Are you buying? :dubious:
The only reason I don’t agree with this is, why didn’t they do this anyway? I don’t think anyone said, “Well, we dodged that bullet, so let’s continue as if nothing happened.”
The Apollo program was already being curtailed prior to the Apollo 13 anomaly, with the Apollo 20 mission formally cancelled and the recognition that Apollo 18 and 19 would need to be cancelled in order to devote budget to what would become the nascent Space Transportation System (‘Shuttle’). While Apollo Applications Program never had any formal missions beyond Skylab, there were a number of proposals for various configurations and missions, including a proposed nuclear thermal upper stage to replace the S-IVB for extended missions, which never went past studies.
It may surprise many but the Apollo program offered essentially nothing in terms of uncrewed Earth surveillance and planetary exploration capability. The vast majority of the effort in the Apollo program (by most estimates, 96% to 98% of engineering and labor hours) was devoted to supporting the crew capability and achieving the required reliability, and very little of that technology development and experience was really applicable to orbital telescopes and uncrewed probes and landers. The real enabling technologies for uncrewed systems were very low power processing and telemetry systems, and the advances in sensors and optics that largely came from satellite surveillance, launch detection, and remote verification systems, i.e. DSP and NRO satellites. Both Hubble and Clementine (among others) used technologies directly adapted from these kinds of systems. Even the massive Apollo booster was never intended or needed for use in launching satellites or probes because of the exorbitant cost and difficultly; instead, boosters developed from ballistic missiles like Titan, Thor/Delta, and Atlas were used for launching nearly all interplanetary missions and most satellites.
Not true. In fact, there was significant opposition to continuing the Lunar program all the way to the top; Nixon viewed the program as both a waste of resources and a legacy of two prior presidents that he viewed dimly. By Apollo 13–only the third flight to the moon–the flights were viewed with such little interest by the public at large that they merited only a short blurb on the evening news on the day of launch and no live coverage on any of the networks. With costs of the Viet Nam conflict rising and the public concerned about the increasing number of deaths of US troops in a conflict that it wasn’t clear we had any strategy to win, plus various domestic issues, the Apollo program was of continued interest only to space enthusiasts and aerospace interests. The Air Force had long before determined that there was little military application for a crewed space station that could not be done more cheaply and reliably by satellites.
The “major program re-direction and several years of soul-searching & re-engineering” following the catastrophic loss of Challenger wasn’t the kind of errant hand-wringing that critics of NASA like to portray. It was rather a recognition that the STS had been developed on a hurried schedule with many latent design problems discovered during the initial and subsequent investigations (there were over a hundred Class 1 engineering change notices to the solid propellant boosters themselves as well as a comprehensive redesign of the RS-24/-25 Space Shuttle Main Engine to fix a variety of well known issues, a major revision of avionics systems, and a thorough review and retesting of the flight system software), had many fundamental problems that would prevent the development of effective abort or crew escape modes for many possible failure modes over most or all of the ascent trajectory, and was not as reliable as the NASA management and Congress felt it should be (though to be fair, the realized reliability fell neatly within the technical prediction of 1:50 to 1:100 catastrophic failures throughout a standard mission profile). The STS was a deeply flawed system, developed by committee and engineered to requirements that were in conflict with one another to perform a set of roles that it was almost never used for.
They didn’t “continue as if nothing happened.” There was a failure investigation (as there always is in any aerospace system failure in my experience, big or small) and the root cause was quickly determined as a requirements mismatch in the electrical system for the LOx tank stirrer and bad wiring routing practices (which had actually been prohibited after the Apollo 1 fire and the problems with the first Grumman LM, but had not been rigorously enforced by the contractors.) In retrospect, there were a lot of known problems with the Apollo CSM, and probably more latent problems or defects that were unknown at the time the program was cancelled. Although aerospace systems in general, and crewed vehicles in particular, are extensively tested in every possible way that engineers can dream up and project managers can find funding and schedule for, these systems are so incredibly complex that there is no way to detect every problem and especially those that only occur in flight conditions which can only be replicated imperfectly in ground testing. We test components and small subsystems to elevated levels and durations compared to the expected loads and environments specifically to try to vet these issues but until there are hundreds or thousands of hours of flight experience in every possible regime it will experience, the potential for undiscovered design problems or latent defects remains.
On the topic of what would have happened to Apollo 13 if the crew had not been able to make an immediate free return trajectory to Earth, it is useful to understand just how fortunate the crew was that the failure occurred when it did, i.e. almost two and a half days into the mission as they were approaching the moon. This meant that the Lunar Module, which was used as a sort of lifeboat (providing all power and environmental control) was attached to the CSM and had maximum reserves. If it had happened to soon before (i.e. just after Trans-Lunar Injection, before CSM/LM rendezvous) or after (Lunar Orbit Injection and especially after the LM had detached and landed) it is likely the crew would have perished without any way to get power or ELCSS for the return trip. In that case, the CSM would have been stuck in Lunar orbit indefinitely, or would be on a free return trajectory back toward Earth where it would likely have gone into a long period elliptical orbit (trajectory adjustments were performed on the return leg to get the desired reentry angle). I think it is implausible that any effort would be made to rendezvous with the capsule or recover the bodies unless to perform an examination on the failure, but again, that was determined quickly from reviewing design specifications and recreating the failure condition in ground testing.
Stranger
Ya know, it never occurred to me that it would have been bad had it happened too soon. I’ve always realized that without the LM’s consumables (i.e. after the landing) they’d have never made it back, but they wouldn’t have had them if they hadn’t extracted & docked with the LM yet either.
I imagine they’d have had two options:
[ul]
[li]Test the SM’s thrusters & systems and see if it could still accomplish the precision extraction & docking maneuver, then do that and continue on as they did.[/li][li]If it couldn’t do that but it could at least turn the CSM 180° around then they’d have to risk trying to fire the SM’s main engine to do a direct abort (negate their velocity and accelerate back toward the Earth).[/ul][/li]If the SM was completely dead (or blew up) I don’t see many options left. Doing an EVA over to the LM, climbing inside, and maneuvering it to the CSM instead sounds beyond the scope of reality (take too much time and oxygen, probably impossible to do to begin with).
I thought that is what the Gemini space walks were all about. :dubious:
The Lunar Module was stowed in the interstage above the Saturn V Instrument Unit. The interstage had to be petalled before the LM could be extracted. This was done just before rendezvous, essentially as shown in the Ron Howard film. Moving from the CSM to the LM would require an untethered spacewalk, something that was neither attempted, nor were the Apollo 13 astronauts equipped for it. The first untethered spacewalk on a NASA mission was on STS-41-B by by Bruce McCandless, who was equipped with the MMU. A previous attempt was made during Gemini 9A by Gene Cernan but proved too difficult to get into the USAF AMU. Most of the “spacewalks” in the Gemini program were just standing up out of the capsule, with just a few tethered spacewalks that turned out to be much more taxing and difficult than anticipated.
Stranger
Ed White used a maneuvering unit in a space walk. He was indeed tethered with umbilical cables. His friend in the other seat had a pair of shears to cut his umbilical if White could not get back into the capsule.
I thought one of the considerations to provide access to the LEM was EVA.
Perhaps the poor performance in space walks changed NASA’s mind, although space walk EVA seems commonplace concerning the space station.
Could you explain this in more detail, as if you are talking to complete techno-peasant (i.e., me? :)) Why would it have been bad if it had happened earlier? or in Lunar orbit? (I think I get why it would be bad if the module had already descended to the Moon; oxygen runs out for the guy in the Command Module, right?)
Does that mean it would have been orbiting the Earth with a long swing out to the moon? Again, techno-peasant. 
As always, I appreciate it greatly when the Stranger weighs in on these abstruse topics.
Ah. When I’ve read about skipping off the atmosphere, it makes it sound as if that’s it, the capsule is now in free orbit around the Sun, never to be seen again. That’s not right? it will come back to Earth again at some point?
Again, techno-peasant seeks ignorance fighting. 
In an emergency if the LM were not able to dock with the CSM, the CSM could be maneuvered near the LM and the astronauts could tether over, but there was never a plan to perform a spacewalk from the CSM to retrieve the LM. One of the Soviet moon programs did have a plan for a cosmonaut to perform a spacewalk from the habit module to their lander, but it was still attached to the habit module; on return, it would dock, and the cosmonaut would spacewalk back over. This eliminated the docking option and a fair amount of mass, but was judged to be more risky overall. Since the Soviet crewed lunar program never got past a development stage this was never performed in reality.
The A7L suits developed for the Apollo program were really designed for Lunar operation and were not well suited for spacewalks. The suits used for Skylab were modified to make them slightly more agile and reduce some of the complexity not needed in freefall, but the still had some issues. For the STS program, the Extravehicular Mobility Unit (EMU) was developed with provisions specifically designed for freefall use and integrated propulsion system (the MMU and the SAFER) to allow untethered operations, although all operations on the ISS are tethered and using the Canadarm2 to maneuver the astronauts around the station, with the SAFER as just an emergency self-rescue system.
And a nitpick, while it is pronounced “Lem” the proper nomenclature for the Lunar Module is LM. Early concepts were dubbed “Lunar Excursion Module” as were some later landing/exploration systems for the now cancelled Constellation program, but the Apollo lander was always designated as the LM.
Stranger
If the rupture of the LOx tank (which provided oxidizer to the fuel cells, and thus provided primary power) had occurred earlier before the CSM separated from the S-IVB and retrieved the LM, the CSM would have lost everything but battery power. Because of the extent of the damage (albeit unknown at the time since the astronauts could not see the Service Module) restarting the SM main engine was not possible. And if it had occurred in Lunar orbit, even before the LM descended, I don’t know if the crew would have been able to make a precise Trans-Earth Injection using the descent engine even if the descent engine had enough impulse. The much smaller ascent engine certainly wouldn’t have had enough impulse to make the course corrections that were made (multiple corrections, unlike the portrayal in the film). As it was, they were in a free return trajectory and only needed the still fully-fueled descent engine to provide fairly minor course corrections. At any other point in the mission from TLI to just up to terminal EDL, the failure experienced by the Apollo 13 SM would likely have been unrecoverable.
An object can only escape the Earth’s sphere of influence (SOI) if it has sufficient orbital energy called the characteristic energy, C3 > 0. The CSM never had enough energy to leave the Earth’s sphere of influence, but if it missed injection point and came in too shallow it would “bounce” and be deflected into a long, elliptical orbit. Remember that to get to the Moon the spacecraft has to essentially match the trajectory of the Moon, most of which is imparted in the TLI maneuver. Upon return, it doesn’t have enough fuel to slow down into a circular Earth orbit; it comes screaming in ass first and burns away all of the excess momentum through compressing the atmosphere in front of it. Come in too shallow of an angle, it loses too little energy and goes into orbit. Too deep of an angle and it sees excessive heating and burns up. It’s kind of a crazy way to run a circus, but until we have technomagical propulsion systems with no impulse limits, it’s about the only way to slow down on return.
Stranger
So what’s happening on the return from the moon is that they’re in a highly elliptical orbit around the earth where the apogee (highest point) is at lunar altitude and the perigee (lowest point) is in the atmosphere. When they enter the atmosphere, the aerodynamic drag results in retrograde acceleration (they slow down). Now, retrograde acceleration at perigee doesn’t much alter your trajectory at perigee (much). What it does is lower your apogee.
So imagine a long ellipse with the earth at the bottom and the moon (or at least where the moon was a couple days ago) at the top. As you slow down via aerobraking, that ellipse shrinks down at the top. The bottom stays much the same, but the ellipse gets rounder and rounder as the apogee gets lower and lower, until, if you’ve entered the atmosphere deep enough, you’re in a circular orbit at probably something like 50km up. Of course, at 50km altitude you’re only technically in orbit. The atmosphere will continue to slow you and now instead of being at the lowest point of your orbit you’ll be at the highest point, still slowing, and you’ll come back to earth.
However, if you didn’t get low enough into the atmosphere, you won’t brake enough to bring your apogee all the way down inside the atmosphere. So maybe your apogee is still halfway to the moon. Now you’ll swing back out of the atmosphere and coast all the way back up to the top of your orbit, and come right back round. Then you’ll enter the atmosphere again and aerobrake some more, further lowering your apogee. Each pass will result in a lower orbit, with less energy.
Unless your perigee was completely out of the atmosphere to begin with, you’re still going to end up either burnt up or on the ground. Eventually. It could take rather a long time, though. Also, although I didn’t mention it above for simplicity’s sake, an aerobraking pass will also lower your perigee to some extent - retrograde acceleration that’s not right at perigee will lower your perigee as well as your apogee, although not to the same extent. This effect might result in your next aerobraking pass being too deep in the atmosphere for your heatshield to handle.
The precise results of course depend on just how deep or not deep into the atmosphere you’re actually going. In any event, “skipping” or “bouncing” are really not the right words. “Not slowing down enough to land this time round” is more like it.
Not all Apollo lunar missions were on a free return trajectory. It depended how close to the lunar equator they wanted to land. Sure, if there was no Lunar Orbital Insertion (LOI) burn a CSM/LM would get slingshotted back towards Earth, but some some more serious mid course corrections might be necessary for an originally non-free return.
That’s true, and was one of the things that limited exploration above certain latitudes. Let’s just say that if something dire happens on the way to the Moon, such as the LOx tank explosion, you are damned lucky if you can actually recover from it. The notion of McGyvering your way out of problems hundreds of thousands of kilometers or more in space is pretty unlikely at best, hence why it costs so much and takes so much effort to send people to space and keep them alive.
Stranger
I read Lovell’s Lost Moon a couple years before the Apollo 13 film came out, so that’s like 25 years ago. I remember in the film Ed Harris’ Gene Kranz simply eliminates the SM main engine from possible use do to its unknown condition. Was it ever proposed/theorized that its engine probably was useless?
Something I remember from the book is that after the explosion, when they were considering the options, a large group of NASA engineers favored a different mode for direct abort. They wanted them to separate the Command and Service modules (with the Command capsule still connected to the LM, like they did right before re-entry) and then use the LM’s descent engine to direct abort back to Earth. Without the huge mass of the SM attached the LM’s engine could have stopped and reversed the LM/CM and returned them to Earth in much less time (like less than a day I think*!*)
However the main (only really) reason they didn’t do this is because it would mean exposing the CM’s heat shield to the thermal extremes of space (+200°F in the light and -200°F in the shadows) for the entire ride back. Like the parachutes, the heat shield ***had ***to work so they didn’t want to risk it.
Speaking of the parachutes, Lost Moon also details how the astronauts were going to jury-rig some ‘jumper cables’ in the CM to bypass the normal firing circuits of the parachutes for fear of the system having been damaged and not working correctly. Lovell says that it was a good thing they couldn’t make it work because it turned out if they had done this it probably would have had the opposite effect*!* (i.e. it would have caused them to not deploy and killed them).