situation. If some LM had landed and was known for some reason not to be able to get back into orbit. Would it have been possible to position the CSM to make a one time try to snag the accent stage on such a suborbital arc? I can imagine positioning the CSM in a orbit just below the peak of the LM arc. Then timing the LM launch so that it would meet up with the CSM providing a limited opportunity for rendezvous. If completed, the CSM would do a burn to stabilize to orbit of the combined ships. I just wonder how much fuel the CSM would burn in this maneuver and weather it would have enough to return to earth. Any one have thoughts on this scenario.
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Bob B.
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Post by Bob B. on May 7, 2008 11:12:42 GMT -4
If some LM had landed and was known for some reason not to be able to get back into orbit. Would it have been possible to position the CSM to make a one time try to snag the accent stage on such a suborbital arc? The CSM itself would, of course, have to slow down to sub-orbital velocity to rendezvous with the ascending LM. The CSM would be in a death dive from which it would very quickly have to recover after docking with the LM. I can imagine positioning the CSM in a orbit just below the peak of the LM arc. Then timing the LM launch so that it would meet up with the CSM providing a limited opportunity for rendezvous. The CSM must to match the LM’s velocity to rendezvous and dock with it. If the LM doesn’t have enough propellant to reach orbital velocity, then the CSM must decelerated to the same sub-orbital velocity to hookup with the LM. This means that both vehicles are about to crash and burn unless the docking can be completed and the SPS used to accelerate the combined vehicles into some type of survivable orbit before impact. I just wonder how much fuel the CSM would burn in this maneuver and weather it would have enough to return to earth. Surely the CSM wouldn’t have anywhere near enough propellant for such a maneuver. At this point in the mission there is just enough propellant left for TEI, mid-course corrections, and a small margin. There is certainly not enough propellant to slow down and rendezvous with the LM and then accelerate both the CSM and LM back to orbital velocity.
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Post by ka9q on May 11, 2008 17:36:38 GMT -4
That is exactly what they did. They took Snoopy down to the 50,000 feet mark, the point where the final Go/No Go is given before entering the landing program, then aborted the landing with the stages separating, a test of the abort systems as well as for LOR. This is space flight, not aviation. Think about velocity, not altitude. Velocity determines whether you're in orbit or not. Snoopy may have been only 50,000 feet above the surface but in terms of velocity (i.e., fuel) it was very far from landing. During the first several landings, the LM first entered an elliptical 'descent orbit' with a pericynthion (lowest altitude) 50,000 feet above the surface and several hundred miles uprange of the landing site. Apollo 11 began powered descent at this point; Apollo 10 did not. Powered descent initiation is a critical moment. Until then, the LM is still in orbit and you can take your time to figure out any problems. But as soon as you begin powered descent, your apocynthion (half an orbit away, on the far side) starts to fall rapidly. Pretty soon you're no longer in orbit, so if the descent engine quits and you do nothing about it, you soon crash. Lots of things have to happen quickly and correctly during powered descent. Entering the descent orbit with its 50,000 foot pericynthion takes relatively little fuel. To actually land you have to cancel your enormous orbital velocity, and that takes almost all of the fuel in the descent stage. And getting that velocity back after a landing takes all the fuel the ascent stage can carry. Apollo 10 was only the second manned flight of the LM. Problems were expected and you needed time to fix them. So the last thing you'd do is to enter powered descent with its extreme time pressure; you stay in orbit where you have time to work things out.
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Post by homobibiens on Aug 11, 2009 19:41:53 GMT -4
The CSM itself would, of course, have to slow down to sub-orbital velocity to rendezvous with the ascending LM. The CSM would be in a death dive from which it would very quickly have to recover after docking with the LM. Forgive the thread necromancy, but doing exercises like this helps me understand orbital mechanics better I am getting that a circular orbit around the moon that just barely clips the tree tops (or would, if there were any trees) would have a period of 1.8 hours. If the lander lacks the energy to achieve this orbit, it can only achieve suborbital trajectory, which follows an ellipse with a smaller semi-major axis than the radius of the moon, with the complicating factor that part of this ellipse lies below the surface of the moon. Since the period of an orbit is proportional to the semi-major axis raised to the power of 1.5, the period of this orbit (if it weren't for the little detail about crashing into the moon at some point) would have to be less than 1.8 hours. So, the absolute upper limit on the amount of time available to pick up the lunar lander from a suborbital trajectory would be 1.8 hours, if it has almost enough fuel to achieve a low orbit. If it has much less fuel, the amount of time available will be much shorter. So in the absolute best case, there would be 1.8 hours to rescue the lander, with one shot - miss it, no chance to try again. Furthermore, if the CSM has enough fuel to swoop down to a suborbital trajectory, pick up the lander, and then zip off again, it's not clear why there's any need for a lander at all - why couldn't the CSM just do the landing itself? (OK, maybe it is hard to steer or maneuver with the level of precision required.) Sound good?
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Bob B.
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Post by Bob B. on Aug 11, 2009 20:56:00 GMT -4
Forgive the thread necromancy, but doing exercises like this helps me understand orbital mechanics better I am getting that a circular orbit around the moon that just barely clips the tree tops (or would, if there were any trees) would have a period of 1.8 hours. If the lander lacks the energy to achieve this orbit, it can only achieve suborbital trajectory, which follows an ellipse with a smaller semi-major axis than the radius of the moon, with the complicating factor that part of this ellipse lies below the surface of the moon. Since the period of an orbit is proportional to the semi-major axis raised to the power of 1.5, the period of this orbit (if it weren't for the little detail about crashing into the moon at some point) would have to be less than 1.8 hours. So, the absolute upper limit on the amount of time available to pick up the lunar lander from a suborbital trajectory would be 1.8 hours, if it has almost enough fuel to achieve a low orbit. If it has much less fuel, the amount of time available will be much shorter. So in the absolute best case, there would be 1.8 hours to rescue the lander, with one shot - miss it, no chance to try again. Furthermore, if the CSM has enough fuel to swoop down to a suborbital trajectory, pick up the lander, and then zip off again, it's not clear why there's any need for a lander at all - why couldn't the CSM just do the landing itself? (OK, maybe it is hard to steer or maneuver with the level of precision required.) Sound good? That sounds generally correct, but in practice I'd say the window to rescue the LM would be far less than 1.8 hours. It is a moot point, however, because in reality the CSM was not in position to reach the LM. The LM was launched into a lower, faster orbit with an insertion point behind the CSM. The LM then had to catch up with the CSM to rendezvous with it. If the LM ended up in a sub-orbital trajectory, I don't think there is any way the two vehicles could be maneuvered into the same space in the time available. I think a rescue might be feasible provided the LM managed to get into some sort of survivable orbit; however, anything sub-orbital would likely mean loss of vehicle and crew.
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Post by homobibiens on Aug 11, 2009 21:12:39 GMT -4
That sounds generally correct, but in practice I'd say the window to rescue the LM would be far less than 1.8 hours. It is a moot point, however, because in reality the CSM was not in position to reach the LM. The LM was launched into a lower, faster orbit with an insertion point behind the CSM. The LM then had to catch up with the CSM to rendezvous with it. If the LM ended up in a sub-orbital trajectory, I don't think there is any way the two vehicles could be maneuvered into the same space in the time available. I think a rescue might be feasible provided the LM managed to get into some sort of survivable orbit; however, anything sub-orbital would likely mean loss of vehicle and crew. I didn't mean to convey optimism about this possibility Anyway, I just found something that says the CSM orbit was at an altitude of 110 km. The difference in energy between an orbit at this altitude and any orbit at all that stays above the moon's surface is fairly small, so I'm not sure the various rescue scenarios had much probability of coming into play at all. There is a pretty narrow range of fuel supply for which it is possible to get into orbit at all, but not possible to get into the planned orbit, if I'm doing this right . . .
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Bob B.
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Post by Bob B. on Aug 11, 2009 23:53:54 GMT -4
Anyway, I just found something that says the CSM orbit was at an altitude of 110 km. The CSM actually changed its orbit serval times. For instance, below are the maneuvers performed by the Apollo 17 CSM with the delta-v and resulting lunar orbit listed. Maneuver | Delta-v (ft/s) | Orbit (n. mi.) | Lunar orbit insertion | 2988 (SPS) | 170 x 52.6 | Descent orbit insertion #1 | 197 (SPS) | 59 x 14.5 | CSM separtion | 1.0 (RCS) | 61.5 x 11.5 | CSM circularization | 70.5 (SPS) | 70 x 54 | Trim | 9.2 (RCS) | 67.3 x 62.5 | CSM lunar plane change | 366 (SPS) | 62.8 x 62.5 | CSM separation | 2 (RCS) | 63.9 x 61.2 | Transearth injection | 3046.3 (SPS) | N/A |
1 nautical mile = 1.852 km 1 ft/s = 0.3048 m/s
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Post by echnaton on Aug 12, 2009 10:07:43 GMT -4
This raises a question about orbital mechanics that I have wondered about. Say Snoopy did land and had to take off in a sub orbital arc to attempt a rendezvous. It could take off, but following any arc it could take, wouldn't it be traveling at slower speed relative to the surface than the CSM? Would there ever be a trajectory during a suborbital arc that the LM and orbiting CSM were traveling at a sufficiently slow speed, relative to each other, that a docking could take place?
This question came to me the other day when a co-worker stated that there were plans to use some derivation of SpaceShip One to go to the ISS. It is simply implausible based on the abilities of SS1 or any ship of similar design.
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Post by homobibiens on Aug 12, 2009 10:48:23 GMT -4
This raises a question about orbital mechanics that I have wondered about. Say Snoopy did land and had to take off in a sub orbital arc to attempt a rendezvous. It could take off, but following any arc it could take, wouldn't it be traveling at slower speed relative to the surface than the CSM? Would there ever be a trajectory during a suborbital arc that the LM and orbiting CSM were traveling at a sufficiently slow speed, relative to each other, that a docking could take place? This question came to me the other day when a co-worker stated that there were plans to use some derivation of SpaceShip One to go to the ISS. It is simply implausible based on the abilities of SS1 or any ship of similar design. I'm not coming up with anything. If the two craft are at the same altitude, then they have the same potential energy. If they are traveling at the same speed, they have the same kinetic energy, so if one has enough energy to be in orbit, so does the other. There are suborbital arcs that have higher energy than a circular orbit in low altitude (go up really really high before returning and crashing into the moon), but if the lander was able to achieve one of the suborbital trajectories, it seems like it should have been able to achieve orbit. (Although I need to rethink this point, at another board, I realized some of my "facts" are based on an inappropriate application of conservation of energy.) The only scenario I can see is one where the lander is just a tiny bit short of the fuel needed to achieve orbit, and can achieve a suborbital trajectory that is nearly circular, and goes almost all the way around the moon. Then its position/velocity shouldn't be far from that of a craft in a circular orbit. But, the CSM would have less than 1.8 hours to drop down a bit, pick up the lander, and get back into orbit before crashing into the moon Maybe they could snag it with a hook and a bungee cord
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