Author Topic: Apollo Q&A  (Read 132611 times)

Offline nicp

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Re: Apollo Q&A
« Reply #320 on: 12/24/2016 06:57 PM »
The Wikipedia article on Apollo 8 mentions that the SPS engine needed 'coating' (edit: with a short burn)
or it could explode in a long burn.
Is that right? I don't recall reading that anywhere else and variants of that engine (which may differ a lot, I've no idea) are used on various rockets and spacecraft.
« Last Edit: 12/24/2016 07:00 PM by nicp »
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Online brickmack

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Re: Apollo Q&A
« Reply #321 on: 12/25/2016 03:43 AM »
The Wikipedia article on Apollo 8 mentions that the SPS engine needed 'coating' (edit: with a short burn)
or it could explode in a long burn.
Is that right? I don't recall reading that anywhere else and variants of that engine (which may differ a lot, I've no idea) are used on various rockets and spacecraft.

Not been able to find any sources mentioning that (and the citation in the wiki article doesn't mention it either)

Offline the_other_Doug

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Re: Apollo Q&A
« Reply #322 on: 08/27/2017 03:17 AM »
The Wikipedia article on Apollo 8 mentions that the SPS engine needed 'coating' (edit: with a short burn)
or it could explode in a long burn.
Is that right? I don't recall reading that anywhere else and variants of that engine (which may differ a lot, I've no idea) are used on various rockets and spacecraft.

Sorry for the really late response.

This story is told in the excellent work "Apollo" by Murray and Bly-Cox.  As the story goes, on Apollo 8, it came out that the combustion chambers of a couple of test SPS engines had failed when they were started with both banks open prior to having been fired after assembly.

It was decided that a short burn on one bank (*) was needed to "wet" (not "coat") the SPS combustion chamber before it was safe to commit to a long two-bank burn, like the lunar orbit insertion.

Since you couldn't guarantee you would need a mid-course correction on the way out to the Moon, you couldn't guarantee you would have a chance to perform a "chamber wetting" burn.  So they had to plan in a combination S-IVB sep maneuver and a counter-maneuver (to undo the first one) to make certain they got the one-bank burn they needed to wet the chamber.

Borman was apparently quite irate about this, but it needed to be done.  On later flights, they got around the issue by starting the first two-bank burn on one bank for several seconds, letting the chamber pressures steady out, and then bringing in the second bank.  But as of the Apollo 8 timeframe, it wasn't clear that this was a safe alternative.  Additional flight experience with the engine proved it safe -- including that gained on Apollo 8.

(*) -- The SPS engine had redundant fuel and oxidizer flow plumbing, down to separate symmetrical sets of injection ports, called Banks A and B.  Each bank was configured to be connected to separate sets of fuel and oxidizer tanks, but the tank-to-bank plumbing was, ISTR, flexible and could be cross-connected.  Each bank had separate sets of valves.  The idea was, if you lost one bank, you could fly the mission completely nominally on the other bank.  Of course, if you were down to one bank, you might be no-go for LOI just on the basis of the one-more-failure rule...
-Doug  (With my shield, not yet upon it)

Offline the_other_Doug

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Re: Apollo Q&A
« Reply #323 on: 08/27/2017 03:30 PM »
A good example of the possibility of the "one more failure" rule making an Apollo mission no-go for LOI just occurred to me -- Apollo 15.

Right after they separated the CSM from the S-IVB and docked with the LM, the 15 crew noticed a light had come on that shouldn't have -- the SPS thrust light.  That light indicated that the SPS was burning, when it wasn't.  By manipulating the various switches and circuit breakers both upstream and downstream of the circuit that lit the SPS thrust light, they were able to determine that there was a short in the upstream leg of the thrust switch (a backup means of starting the engine -- they could, with the right configurations, have started the engine just by flicking that one switch).  The short only led into the Bank B wiring, through what was called the Entry Monitoring System (EMS), a secondary system of accelerometers that was primarily used during entry for a control mode called "riding down the G's", to be used if the primary (CMC) and secondary (SCS) systems went out.

The upshot of it was that, when the engine was completely armed (i.e., the pilot valves were opened), Bank B would open up and start the engine before the computer commanded it.  But the wiring into Bank A was fine.  So, they ended up doing all of their two-bank burns by disarming Bank B, starting the engine normally under computer command, then bringing Bank B on by opening its pilot valves.  And about 10 seconds before the commanded end of the burn, they would disable Bank B.

Had the short circuit been in a different spot and Bank B been completely disabled, they were indeed looking at a mission rule that would have made them no-go for LOI.  It was a huge sigh of relief when they discovered that the glitching bank was still usable, and just required a small change in procedures for near-nominal engine firings.  And had Bank A gone out, Bank B was still quite usable; it just required a manual start and stop, something the crew proved they could do when they tested Bank B with an "SPS test" that was actually a mid-course correction maneuver... hit it manually, operating just the pilot valves via their circuit breakers, to within a tenth of a foot per second.

All that said, I imagine Dave Scott would have argued strenuously for a relaxation of the rules, so he and his crew could be allowed to proceed with the mission.  After all, it was Dave who, during Apollo 16, was in the MOCR arguing strenuously with Jim McDivitt that 16's degraded SPS secondary control mode was good enough to serve as backup, to allow that mission to move forward to a landing.  And he had a lot more skin in the game on 15 than he did sitting on the ground nine months later... ;)
« Last Edit: 08/29/2017 03:57 PM by the_other_Doug »
-Doug  (With my shield, not yet upon it)

Offline envy887

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Re: Apollo Q&A
« Reply #324 on: 09/19/2017 05:51 PM »
I would like to know about the micrometeoroid impact probability and the dangerous physical phenomenons the astronauts where exposed to (cosmic rays, radiation, etc).

Lots of people argue that we never went to the moon because of these things... I have no doubt we went to the moon, but would really like to know more about this since I know nothing...

The quick and dirty summary:

Van Allen belts - occurred soon after TLI and soon before entry, so velocity was still relatively high (passed through belts quickly); during outbound leg crew was confined to the (relatively) better shielded CM to minimize exposure.

Solar flares - just plain lucky that one didn't occur during an Apollo mission. There was one observed flare (between 16 and 17, I believe) that would likely have killed a crew had there been one up at the time.

Cosmic rays - crew undoubtedly took doses; effects minimized due to short missions.

Micrometeoroids - odds of impact very low over mission duration.

Bottom line is that future missions will still need to get crews through the Van Allen belts quickly (rules out low-thrust propulsion for crew) and will need better shielding for long-duration surface stays.

Re. the Van Allen belts: the Apollo TLI trajectory was quite inclined, and passed over the highest radiation parts of the belt which are more equatorial.

http://www.braeunig.us/apollo/apollo11-TLI.htm
« Last Edit: 09/19/2017 05:52 PM by envy887 »

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