Eve ascent efficiency.

[Vaporized Steel]

I've set myself a challenge a few days ago. I want to build a single launch rocket capable of landing 3 kerbals to eves surface (sea level or anything close within 300+/- altitude) and take them back to kerbin without using mods besides procedural parts and flight management mods like protractor, mechjeb and sort alike. and only by the use of chemical combustion (no nuclear rockets, ion engines. In my case that means any vessel under 500-600 parts upon launch considering my computer hardware which is quite decent. The last stage on the rocket is occupied by 3 external command seats. Some people argue that's unrealistic because of atmospheric conditions blasting on the pressure suits during ascent. I say bwah,... that's just my imagination of 23rd century pressure suits. I basically transformed my ascent module into a rocket stage with 3 seats with 2500m/s delta v excluding any kerbals sitting in it.

I will obviously use a eve rendezvouz aproach where I will leave the Kerbin return vehicle in eve orbit. I said Obvious because any other chemical combustion method is less efficient and I should change the topic title:sticktongue:

I'm really taking this "efficiency" goal quite seriously. So seriously in fact that I have come up with the following question.

At first I wondered what is the best TWR for each region of Eves atmosphere? I've come up with a chart of terminal velocities upon ascending from eves surface but don't know how to calculate The amount of TWR needed to reach that terminal velocity because I want to cut on total engine mass as much as I can.

If you want to share any additional tips on increasing efficiency which I might now know please dont hesitate to reply. By efficiency I mean "Mass" from Eves surface to orbit (not part count or size) So far I got a 25.8 ton vehicle with 10km/s delta v (2-2.5 km/s short for a sea level ascent)

Edited January 12, 2015 by Vaporized Steel

[OhioBob]

I haven't lifted off from Eve yet so take this advice with caution. A short time ago I was planning a manned mission to Eve that I haven't flown yet. In preparation for that I did some computer simulations to see if I could figure out the best TWR. I used three core stages stacked is series and three pairs of asparagus strap-ons. The following is what I came up with, in graphical form. TWR is based on Eve gravity. The first three peaks represent burnout of the strap-on pairs, while the last three peaks represent burnout of the core stages.

Note that my initial plan was to land at a high elevation somewhere, so the above is based on taking off from an elevation of 3000 meters. One of the reasons I never flew the mission was because I decided to change my design to land near sea level. I'm not sure how landing at the lower elevation will change the TWR. I was planning to rerun the simulations but I haven't gotten around to it yet.

Edited January 12, 2015 by OhioBob

[FlyingPete]

I've not attempted to land and return from Eve yet- though I've sent some one-way colonists, hopefully to be joined by more as the next launch window opens. Really the best way would be to have a dirigible of some sort to take the vehicle out of most of the atmosphere.

[Vaporized Steel]

According to your simulation you focus on a TWR between 1.5 through 2.5 on ascent throughout the atmosphere. That sends me back to my Original question which is what is the best TWR for every specific region of eves atmosphere.

In my scenario I have build a 4 core stage ascent vehicle with 2 strap-ons on the lower stages with the aforementioned delta-v with a TWR ranging from 1.35 at liftoff through 1.9 across all stages (around 1.5twr on average). The question remains what is the best TWR for every specific region of eves atmosphere.

Is your graph in fact simulated to be the best use of TWR taking into account atmospheric pressure (density altitude) at a given height (in your case 3000 meters)

If yes then I get a rough idea on what I should aim for.

The goal here is to cut on engine mass obviously to reduce engine weight and consequently fuel weight.

[OhioBob]

Is your graph in fact simulated to be the best use of TWR taking into account atmospheric pressure (density altitude) at a given height (in your case 3000 meters).

Essentially yes. What I did first was to settle on a basic design configuration - in this case it was the three central stages with the three pairs of asparagus strap-ons. I choose this arrangement because it gave me the ÃŽâ€V I needed. I then ran a series of simulations in which I could alter two key parameters: (1) the starting TWR of each stage, and (2) the distribution of propellant between the stages. I went through repeated iterations, adjusting one parameter, then the next, and then back to the beginning for another series of iterations. I just kept making adjustments until I could obtain no further improvement in performance. I figured at that point I had pretty much optimized the TWR because any further change I would make would lessen the performance. In the end my basic conclusions were the following:

Propellant should be distributed between the stages in roughly to the following ratios:

* Stage 3 = 1 (uppermost stage)

* Stage 2 = 5

* Stage 1 = 14 (core stage onto which the strap-ons are mounted)

* Strap-ons = 7 x 6 each

Liftoff TWR, all strap-ons + Stage 1: 1.60

Stage 1 TWR after jettison of the last strap-on pair (propellant tanks full): 1.40

Stage 2 TWR at ignition: 1.235

Stage 3 TWR at ignition: 0.80

Of course this is only theoretical and hasn't been proven in practice. I should also note that this is based on an ISP of 320 s sea level and 370 s vacuum for all engines. Results may vary with different engines, such as the aerospike.

Let me also specify that my definition of 'optimum performance' is 'maximum payload fraction'. For the simulation that I've been talking about, the payload fraction is about 0.00565. That's miniscule, but then Eve is a particularly tough place to launch from. The ÃŽâ€V of the simulated launch was about 10,770 m/s (from 3000 m elevation).

Although I haven't tested the above recommendations within the game, I have used a the same simulation techniques to try to optimize my Kerbin launch vehicles. I found that when I put those results to the test in gameplay, the launch vehicles performed identical to the simulations.

(edit to add)

Note that during the simulation I assumed that all engines are operating at 100% throttle all the time. I just took a look at my simulation results and I see that toward the end of Stage 1 burn (about 124-149 s elapsed time) the launch vehicle exceeds terminal velocity. In practice the engine should probably be throttle back during this time just enough to keep at or below terminal velocity. This would truncate that first tall peak in the TWR graph. By the time of the second tall peak at Stage 2 burnout, terminal velocity is large enough that there is no need to throttle back. This change should provide a small improvement in performance.

Edited January 13, 2015 by OhioBob