Optimal Ascent Profile for this spacecraft

[closette]

I\'m trailing behind in the non-MechJeb leaderboard, but I bettered my own record to get 79.02683 kg remaining, with a 79.0 x 79.4 km orbit.

Yes I overshot again, which seems to be easier for me than going under 75km (where one barely scrapes out of the atmosphere at a shallow angle before doing the circularization burn).

Sorry I can\'t right-click to verify that fuel status on my MacBook, but I\'ve nothing to gain from fooling anyone!

[jqhullekes]

Any chance you could upload a video of your ascent, please? A 'boring' one with no changes to the camera angle, so that we can see how the pitchover angle varies with time. (And in fact there might be a way to measure it from the video).

After your response, I\'ve tried to make a video with Windows Media Encoder. I had all kinds of difficulties, crashes, hanging, slow game, really really bad quality video, no audio, until I gave up. I had a 'video' of a 88.3kg run, but it was soo low quality, I wouldn\'t show it my dog.

Has anybody got any suggestion for a free and fairly good video encoder for KSP? My PC isn\'t that good, so it has to be lightweight. The game doesn\'t run well on 1680x1050, so I usually run it on 1024x768.

Thanks in advance.

Otherwise I could simply do some screenshots along the way (but I\'m really focused during the flight, making F1\'s continuously a bit distracting...)

[jqhullekes]

There you go:

This is a video from a ascent with 88.4kg left (at 75.1 x 75.8 ). So quite a good one.

I\'ve added some annotations to the video, at crucial points.

And I recommend to watch it in 720p. Added some music.

[closette]

Nice! What video capture software did you end up using?

I see you were very careful with the circularization burn(s), I\'ll be more careful too.

[PakledHostage]

This is a video from a ascent with 88.4kg left (at 75.1 x 75.8 ). So quite a good one.

Nice flying! I tried a bunch of launches last night and got to the point where I can reliably get into orbit with about 87 kg fuel remaining, but I wasn\'t able to beat your or Tarmenius\' efforts. My results are probably still useful when compared to yours, though. I have been starting my pitchover into a gravity turn about 2500 m higher than you do; I\'m probably on the other side of optimal.

I\'ll try again this evening with your method.

[jqhullekes]

Nice! What video capture software did you end up using?

Fraps. But my files got too large, so had to do it over again, with a lower fps when recording, and then use VirtualDub to reencode the video, to make it smaller.

Ah well. A bit of a hassle, but it kinda works. Sound recording didn\'t work, but I got the music from youtube featured music. So it\'s a nice video.

[jqhullekes]

Nice flying! I tried a bunch of launches last night and got to the point where I can reliably get into orbit with about 87 kg fuel remaining, but I wasn\'t able to beat your or Tarmenius\' efforts. My results are probably still useful when compared to yours, though. I have been starting my pitchover into a gravity turn about 2500 m higher than you do; I\'m probably on the other side of optimal.

I\'ll try again this evening with your method.

I\'m actually quite unsure what the optimal heights are. I think there is a bit of room for improvement still. Although 100kg feels impossible. You never know though...

[stucker]

I have a 87.3kg Mech Jeb launch.

[Wild Card]

And yet, you didn\'t post the ascent profile?

[closette]

82.1 kg trying to follow the ascent profile in jqhullekes\' video, and I got the orbit in range for once at 75.9x75.5km (so the superscript can be removed from my leaderboard entry), and I\'m in the 80-something kg club at last!

But that\'s not why I\'m posting. A few things I noted:

1. During the coasting phase in one of my attempts, I saw the apoapsis 'decay' from 75500 m down to below 73000 m. I then had to decide whether to (a) re-boost while in the atmosphere (at ~30 000m) to an apopapsis above 76000m and let it decay back down, or ( wait until I was above 70 000m and boost then, or © something in between, i.e. wait until the atmosphere got thinner, then reboost.

Not sure what I did, but I was already out of contention on that attempt. Ideas as to which option would have been best, and why?

2. On my 82.1kg ascent I tried to follow the video closely, including the advice to keep the heading slightly to the left of the edge of the yellow ball. This means a slightly nose-up attitude in flight, perhaps more so than a real rocket can do due to aerodynamic stresses. Technically, a gravity turn 'follows the ball' exactly so that the most kinetic energy is gained per delta-v, which is what I was doing before, but this is clearly not optimal.

3. The initial pitchover at 8km has to happen fast and be 'just right' to set up the rest of the ascent profile. I suspect that may be the discriminating factor between the top few entries on the leaderboard.

Comments anyone?

[PakledHostage]

82.1 kg trying to follow the ascent profile in jqhullekes\' video, and I got the orbit in range for once at 75.9x75.5km (so the superscript can be removed from my leaderboard entry), and I\'m in the 80-something kg club at last!

Done.

1. During the coasting phase in one of my attempts, I saw the apoapsis 'decay' from 75500 m down to below 73000 m. I then had to decide whether to (a) re-boost while in the atmosphere (at ~30 000m) to an apopapsis above 76000m and let it decay back down, or ( wait until I was above 70 000m and boost then, or © something in between, i.e. wait until the atmosphere got thinner, then reboost.

I usually do it by method ( and it looks to me like jphullekes does too in his video. His initial apoapsis at MECO1 is above 73 km but his periapsis has decayed into the 72 km range when he\'s finished his orbit insertion burn. I\'ve tried (a) and © without seeing any improvement. I have a sense that the 'curvature' of the ascent trajectory between pitchover and orbit insertion burn is critical to realising the final few kg of improvements that are possible for this rocket.

[closette]

Thanks for the update and insightful comments. I am still left with a lingering question of 'What is the optimal ascent profile?', which would take some gnarly optimal control theory to solve. Are we all missing something that could put us in orbit with 90kg of fuel remaining?

It\'s a tough problem to solve. Most of the papers I\'ve read are quick to make a 'flat Earth' assumption, which does not work for a small planet like Kerbin, and of course they use a different drag law. Also most of them have constraints that we do not, such as on the allowed deviation of the velocity and pitch attitude for mechanical reasons.

I believe there is MATLAB code out there for solving the ascent problem (as a two-point boundary value problem) which could be adapted, if we have any MATLAB experts on the forum...

[PakledHostage]

Thanks for the update and insightful comments.

Or not... It was probably too obvious to say that the remaining optimisation can be realised by tweaking the ascent profile. But I\'m afraid that the mathematics of 'gnarly optimal control theory' are beyond me so my only remaining approach to optimising the ascent profile is through experimentation. Looking at jqhullekes\' video again, I see that he is more precise with his orbit insertion burn than I am. Even so, I am getting close to his results. I will give my pilot some training and give him another go.

In the mean time, I hope one of the experts out there will take on solving this problem analytically.

[Tarmenius]

'What is the optimal ascent profile?'

I personally think we\'re all pretty much there. Like I said in an earlier post, all of the non-MechJeb positions are within one second\'s burn time (at full thrust) of each other. Without the use of computer-controlled programs for throttle and heading changes, any ascent profile can only be fine-tuned so far. Of course, despite all that I\'m still going to try breaking the 90kg mark

As for your question about the coasting phase, option 'a' seems to imply that your MECO is happening below 30,000m. If that\'s the case, your ascent may be too steep. On my ascents, MECO was around 43,000m and my velocity was about 1400 m/s. While coasting, I would only lose 300 or so meters from my Ap. I\'ll be making another attempt or two in a few minutes to get more exact numbers because my memory could be a bit off and it bothers me some when I\'m that vague . At Ap -20s I would start circularizing, pitching up very slightly to raise Ap if needed. Toward the end of circularization, thrust is minimized then made in quick bursts to prevent Ap from running away from me. The key there is to keep Ap just off the nose of the spacecraft.

[Edit]: Fixed the numbers

[Kosmo-not]

I\'m curious about comparing the specific orbital energies just before the circularization burn.

Basically, I would be asking for a table of the specific energy before circularization and the amount of fuel remaining after circularization.

[tjoreilly]

I had an 89.35 with just a little over 2,000 m/s prior to the circularization burn. Kosmonot, I had been starting to get curious about the same thing. I killed the throttle at about 50km thinking I might try to shorten the length of time between raising the apoapsis to 75.5km and starting the circularization burn. This put my apoapsis at around 70km. I thought that, perhaps, I would try to incorporate as much altitude gain with circularization after getting beyond severe atmospheric drag.

[tjoreilly]

Oh! pictures.

Ascent profile: gravity turn starting at about 9km, full throttle until craft reached altitude of about 50km (apoapsis at about 70km). Then, modulated throttle just enough to keep from losing too much altitude from remaining atmospheric drag until I reached apoapsis. Then modulated throttle some more to circularize, never letting the apoapsis get more than +/- 5 seconds from craft. Very slight correction burn (200m) at periapsis.

[PakledHostage]

I\'ve updated the leader board, but that\'ll be it for me until after the weekend. Maybe a new record will be set by the time I get back.

Until then, Happy Canada Day to my fellow Canadians and an early 'Happy Independence day' to our neighbours to the south!

Canadian National Canthem

[Kosmo-not]

After I get home (whenever that may be), I\'ll be working on posting a new record. My first few tries on my laptop resulted in a personal record of about 82kg. I intend to do much better.

[Zephram Kerman]

I\'m curious about comparing the specific orbital energies just before the circularization burn.

Basically, I would be asking for a table of the specific energy before circularization and the amount of fuel remaining after circularization.

Challenge accepted. I plan to experiment with some ascent profiles using 8km turn start altitude, manually varying the aggressiveness of the turn start, and then a consistent MechJeb curve for the remainder of the ascent. For your data, I will add to this a measurement of S.E. before and after circularization.

...Canadian National Canthem

That\'s awesome.

[Kosmo-not]

I will add to this a measurement of S.E. before and after circularization.

Unnecessary. The S.E. after circularization will be approximately equal in every case.

[Zephram Kerman]

I\'m curious about comparing the specific orbital energies just before the circularization burn.

Basically, I would be asking for a table of the specific energy before circularization and the amount of fuel remaining after circularization.

Ok, here is a first run of the data you requested.

This experiment combined two subjects of interest: the effect of gravity turn start aggressiveness, and Specific Energy just prior to circularization.

The process for each run was to set up MechJeb for 75.5 km orbit, with Turn Start Altitude at 8 km, and the 'Turn Shape' slider at about 33%. I then turned on MJ Surface Mode at 90° pitch from launch until 8 km. Then I changed the surface mode pitch to the test value, and hit 'execute' at 8 km. When altitude reached 10 km, I switched on MJ Ascent Autopilot using the values set up earlier. Upon reaching 75.0 km, I took a screenshot to record the velocity and altitude, for use in calculating the Specific Energy.

I had trouble with the Excel graph not displaying the data point for the 80° run. Those values are: S.E. 1,133,270.355 J/kg, Fuel remaining 84.6 kg. Also of interest are the twin 60° data points; the lower-performance one was a MechJeb-only ascent as a control group, while the higher-performace one was my attempt to match it using Surface Mode to be consistent with the other runs.

The results are not very dramatic. The S.E. and fuel varied only slightly, probably within the precision of my button-click timing. I zoomed in the graph for better detail, but really it\'s almost flat. So it appears the aggressiveness of the initial turn is not as important as I had thought. If this is true, it brings more evidence to support PakledHostage\'s theory:

I have a sense that the 'curvature' of the ascent trajectory between pitchover and orbit insertion burn is critical to realising the final few kg of improvements that are possible for this rocket.

I think what we really need is a repeatable way to evaluate more curvature options than are available with MechJeb.

I\'ll hold on to the raw data for a while, just in case anyone has questions or I\'ve made a mistake somewhere.

[PakledHostage]

The results are not very dramatic. The S.E. and fuel varied only slightly, probably within the precision of my button-click timing. I zoomed in the graph for better detail, but really it\'s almost flat. So it appears the aggressiveness of the initial turn is not as important as I had thought.

You certainly stand out as the most methodical of the contributors to this challenge! Thanks for your efforts and for sharing your results.

Empirically, I\'ve found that my best results occur when I get set up on a trajectory that gets me above 38 km altitude relatively early in the flight, yet with low enough vertical velocity that I can continue to accelerate down range for an extended period before my apoapsis reaches 75 km. That requires a later pitchover and a more aggressive down range turn than just a gravity turn. A couple of times, I tried modulating the throttle to more closely follow a gravity turn trajectory, but it didn\'t work very well. I am able to reliably get into the 87 kg range, and I once achieved an 88.3 kg flight, but I haven\'t matched the top results yet.

That said, I agree with Tarmenius that we are definitely down to the short strokes here. The difference between an 88 kg flight and an 89 kg flight is only an eighth of a second at full throttle!

[bocaJ]

Hello all!

I was attempting to complete this challenge with the unofficial fuel consumption bug-fix in KSP v. .16, and I can't get into orbit with this ship configuration; either manually or with MechJeb. Am I doing something wrong, or is this challenge not achievable with my configuration?

[PakledHostage]

Welcome to the forums!

This rocket configuration doesn't have enough delta-V to reach orbit in v0.16 of the game. I am sure that efficiency challenges like this one will see a renaissance when the fuel bug gets fixed in v0.17. Until then, if you really want to try this challenge, you could download the demo version of the game and try it in that version...