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View Full Version : Why did Apollo spacecraft have to turn around and dock in space?


JKellyMap
01-20-2009, 11:48 AM
When the Apollo spacecraft components were launched in the Saturn V rocket, they were arranged so that a tricky "separate, turn around, and dock" maneuver, in space, was necessary in order to get two of the three astronauts from the command module into the lunar module. Why bother? Why not have the two components attached correctly, "nose to nose", at launch? So what if it "looks funny" to have an "upside-down" lunar module, with no one inside it, as the uppermost component inside the Saturn V?

I've read plenty of material, but I've come across nothing which addresses this decision. I'm guessing that maybe it has to do with the mini-escape-rocket which would clear the astronauts away from an emergency during the first minute of launch. Were the lunar module in the position I suggest, the mini-escape-rocket would have to be powerful enough to propel its "dead weight" in addition to the capsule containing the astronauts.

Another factor might be that the lunar module was housed in a special, separate casing, since it wasn't designed to deal with the conditions of Earth's atmosphere. But that alone shouldn't necessitate a "turn around and dock" maneuver, so unless someone has a different answer, I'm going to assume it's because of the mini-escape-rocket system.

Anyone know if I have guessed correctly?

(By the way, this won't be a factor if Ares/Orion goes according to plan, because humans and lunar modules will be sent up at different times, with very different rockets.)

Keeve
01-20-2009, 12:01 PM
I'm guessing that maybe it has to do with the mini-escape-rocket which would clear the astronauts away from an emergency during the first minute of launch. Were the lunar module in the position I suggest, the mini-escape-rocket would have to be powerful enough to propel its "dead weight" in addition to the capsule containing the astronauts.In my view, that's the second-best answer.

The #1 answer, it seems to me, has to do with the aerodynamics at liftoff. The cone shape of the command module is perfect for the top of the rocket. If the LM were on top, they'd need another cone further up, making the whole thing even taller, and even heavier.

mlees
01-20-2009, 12:30 PM
The astronauts handle the G-forces best if seated with thier backs to the engines that provide the kick during liftoff and manuevers, don't they?

So the astronauts would be facing the engines of the CM module in your new configuration. When they use those engines for manuavering in space, they are now facing the wrong direction. (Swivels posts and lock down clamps for the seats would be additional weight that really could be avoided.)

JKellyMap
01-20-2009, 12:46 PM
Thanks, both of you! Both your points make sense to me. Any one of these issues alone may not have sufficed to make the turn-around-and-dock configuration be worth the risk, but put all three reasons together, and it seems more than worthwhile to me.

tallcoldone
01-20-2009, 12:51 PM
The crew also needed to reenter Earth's atmosphere. The command module's (CM) shape and structural strength were, IMHO, determined by this. The LEM was designed only for use in outer space and was relatively fragile. It needed protection during launch.

This along with the other ideas mentioned make putting the CM's point at the top of the rocket the most efficient. The craft needed to be maneuverable anyway, so docking in orbit, while risky, was well within the skill set astronauts needed.

Saltire
01-20-2009, 12:57 PM
It would be nice if one of the designers would pop in with the actual answer, but until then, I'll give yet another resonable guess.

It might be that the stress of launch could damage the dock, if the two modules were connected. There'd be a lot of mass hanging on that connection, necessitating heavy struts somewhere to hold it steady and keep it from torquing the dock sealing system. If they aren't connected during the launch, the dock has no stress, and is unlikely to take damage. Since that seal is keeping the astronauts alive, any risk to its integrity is to be avoided.

Besides, astronauts are pilots, and I'm sure they loved doing that maneuver.

Icerigger
01-20-2009, 12:58 PM
In Michael Collins (Apollo 11 CMP) book Carrying the Fire he goes into this maneuver quite a bit as he was the one trained for it. It was very tricky to get the probe and drogue in the correct alignment and he fretted about screwing it up and was relieved when the operation was done.

Chronos
01-20-2009, 01:05 PM
The docking maneuver might be a bit tricky, but I have a hard time seeing how it could be risky. What's the worst that could happen?

ElvisL1ves
01-20-2009, 01:08 PM
You collide with the LEM and puncture the pressure vessel wall, terminating the mission.

billfish678
01-20-2009, 01:33 PM
As for the aerodynamics of it.

A rocket's aerodynamics as it EXITS through the earths atmosphere are almost a trivial aspect of the problem in terms of energy used to get into orbit. Though I could be wrong on this.

If they had it stacked the way they did, I'd suspect it was mainly for structural reasons. As in which way G loads and forces are oriented. In other words, you can certainly design something that will take several G's in one direction and less, if not significantly less, in the other.

Upon reflection, werent the Lunar module and the capsule oriented in a way so that the heavy loads they experienced in launch in the same direction they would experience in USE?

I saw Apollo 13 again just the other day and this question crossed my mind as well.

Voyager
01-20-2009, 02:48 PM
One more thing, though I admit I've never seen this mentioned in all the research I've done on the Apollo missions: The docking had to be done anyway after the LEM returned from the lunar surface. If there was a problem preventing the docking from happening, it is much better to find this out in earth orbit as opposed to lunar orbit.
Not all the Gemini dockings went well, to put it mildly.

I've also read that Collins was nervous - but any one doing a tricky operation like that with the whole world watching was likely to be nervous.

Voyager
01-20-2009, 02:53 PM
As for the aerodynamics of it.

A rocket's aerodynamics as it EXITS through the earths atmosphere are almost a trivial aspect of the problem in terms of energy used to get into orbit. Though I could be wrong on this.

I suspect the impact of bumpy bits on the course of the rocket heading into orbit was far more important than increase air resistance, something only a factor in the first few seconds of flight in any case. If unequal airflow caused the rocket to start oscillating or something during ascent, there could be a problem. I think you would also want some degree of radial symmetry to make it simpler to get inserted into orbit right where you wanted to be. However, I think the other factors were probably more important.

Stranger On A Train
01-20-2009, 02:56 PM
I've read plenty of material, but I've come across nothing which addresses this decision. I'm guessing that maybe it has to do with the mini-escape-rocket which would clear the astronauts away from an emergency during the first minute of launch. Were the lunar module in the position I suggest, the mini-escape-rocket would have to be powerful enough to propel its "dead weight" in addition to the capsule containing the astronauts.This, along with packaging and previous design, where the primary drivers in how the Apollo Lunar Mission vehicles were configured. As noted above, the Apollo Command Module had a nose-mounted rocket, called the Launch Escape System (http://apollosaturn.com/asnr/escape.htm), which would pull it up any away from the Saturn rocket stack during an early launch time abort regime. Having the CM as a forward mounted, separable vehicle was required by this concept. In addition, this was the same basic configuration as the Mercury/Redstone and Mercury/Atlas systems. (Gemini, launched atop a Titan II booster, used ejection seats for abort.)

While it might be possible to pre-assemble the CM and Lunar Module, this would have placed the Lunar Module way forward, making it difficult to access the CM and requiring a support and protective aerostructure to go around the LM. You could not arrange the vehicle the opposite way with the CM facing aft because the Service Module (containing the SPS rocket motor, reaction and control systems, extended life support, fuel cells, guidance & navigation systems, and long range communications) would have to sit forward of that, which is obviously unworkable.

There is another functional advantage to dock with an extract the LM from its bay behind the CSM; by having to dock with the LM before entering lunar orbit, it provides a final, realistic checkout of CSM and LM docking and propulsion systems in a way that can't be fully tested on the ground. If the pilot can't dock due to damage or misassembly, that should become apparent, and the astronauts simply stay in their Lunar swingby/Earth return trajectory instead of landing. Bummer, but better than going down and then not being able to dock on return.

The use of a separate LM and Lunar orbit rendezvous was actually highly controversial and initially disfavored, as it would result in considerably more inert mass on launch and would require a crew member to stay in orbit (automatic avionics systems of the day being rather primitive). However, as mission conceptual design and trade studies were refined it became apparent that building a crew vehicle capable of both lunar descent/ascent and Earth reentry would be prohibitively complex and expensive, whereas a fairly sophisticated primary CSM plus a 'bare bones' lunar lander would give far more capability. As it turned out with Apollo XIII it also provided a margin against unforeseen failure of the SM life support and propulsion systems.

A rocket's aerodynamics as it EXITS through the earths atmosphere are almost a trivial aspect of the problem in terms of energy used to get into orbit. Though I could be wrong on this.The aerodynamic losses on an ascent vehicle are small, but hardly trivial. Given that an acceptable ascent margin is generally just a few percent of the total energy expenditure, even modest losses to drag can become prohibitive, and of course, will reduce overall payload capacity. In addition, a blunt-nosed vehicle will be difficult to control due to offset forces, hence why most rockets have conical nose cones.

Stranger

rbroome
01-20-2009, 07:12 PM
When the Apollo spacecraft components were launched in the Saturn V rocket, they were arranged so that a tricky "separate, turn around, and dock" maneuver, in space, was necessary in order to get two of the three astronauts from the command module into the lunar module. Why bother? Why not have the two components attached correctly, "nose to nose", at launch? So what if it "looks funny" to have an "upside-down" lunar module, with no one inside it, as the uppermost component inside the Saturn V?

I've read plenty of material, but I've come across nothing which addresses this decision. I'm guessing that maybe it has to do with the mini-escape-rocket which would clear the astronauts away from an emergency during the first minute of launch. Were the lunar module in the position I suggest, the mini-escape-rocket would have to be powerful enough to propel its "dead weight" in addition to the capsule containing the astronauts.

Another factor might be that the lunar module was housed in a special, separate casing, since it wasn't designed to deal with the conditions of Earth's atmosphere. But that alone shouldn't necessitate a "turn around and dock" maneuver, so unless someone has a different answer, I'm going to assume it's because of the mini-escape-rocket system.

Anyone know if I have guessed correctly?

(By the way, this won't be a factor if Ares/Orion goes according to plan, because humans and lunar modules will be sent up at different times, with very different rockets.)

I assume that you postulate a windscreen or cover over the LM for this configuration?
In that case it is a simple matter of weight. A windscreen capable of withstanding launch will weigh a lot, certainly more than the weight of the inline cowling that covers the LM behind the CM. Interesting tidbit: on the space shuttle main fuel tank, the insulation on the nose is approximately 2 inches thick. On one side is liquid oxygen at -293F and the other side heats up to 800F due to friction. Clearly this is not an indication of the conditions the CM or any other vehicle experiences. Each vehicle is unique. But it shows the kind of conditions expected.

After that there is the simple argument that the escape rockets needed to be at the very top.
And since the LM and CM had to go through at least one docking and undocking, why not make it two and save a lot of weight and trouble?

I can imagine no reason to try to put the LM ahead of the CM.

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