ApolloHoax.net
Apollo Discussions => The Reality of Apollo => Topic started by: ka9q on February 10, 2015, 11:39:21 AM
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Ever since I was a kid watching launches in real time, I've always wondered how and why certain pre-launch events are scheduled as they are. Obviously things like propellant loadings have to be done on a certain sequence well in advance of launch, with time allowed for chilldown and such, but what about the timing of events closer to launch?
I'm thinking of the swingarm retractions, the switch to internal power, and the switch to internal guidance (which I assume means "guidance release"). The CM access arm was retracted several minutes before launch, one of the S-IC arms retracted some seconds before launch, but most didn't retract until after first motion. That has always seemed somewhat dangerous to me, given the consequences of either an umbilical disconnect failure or failure of an arm to retract. These risks would be avoided by retracting the arms just before liftoff so you could still abort the launch if one fails to do so.
It just occurred to me that if you did retract the arms before liftoff and then had to hold, how would you empty the propellant tanks for a scrub? Maybe that's the reason -- once the arms retract, you're committed so you just have to make sure they work.
But why was the CM access arm retracted so soon? Was this to get out of the way of an LES firing in the event of a sudden emergency in which they wouldn't have time to climb out of the cabin anyway?
The switch to battery power occurs at T-50 seconds. Here the tradeoff is fairly obvious: you want to transfer as late as possible to conserve them, especially if you then have a hold, but at the same time you want to see them perform under load long enough to see that they're in good shape, so you can hold if they're not.
I assume "internal guidance" or "guidance release" refers to the moment that the inertial platform in the IU is physically uncaged and allowed to do its thing. I believe it's also the moment at which the launch REFSMMAT (the space-fixed coordinate system used by the platforms in the IU and CSM) has its X axis pointed straight up from the launch site. I can see why you wouldn't want to do this too early, as it would allow the platform to drift, but why T-17 sec, specifically?
I can imagine that because there was so much manual prelaunch monitoring of the Saturn V (remember those shots of rows and rows of engineers at consoles in the KSC firing room) enough time had to be allowed after each event for the person monitoring it to call a hold if necessary. I suspect that with computer monitoring of modern launch vehicles, a lot of these events can be scheduled much more tightly with respect to liftoff.
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I seem to remember a shuttle launch being scrubbed, after the shuttle engines were lit.. Or am I imagining that? Must have been pretty late on the countdown clock.
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It just occurred to me that if you did retract the arms before liftoff and then had to hold, how would you empty the propellant tanks for a scrub? Maybe that's the reason -- once the arms retract, you're committed so you just have to make sure they work.
That is what I have understood from reading about Apollo. They could safely scrub the launch any time before the hold down release and empty the tanks before sending in a ground crew to remove the astronauts. But what a nervous time that would be sitting on top of that now failed candle with no goal to occupy/distract your mind but getting out of it.
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I seem to remember a shuttle launch being scrubbed, after the shuttle engines were lit.. Or am I imagining that? Must have been pretty late on the countdown clock.
Flight 41D on 26 June 1984. Canceled at t-6 due to failure of an engine to start due to a stuck LOX valve. Apparently a fire burned at the bottom for some time because of the excess hydrogen in the pad area.
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I seem to remember a shuttle launch being scrubbed, after the shuttle engines were lit.. Or am I imagining that? Must have been pretty late on the countdown clock.
Flight 41D on 26 June 1984. Canceled at t-6 due to failure of an engine to start due to a stuck LOX valve. Apparently a fire burned at the bottom for some time because of the excess hydrogen in the pad area.
Thanks for confirming I'm not senile (yet), just found the video of the scrub on youtube too. :)
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I seem to remember a shuttle launch being scrubbed, after the shuttle engines were lit..
Gemini 6 as well. Its first launch attempt on 12-Dec-65 was scrubbed when the first stage was automatically shut down one second after ignition. The automatic shut down occurred because an electrical umbilical separated from the vehicle prematurely. The successful launch of Gemini took place three days latter on 15-Dec.
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That is what I have understood from reading about Apollo. They could safely scrub the launch any time before the hold down release and empty the tanks before sending in a ground crew to remove the astronauts.
The timing of the hold down release amazes me, especially over the last few seconds with the retraction of the remaining swing arms. If anyone gets a chance to watch Moon Machines - Saturn V, the engineers talk about the complexity of the problem. It's incredible to hear their account of the engineering.
But what a nervous time that would be sitting on top of that now failed candle with no goal to occupy/distract your mind but getting out of it.
I would just twiddle my thumbs. Honestly :o
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I seem to remember a shuttle launch being scrubbed, after the shuttle engines were lit.. Or am I imagining that? Must have been pretty late on the countdown clock.
AFAIK, this happened five times on the launch pad in the Shuttle's history
http://www.spacesafetymagazine.com/spaceflight/launch/shuttle-launch-pad-aborts/
STS 41D (Discovery) 26/06/1984 - #2 and #3 started, #1 didn't.
STS 51F (Challenger) 12/07/1984 - Coolant valve problem with #2
STS 55 (Columbia) 22/03/1993 - #3 engine failed to start correctly. Launch pad abort 6.5 second before SRB ignition.
STS 51 (Discovery) 12/08/1993 - Main engine start abort due to instrument failure.
STS 68 (Endeavour) 18/09/1994 - Triple engine shutdown 1.9 seconds before SRB ignition.
In all cases these aborts happened after the main engines started and just a few seconds before SRB ignition. As near as I can tell, all these shutdowns were commanded autonomously by computer. There are people here more knowledgeable than me who could probably elaborate on what would/could have happened if these engine shutdowns occurred after SRB ignition (or if that was even possible), but I can't imagine it would be anything good.
NOTE: There was also an "Abort to Orbit "with STS 51F when it eventually lauched two weeks later when No 1 engine failed almost 6 minutes into the flight. This resulted in the planned orbit not being reached. AFAIK the lower orbit was not mission critical.
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STS 68 was the one I had in mind, just watched the video.. :) Thanks for that.. :)
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STS 68 was the one I had in mind, just watched the video.. :) Thanks for that.. :)
I would NOT want to be sitting in that cockpit when that happened.
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There are people here more knowledgeable than me who could probably elaborate on what would/could have happened if these engine shutdowns occurred after SRB ignition (or if that was even possible), but I can't imagine it would be anything good.
Once the solids ignite, there's no turning back. (You hear the term "launch commit" during Saturn V launches, but I don't think I've ever heard it during a shuttle launch. Nevertheless, you are definitely committed once the solids ignite. I think the hold-down bolts are fired at almost exactly the same time, but I'm not sure which is first.)
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Once the solids ignite, there's no turning back.
I know the design of SRBs is not simple, for example there was a recent discussion about the propellants being shaped to give a thrust profile. Crudely speaking, are you saying that once lit there is not much difference between an SRB and a firework or a homemade glucose rocket motor?
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Once the solids ignite, there's no turning back.
I know the design of SRBs is not simple, for example there was a recent discussion about the propellants being shaped to give a thrust profile. Crudely speaking, are you saying that once lit there is not much difference between an SRB and a firework or a homemade glucose rocket motor?
Once fired, they either burn to completion or are destroyed by the range safety system. You can't stop them.
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The STS SRB's when firing seem to me to be just barely short of an explosion. Many observers have been critical of their use on safety grounds (for the reasons stated by ka9q and AllanF) yet ones of a similar design look set to be used in the SLS that will put the Orion spacecraft in orbit with a crew in the next decade
(http://upload.wikimedia.org/wikipedia/commons/9/9d/SLS_configurations.png)
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The STS SRB's when firing seem to me to be just barely short of an explosion. Many observers have been critical of their use on safety grounds (for the reasons stated by ka9q and AllanF) yet ones of a similar design look set to be used in the SLS that will put the Orion spacecraft in orbit with a crew in the next decade
At least with a capsule configuration you have abort modes while the solids are firing. With the shuttle, the crew was along for the ride until they burned out.
IMHO the main reason for using large solids is to form a subsidy to Thiokol, keeping them in the large solid rocket business so they can manufacture storable SLBMs. Many would disagree with that opinion though.
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The STS SRB's when firing seem to me to be just barely short of an explosion. Many observers have been critical of their use on safety grounds (for the reasons stated by ka9q and AllanF) yet ones of a similar design look set to be used in the SLS that will put the Orion spacecraft in orbit with a crew in the next decade
Yes, and this also bothers me. To see why, watch this video of a failed Delta-II/GPS launch (you may have already seen it):
This is a much more typical SRB failure than what happened on STS-51L (Challenger). I believe one of the things that doomed the Aries 1 was an Air Force study that showed this kind of SRB failure would probably be unsurvivable even with a working LES because the parachutes would likely be destroyed by burning chunks of solid propellant. I don't know if there's been a corresponding analysis of the Orion atop SLS.
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Of course, not every failure of a rocket with SRBs has been their fault. This 1986 Delta failure was caused by premature shutdown (at 1:55) of the liquid fuel engine in the core stage. Its gimbal provided the only roll and pitch control. This is an excellent demonstration of what happens when the angle of attack gets too high around the time of max-Q.
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The STS SRB's when firing seem to me to be just barely short of an explosion. Many observers have been critical of their use on safety grounds (for the reasons stated by ka9q and AllanF) yet ones of a similar design look set to be used in the SLS that will put the Orion spacecraft in orbit with a crew in the next decade
Yes, and this also bothers me. To see why, watch this video of a failed Delta-II/GPS launch (you may have already seen it):
This is a much more typical SRB failure than what happened on STS-51L (Challenger). I believe one of the things that doomed the Aries 1 was an Air Force study that showed this kind of SRB failure would probably be unsurvivable even with a working LES because the parachutes would likely be destroyed by burning chunks of solid propellant. I don't know if there's been a corresponding analysis of the Orion atop SLS.
Holy Moly!!
I suppose the "fireworks" display is the aforementioned chunks of burning solid propellant falling back to earth.
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IMHO the main reason for using large solids is to form a subsidy to Thiokol, keeping them in the large solid rocket business so they can manufacture storable SLBMs. Many would disagree with that opinion though.
Not sure it is the main reason, but probably a consideration. There is a need in government contracting to spread the money around among various states and to appeal to various congress critters with broader agendas than the single project. And in this case, the work can fund research and other needs that seem sure to be applicable to the Defense department through the NASA budget. In the same view as your comment, the entire space program can be viewed as a corporate welfare/jobs program for aerospace engineers to help maintain the defense program.
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Wasn't the option of SRB's the only economically viable option to get around the thrust required for the shuttle? I expect it wasn't a decision that NASA necessarily liked.
Apology's for taking this thread a little bit away from the OP. :(
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Of course, not every failure of a rocket with SRBs has been their fault. This 1986 Delta failure was caused by premature shutdown (at 1:55) of the liquid fuel engine in the core stage. Its gimbal provided the only roll and pitch control. This is an excellent demonstration of what happens when the angle of attack gets too high around the time of max-Q.
I'm going to use this one to explain why they should pay attention to the NOTAM's around KSC and Vandenberg, among other places. It'd suck to have all that fall out the sky on to your aircraft.
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Here are two more launch failures involving launchers with SRBs:
18 April 1986, Titan 34D, Vandenberg AFB, KH-9 payload:
This was another SRB failure, caused by separation of insulation within one of the SRBs near a field joint and burn-through during flight, somewhat similar to the Challenger STS-51L failure.
12 Aug 1998, Titan IVA, Cape Canaveral AFS, NRO payload:
This one was more like the Delta/GOES-G failure in 1986 in that the cause was an electrical short, not an SRB failure. Power was momentarily lost to the guidance computer, which in turn provided control signals to the guidance platform. The platform tumbled. When power came back, the guidance computer commanded a hardover of the engines to compensate for a large, illusory attitude error and aerodynamic forces ripped the vehicle apart starting with one of the SRBs. This triggered the destruct system.
Destruct ordnance can be fired not only by a command from the ground, but automatically by the vehicle itself when breakwires detect that it's coming apart anyway.
Since these were both Titans that use hypergolic propellants in their core stages, the explosions produced huge clouds of toxic brown N2O4 gas (actually a mixture of NO2 and N2O4). This is the same stuff that gives smog its yellow-brown color. I don't think any western launch vehicle lower stages use it anymore, though it's still common in upper stages and in spacecraft.
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Here are two more launch failures involving launchers with SRBs:
Ya gotta luv rocket porn.
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I seem to remember a shuttle launch being scrubbed, after the shuttle engines were lit.. Or am I imagining that? Must have been pretty late on the countdown clock.
Mike Mullane described such an event in his memoirs. In particular, he described the effect on his wife who was watching several miles away.
You hear the ignition then all goes quiet, so it sounds a bit like an explosion. Not pleasant for the wife.
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Ya gotta luv rocket porn.
I've typed my witty reply once, but in good taste I'm not sure I can post it, so I won't risk a reprimand from LO.
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The STS SRB's when firing seem to me to be just barely short of an explosion. Many observers have been critical of their use on safety grounds (for the reasons stated by ka9q and AllanF) yet ones of a similar design look set to be used in the SLS that will put the Orion spacecraft in orbit with a crew in the next decade
The initial versions are to use Shuttle-derived solids. The later versions are to have higher performance boosters, two of the proposed options being liquids, one using F1B engines derived from the F1 used on the Saturn, one using a domestic, uprated version of a NK-33. The third option is another ATK solid.
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Whatever happened to earth orbit rendezvous? In the 1960s, you could make a reasonable argument that rendezvous is a tricky, difficult procedure you do only when you have to, and the one in lunar orbit was enough since it greatly reduced the total mass you had to lift into space.
But rendezvous is now utterly routine, unlike launching Saturn-V-class (and larger) rockets, and you could assemble a spacecraft of any desired size from enough launches on smaller (and already available and proven) launch vehicles. Case in point: the ISS.
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I thought it was a cost thing. Having a large LV capable of launching your entire payload in one go means cheaper operating costs that having to manage too missions.
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I thought it was a cost thing. Having a large LV capable of launching your entire payload in one go means cheaper operating costs that having to manage too missions.
I thought it was both factors to be quite honest, cost and lift. Without LOR it would have required two or more lifts to get the lunar package into space (mass factor), which would have cost more.
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That has always seemed somewhat dangerous to me, given the consequences of either an umbilical disconnect failure or failure of an arm to retract. These risks would be avoided by retracting the arms just before liftoff so you could still abort the launch if one fails to do so.
The swingarms are an important part of stability under unexpected wind loads right up to the instant of flight and engine thrust disagreement during ignition and runup. Umbilical disconnection is an art. Which is to say, it's an engineering discipline within rocket science in and of itself. Understandably it's biased toward fail-safety. Detanking is a big issue, as you guessed. But also with cryogenic propellants you typically want the option of restarting a topoff procedure if you need to cycle the countdown. At a certain point you close off the venting and allow the tanks to naturally pressurize, but for a recycle you may want to reopen the vents. That means you need the propellant lines connected right up to the scrub point.
Consider also deadfacing, which is often best accomplished with one-time actuators. This leads to the larger issue in that umbilical mating is a laborious process. It's not just a matter of swinging the arm back into place.
But why was the CM access arm retracted so soon? Was this to get out of the way of an LES firing in the event of a sudden emergency in which they wouldn't have time to climb out of the cabin anyway?
My impression is that the service swingarms (including the CM's) were very much more massive and couldn't benefit from the retraction methods used for the umbilical swingarms.
I can see why you wouldn't want to do this too early, as it would allow the platform to drift, but why T-17 sec, specifically?
You need several seconds of good data to know that the platform is released and responding. And beginning a few seconds prior to ignition, the telemetry bus starts getting saturated with propulsion system events. Even today we stagger individual engine ignition commands. And at T-7 seconds or so the vehicle is in full ignition rattle -- not a time when you want to be trying to calibrate the guidance platform. If you aren't sure by then whether you've got a good guidance system, then you don't want to light off the engines.
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I thought it was a cost thing. Having a large LV capable of launching your entire payload in one go means cheaper operating costs that having to manage too missions.
I thought it was both factors to be quite honest, cost and lift. Without LOR it would have required two or more lifts to get the lunar package into space (mass factor), which would have cost more.
I've read that the end-of-the-decade deadline was also a big factor in the decision to go with LOR using a single launch vehicle.
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LOR remains a good idea; it did save hugely on total launch mass, which made it possible to launch a mission on a single Saturn V. (It did make many of the crews rather sad; Armstrong and Aldrin said they wished they could have brought Eagle back to a museum).
My point is that the Saturn V remains an impracticably huge rocket, as shown by the fact that no other rocket that large has been built and successfully flown since Apollo, not even the short-lived Energia. It's not impossible to build one even bigger, but is that optimal? Even with LOR, nowadays you might want to add EOR just to be able to use smaller existing launch vehicles already regularly flown for other purposes, such as communication satellite launches.
Even if in theory a larger launch vehicle could yield a lower per-kg launch cost, this may not be borne out in practice if so few missions need it that it rarely flies.
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I thought it was a cost thing. Having a large LV capable of launching your entire payload in one go means cheaper operating costs that having to manage too missions.
I thought it was both factors to be quite honest, cost and lift. Without LOR it would have required two or more lifts to get the lunar package into space (mass factor), which would have cost more.
I'm sure that I read somewhere that NASA wasn't confident of getting multiple launches into LEO within whatever time-window was required. look at the problems that they had with Gemini when they were practising rendezvous with trying to get both launches off the pad in time. The cost of failure would have been high, as if the either launch had to be scrubbed then potentially you could lose both missions.
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I've read that the end-of-the-decade deadline was also a big factor in the decision to go with LOR using a single launch vehicle.
Yes, a little bit of research on my part and I would have found this. My reading of the situation now is that LOR enabled the use of rocket technology at the time to be used, thus meeting JFK's end of the decade goal. I don't think it is fair to say that there was one factor in making the LOR decision. LOR would have been financially appealing too given that it meant it meant one launch vehicle. I would guess that some cost analysis informed decisions during Apollo (defers to others).
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I thought it was a cost thing. Having a large LV capable of launching your entire payload in one go means cheaper operating costs that having to manage too missions.
I thought it was both factors to be quite honest, cost and lift. Without LOR it would have required two or more lifts to get the lunar package into space (mass factor), which would have cost more.
I'm sure that I read somewhere that NASA wasn't confident of getting multiple launches into LEO within whatever time-window was required. look at the problems that they had with Gemini when they were practising rendezvous with trying to get both launches off the pad in time. The cost of failure would have been high, as if the either launch had to be scrubbed then potentially you could lose both missions.
Was that really a problem? Sure the Atlas was flaky, but in a EOR situation, they would be using a man rated rocket that you'd like to think would have better reliability.
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Ever since I was a kid watching launches in real time, I've always wondered how and why certain pre-launch events are scheduled as they are. Obviously things like propellant loadings have to be done on a certain sequence well in advance of launch, with time allowed for chilldown and such, but what about the timing of events closer to launch?
I'm thinking of the swingarm retractions, the switch to internal power, and the switch to internal guidance (which I assume means "guidance release"). The CM access arm was retracted several minutes before launch, one of the S-IC arms retracted some seconds before launch, but most didn't retract until after first motion. That has always seemed somewhat dangerous to me, given the consequences of either an umbilical disconnect failure or failure of an arm to retract. These risks would be avoided by retracting the arms just before liftoff so you could still abort the launch if one fails to do so.
It just occurred to me that if you did retract the arms before liftoff and then had to hold, how would you empty the propellant tanks for a scrub? Maybe that's the reason -- once the arms retract, you're committed so you just have to make sure they work.
But why was the CM access arm retracted so soon? Was this to get out of the way of an LES firing in the event of a sudden emergency in which they wouldn't have time to climb out of the cabin anyway?
The switch to battery power occurs at T-50 seconds. Here the tradeoff is fairly obvious: you want to transfer as late as possible to conserve them, especially if you then have a hold, but at the same time you want to see them perform under load long enough to see that they're in good shape, so you can hold if they're not.
I assume "internal guidance" or "guidance release" refers to the moment that the inertial platform in the IU is physically uncaged and allowed to do its thing. I believe it's also the moment at which the launch REFSMMAT (the space-fixed coordinate system used by the platforms in the IU and CSM) has its X axis pointed straight up from the launch site. I can see why you wouldn't want to do this too early, as it would allow the platform to drift, but why T-17 sec, specifically?
I can imagine that because there was so much manual prelaunch monitoring of the Saturn V (remember those shots of rows and rows of engineers at consoles in the KSC firing room) enough time had to be allowed after each event for the person monitoring it to call a hold if necessary. I suspect that with computer monitoring of modern launch vehicles, a lot of these events can be scheduled much more tightly with respect to liftoff.
Murray and Cox go into this in considerable detail in "Apollo - The Race to the Moon", in the chapter covering the launch of Apollo 4. They foreshadow it several chapters earlier when they describe the MR-1 "four inch flight" mission which occurred in 1960 (http://en.wikipedia.org/wiki/Mercury-Redstone_1). The upshot of that mission's events was that Mission Control had a fully fuelled spacecraft sitting on the launch pad, the capsule's parachute hanging off the side just waiting for a wind to catch it and pull the spacecraft over, and no electrical connection between the spacecraft and the launch pad.
Essentially, the way M&C put it, until the Saturn V was completely ready to leave the launch pad (that is, five engines at full thrust), electrical connections had to be maintained so that Mission Control could regain control of the spacecraft if for some reason they had to abort before the hold-down clamps released; once the Saturn V moved even fractionally, it had to keep going - there was no way it was going to settle back on its pad like MR-1 had.
ETA: Murray and Cox also describe how the start-ups of the five engines were slightly staggered so that the spacecraft didn't have to deal with the shock of all five engines coming up to full power simultaneously. The Apollo Flight Journal web-site shows this in a graph on their page for the launch of Apollo 8 (along with so much other cool stuff): http://history.nasa.gov/ap08fj/01launch_ascent.htm
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Wasn't the option of SRB's the only economically viable option to get around the thrust required for the shuttle? I expect it wasn't a decision that NASA necessarily liked.
Apology's for taking this thread a little bit away from the OP. :(
I remember reading somewhere that NASA originally intended for the Shuttle to have liquid fuel boosters rather than SRBs. They would have been cheaper to operate than the SRBs, but they would have cost more to develop. But given NASA's lack of funds at the time the booster decision was made, they felt they had to go with the booster which was cheaper to develop.
I'm happy to be corrected by others with more knowledge.
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Ever since I was a kid watching launches in real time, I've always wondered how and why certain pre-launch events are scheduled as they are. Obviously things like propellant loadings have to be done on a certain sequence well in advance of launch, with time allowed for chilldown and such, but what about the timing of events closer to launch?
I'm thinking of the swingarm retractions, the switch to internal power, and the switch to internal guidance (which I assume means "guidance release"). The CM access arm was retracted several minutes before launch, one of the S-IC arms retracted some seconds before launch, but most didn't retract until after first motion. That has always seemed somewhat dangerous to me, given the consequences of either an umbilical disconnect failure or failure of an arm to retract. These risks would be avoided by retracting the arms just before liftoff so you could still abort the launch if one fails to do so.
It just occurred to me that if you did retract the arms before liftoff and then had to hold, how would you empty the propellant tanks for a scrub? Maybe that's the reason -- once the arms retract, you're committed so you just have to make sure they work.
But why was the CM access arm retracted so soon? Was this to get out of the way of an LES firing in the event of a sudden emergency in which they wouldn't have time to climb out of the cabin anyway?
The switch to battery power occurs at T-50 seconds. Here the tradeoff is fairly obvious: you want to transfer as late as possible to conserve them, especially if you then have a hold, but at the same time you want to see them perform under load long enough to see that they're in good shape, so you can hold if they're not.
I assume "internal guidance" or "guidance release" refers to the moment that the inertial platform in the IU is physically uncaged and allowed to do its thing. I believe it's also the moment at which the launch REFSMMAT (the space-fixed coordinate system used by the platforms in the IU and CSM) has its X axis pointed straight up from the launch site. I can see why you wouldn't want to do this too early, as it would allow the platform to drift, but why T-17 sec, specifically?
I can imagine that because there was so much manual prelaunch monitoring of the Saturn V (remember those shots of rows and rows of engineers at consoles in the KSC firing room) enough time had to be allowed after each event for the person monitoring it to call a hold if necessary. I suspect that with computer monitoring of modern launch vehicles, a lot of these events can be scheduled much more tightly with respect to liftoff.
Murray and Cox go into this in considerable detail in "Apollo - The Race to the Moon", in the chapter covering the launch of Apollo 4. They foreshadow it several chapters earlier when they describe the MR-1 "four inch flight" mission which occurred in 1960 (http://en.wikipedia.org/wiki/Mercury-Redstone_1). The upshot of that mission's events was that Mission Control had a fully fuelled spacecraft sitting on the launch pad, the capsule's parachute hanging off the side just waiting for a wind to catch it and pull the spacecraft over, and no electrical connection between the spacecraft and the launch pad.
Essentially, the way M&C put it, until the Saturn V was completely ready to leave the launch pad (that is, five engines at full thrust), electrical connections had to be maintained so that Mission Control could regain control of the spacecraft if for some reason they had to abort before the hold-down clamps released; once the Saturn V moved even fractionally, it had to keep going - there was no way it was going to settle back on its pad like MR-1 had.
ETA: Murray and Cox also describe how the start-ups of the five engines were slightly staggered so that the spacecraft didn't have to deal with the shock of all five engines coming up to full power simultaneously. The Apollo Flight Journal web-site shows this in a graph on their page for the launch of Apollo 8 (along with so much other cool stuff): http://history.nasa.gov/ap08fj/01launch_ascent.htm
The first time I read their description of MR-1, I had to laugh out loud when they said "and then everyone realized the escape rocket had to come back down."