Constellation Lunar Launch Options

[tuvas]

Just saw this video, thought I'd hear what you all had to say about it.

http://www.space.com/php/video/player.php?...stellation-rock

It looks to be quite interesting. I must say, the computer animation is amazing too!

[dvandorn]

Well, let's do a little back-of-the-envelope calculating, here, then.

In Carrying the Fire, Mike Collins notes the formula for acceleration G forces -- his SPS engine on Apollo 11 developed about 20,500 lbs of thrust (sorry for the Imperial units, they're what I learned all this stuff in) and when it was used to slow Columbia and Eagle into lunar orbit, it generated roughly one-fifth of a G. Collins set it up thus: 20,500 lbs of thrust is about one-fifth of the total mass of the spacecraft at ignition (something over 100,000 lbs), and so the G force of the acceleration will be about one-fifth.

So, the uprated J-2X engine generates what, about 240,000 lbs of thrust? The heaviest Apollo TLI stage massed, at ignition, something on the order of 317,000 lbs. I have no idea what the current concept is for the mass of the Constellation TLI stage at ignition, but I'd have to guess it in the range of 500,000 lbs or less.

So, at ignition, with a J-2X and a roughly 500,000 lb TLI stage, acceleration at ignition would be roughly a half a G. That would increase to something like two-thirds of a G to nearing a full G by the end of the burn, I would imagine.

It might not be quite as much of a shock as depicted in this excellent CGI animation, but still -- it's not trifling. The best analog anyone has experienced would be the Gemini flights in which the Agena main engines were ignited while docked. Those crews hung on through one-G eyeballs-out burns, plastered to their straps. And no one ever complained.

-the other Doug