-
Best efficiency of rocket fuel expenditure (Q&A) by aero on 18 Jun, 2012 00:23
-
Given a two stage rocket with identical engines, and 3 identical tanks of fuel, will I get more payload with 1 tank of fuel on the first stage, and two tanks on the second, or will it be better the other way around, ie. two tanks on the first stage and one on the second?
I've always ass.u.me.d that I should put the bulk of the fuel in the first stage, but now I'm wondering about that. I've been playing this game,
http://forum.nasaspaceflight.com/index.php?topic=25943.0
and the game rockets work the other way. That is,
One tank second stage over a two tank first stage gives 164.2 km altitude
Two tank second stage over a one tank first stage gives 228.0 km altitude
I get a similar result when I configure 5 tanks ratioed 2/3 and 3/2, but I have to use boosters to get it off the launch pad. Oh, and all these launches were straight up, like sounding rockets.
So my question is, "Does this game reflect physics as it is known, or what?" -
#1 by Lars_J on 19 Jun, 2012 00:18
-
In your example - Are all other variables the same? Do both stages use the same engines at the same thrust setting?
-
#2 by aero on 19 Jun, 2012 02:12
-
In your example - Are all other variables the same? Do both stages use the same engines at the same thrust setting?
Yes, all like components are identical with the same set on each rocket, with the exception of the switch of one tank of fuel between stages. -
#3 by Lars_J on 19 Jun, 2012 04:11
-
I ran your tests, and got similar results. As other's have mentioned, the game is not 100% realistic - staging has heavy penalties. The game authors admit this.
To prove that point, I ran a 3rd test - with three fuel tanks on a single stage rocket. And it outperformed both the 1/2 an 2/1 fuel tank combinations.
(EDIT, and with a 4 tank SSTO I reached escape velocity) 
But it still is fun to play around with. (there may well be modded parts for the full version that have more realistic mass figures) -
#4 by aga on 19 Jun, 2012 06:20
-
(there may well be modded parts for the full version that have more realistic mass figures)
there are, or at least were, modded parts even for the free version that have/had more realistic mass and Isp figures
-
#5 by aero on 20 Jun, 2012 13:00
-
Thanks everyone. I went ahead and used the rocket equation to check out my problem. The answer is "Yes," you do get more Delta V in the real world by putting more of the available fuel mass on stage 2.
The key variable in my example is in the Isp of the engine at altitude (stage 2 engine). The Isp at altitude will be higher than the Isp of the engine on the launch pad (stage 1). This will be true, more or less, even with identical engines. So, yes, burn the fuel at altitude whenever possible.
(EDIT, and with a 4 tank SSTO I reached escape velocity) 