Talk:ITS launch vehicle/Archive 1

Archive 1

Raptor's isp numbers.

@Solardays, N2e, BatteryIncluded, and JFG:, I believe that SpaceX did, in fact, disclosed all relevant isp numbers. The first stage isp is revealed as 334 seconds. Then, for the second stage, they disclose the vacuum isp for sea level and vacuum version of the Raptors. Thus, I say that we know the isp at SL and Vacuum for the first stage. The problem for the two different upper stages, is that they have multiple isp. If they use only the vacuum optimized, they would get 382s, if they used only the sea level optimized, they would get 361s, but using both they would get something like 374.7seconds. — Baldusi (talk) 20:37, 3 October 2016 (UTC)

Now that I've been pointed out on another forum, it would seem that the whole confusion is from the fact that in the ITS presentation, the upper stage isp is presented in a confusing way:

Raptor Engines: 3 Sea Level 361 sec
9 Vacuum 382 sec

For a neophyte it would seem like they stated the isp at sea level and in vacuum. But from the context, and the fact that you simply can not reach 361 seconds at sea level with methane/LOX propellant, it is clear that they meant the vacuum isp for the Sea Level and Vacuum version of the Raptor engines. That and the fact that they start the line with the number of each type of engine and the upper stage is supposed to be used in vacuum. — Baldusi (talk) 21:19, 3 October 2016 (UTC)

@Baldusi: all good! You corrected me when I mistakenly put the 381s value back for the first stage, so thank you. It's this kind of friendly back & forth that collectively weeds out errors and makes collaborative endeavors worthwhile.  :) Solardays (talk) 21:32, 3 October 2016 (UTC)

And yes, I am reading the PDF slightly differently than you, which is fine. When SpaceX claims 'sea-level 361s ISP' I read it as exactly that. Consider this: 1) these are targets and ambitions, obviously, and not yet realized, and 2) there are some interesting qualifiers involved. Specifically, the fact that those three engines will never be used, at Earth sea-level, for lifting mass from velocity=0. The only lifting will take place on Mars. The only time those engines will be operating at Earth sea-level is in the final moment before the craft lands.Solardays (talk) 21:46, 3 October 2016 (UTC)

Yes, that is the strength of Wikipedia. Cheers, BatteryIncluded (talk) 21:46, 3 October 2016 (UTC)

BTW, there is a quick way to get the vacuum isp of the ITS. You have the Sea Level isp (334s), plus the sea level thrust (128MN) and the vacuum thrust (138MN). So you just do 334s × (138MN/128MN), and you get 360.1s. Given that those numbers are surely rounded up, I say that we keep the 361 of vacuum performance in the first stage.