Countdown event timing

[BazBear]

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.

[ka9q]

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.

[Zakalwe]

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.

[Luke Pemberton]

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).

[Glom]

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.

[Peter B]

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

[Peter B]

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.

[Sus_pilot]

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."