Optimal Ascent Profile for this spacecraft

[jqhullekes]

Finally! Collected a bunch of data points, using two of MechJeb\'s ascent profile settings. Lots of data, so I made a graph.

Sorry jqhullekes, but it turns out the right side of the \'Turn Shape\' slider is toxic! (Basically, as the slider gets moved to the right, for steeper angles, the graph peaks move left towards ground level and below it.) On the other hand, if your manual curve shape works better than MechJeb\'s, it might be just fine.

The best shape for MechJeb seems to be around 1/3 from the left. With this setting, and a \'Turn Start\' altitude of 9 km, my best run was 84.7 kg fuel remaining.

Cool data. Thanks.

[PakledHostage]

Finally! Collected a bunch of data points, using two of MechJeb\'s ascent profile settings. Lots of data, so I made a graph.

Awesome! Interesting that at least one of the human pilots is still doing better than MechJeb. I\'ll have to try again myself over the weekend.

I\'ve updated the leaderboard.

[Tarmenius]

Yes, very nice work, Zephram!

Now I\'m curious about whether different rockets really do have differnt ideal ascent profiles. Maybe I\'ll try a simple, two-stage setup and see how it compares...

[PakledHostage]

Now I\'m curious about whether different rockets really do have differnt ideal ascent profiles. Maybe I\'ll try a simple, two-stage setup and see how it compares...

My gut tells me that it will depend on the thrust-to-mass ratio of the stack. Rockets with lower ratios will probably do better with a longer vertical ascent phase. There\'s also the added complexity that modulating the throttle will help on rockets that have high thrust-to-mass ratios. (Ref. Mini Challenge: Max Altitude with this supplied spacecraft)

I\'ve been wrong plenty of times before, though... Let us know what you find out?

[Tarmenius]

Hmm, you\'re probably right about that. Instead of a two-stage rocket, I think I\'ll test the trust-to-weight ratio variable instead. Have one rocket with a high ratio and one with a low, then see whether their ideal ascent profiles are much different. And I\'ll definitely let you guys know what I find.

[closette]

Great work Zephram - you had to do about 40 launches to orbit as far as I can tell. Nicely presented data as well.

With this rocket\'s picthover start altitude being so low, I\'ve noticed that after I apply full thrust at ~ 10 km and then coast, the apoapsis starts decaying fairly quickly (say from ~80 km to 75 km if I judge it right). This makes me wonder if a better strategy might be to either:

- pitchover, but reduce thrust or even coast to a higher, less dense altitude for a while , and then throttle back up to 100%

and/or

- aim for a higher apoapsis in the first place, which takes the rocket on a shorter, steeper path out of the air. There might even be a small fuel savings if one does this. Imagine aiming for an apoapsis of 70 100 m for instance - you\'d be stuck in the atmosphere for a long distance and path length.

I\'ll be giving these a try, but sadly not as systematically as I would like. If someone can explain why a coasting segment in the atmosphere could never be optimal I would like to know!

Also, on the subject of other rockets, TWR is a useful parameterization but without editing .cfg files I don\'t see how to change the mass or weight without also affecting the drag.

[Cruisix]

I just started playing the other day and having a blast so i could do with a challenge. I do think i messed it up or did\'nt get it right to start with but give a fresh kerbal a chance so to speak.

I used MechJeb and ended up slight of 75k (i need to figure out course control actions still) and started a turn at 4km\'s up on the midlength turn and ended up with 83.4kg left to spare on fuel. I hope i fared well for a first attempt at defining kerbal laws..

UPDATE: Lost 12kg of fuel getting it to 75.5 so i need to retry that.. hehe

[Tarmenius]

Well, after doing several trial launches (nothing so extensive as Zephram\'s ) I found that as far as I could tell, the optimal ascent paths were identical in both the High and Low thrust-weight ratio rockets. The high TWR rocket just burned through fuel too fast. It had a hard time even reaching orbit unless the ascent profile was pretty near ideal.

closette- If you pitchover early then reduce thrust through denser atmosphere, you may lessen the force of drag moment-by-moment, but will end up increasing overal drag losses due to being in the atmosphere longer and slightly increase gravity losses due to a slower ascent. Coasting through the denser parts will mean having to spend extra fuel because you\'d end up fighting a certain amount of drag and gravity twice. I have also found that a steeper path out of the air will certainly reduce drag, but will increase gravity losses by even more and typically add some steering losses as well.

At least, those are my thoughts based on observations made during my trials in this challenge. Feel free to prove me wrong, it\'s been known to happen ;P

Cruisix- You did indeed fair well in your first attempt! Keep in mind that 10kg of fuel goes by in around a second at full throttle (don\'t know the exact time for sure), so we are all pretty close to the same efficiency.

[Cruisix]

Cruisix- You did indeed fair well in your first attempt! Keep in mind that 10kg of fuel goes by in around a second at full throttle (don\'t know the exact time for sure), so we are all pretty close to the same efficiency.

Thanks.. Tommorow i\'ll go have a more organised attempt at it, mainly going manual to see if i can reach Ap on a more enviromental effort of saving fuel.

[Zephram Kerman]

Thanks for the compliments everyone. I\'ve been wanting to investigate gravity turns ever since we tackled the Goddard challenge. So putting some time into it was just fun for me. (That, and I needed practice with Excel graphs.)

I\'m just speculating here, but the ideal curve probably varies according to both thrust/weight ratio and coefficient of drag:

• High TWR and low drag would probably balance out.

• If both TWR and drag are high, that craft would prefer a steeper ascent curve, to minimize air drag.

• If both TWR and drag are low, that craft would prefer a shallow-ish ascent curve, to minimize gravity drag while slower than terminal velocity.

So instead of being a speed limit, I\'m starting to think of terminal velocity as the target speed. Go full thrust whenever possible. If speed > Vt, pitch up (or throttle down if already vertical). If speed < Vt, pitch down (but only down to horizontal). Adjust this to compensate for your craft\'s particular coefficient of drag.

Does this sound right to anyone else?

[Cryphonus]

Hello!

Here is my first attempt with Mechjeb, Got 75.0 Kg Fuel left you can see the ascent stats and orbital information from screenshot, I will surely be trying for a more efficient ascent.

[Cryphonus]

Here is some improvement:

This time i had 81.9 Kg of fuel left in the tank. Again another screenshot included. This is just a matter of trade offs between 3 parameters. You gotta find the balance ;P

[Cryphonus]

Just did better with 85.5 Kg of fuel left. Is there anyway to attach a pic with modifying my last reply. I\'m a forum newb :/

[jqhullekes]

New record 86.3kg - no MechJeb

75.0 x 75.2 km

I did pretty much the same as last time.

[PakledHostage]

New record 86.3kg - no MechJeb

MechJeb fears you the way us mortals fear Chuck Norris!

[Cruisix]

Right.. I did\'nt have much time today but had a shot at getting at least more fuel conserving and this is what i ended up with. A nice coast with still 83.1kg to spare and still using Jebs brain. Next try will be without him

[Tarmenius]

Does this sound right to anyone else?

Sounds right to me, in a real-world sense. But if I remember how a craft\'s total drag is determined (the sum of each part\'s rating regardless of position), then the High TWR rocket I used would have also had a higher drag rating than the Low TWR rocket and therefore would have preferred a steeper ascent. But both ended up with the same profile as optimal, though given the fact that my tests weren\'t terribly extensive (six or seven launches each), that conclusion could very well be flawed. Perhaps some further testing is in order... *scratches chin*

Is there anyway to attach a pic with modifying my last reply. I\'m a forum newb :/

When you modify your post, there is the option to add attachments. Also, if you have the pictures hosted on a website like imgur or photobucket, you can insert them directly into the post by using the spoiler button (looks like a radioactivity hazard sign) then the image button (looks like a painting). You will end up with this: [ spoiler][ img] link to picture here Also, good job on the 85.5kg remaining!

[Edit]: I tried the original craft again without MechJeb. At an orbit of 75,387 x 75,954 I had 88.7kg remaining fuel

Of course, without the spaces

[closette]

That\'s a big improvement Tarmenius, I bet it will be hard to beat. Regarding TWR and drag, we know that for a vertical ascent, the fuel-optimum speed vs altitude results in an equal 'gravity loss' vs. 'drag loss', and I\'ve seen it suggested that the same 'equipartition' might also hold for an optimum 2D ascent.

So could I suggest that some of us non-Mechjeb leaderboard winners try the ascent with a MechJeb module installed and the 'Ascent stats' turned on, so that we can see how gravity loss and drag loss compare on these near-optimal paths?

[Tarmenius]

Thanks, closette. I was actually surprised when it turned out that well. It started as a 'what the heck, I\'ll try again for laughs' sort of thing.

I used the Ascent Stats when doing all my MechJeb trials, which is what I would base my decisions on for the following launch. I started with a profile that gave good results, then tweaked it little-by-little to watch the affects and scribble them on the notepad I keep on my desk. The effects were usually something like 'Well, I shaved a couple hundred m/s from the Gravity Losses, but gained it all back with interest in Drag and Steering losses.' Or vice-versa. Of course, like I said in an earlier post, I am still new to using MechJeb so there are quite possibly things I overlooked or failed to account for. For example, it wouldn\'t give the stats for ascents flown manually. I really wanted to know how my own method fared.

Anyway, I highly recommend using the Ascent Stats to help figure out where to try and shave off those precious m/s. It was an invaluable tool in my process.

[Cryphonus]

Thanks, closette. I was actually surprised when it turned out that well. It started as a 'what the heck, I\'ll try again for laughs' sort of thing.

I used the Ascent Stats when doing all my MechJeb trials, which is what I would base my decisions on for the following launch. I started with a profile that gave good results, then tweaked it little-by-little to watch the affects and scribble them on the notepad I keep on my desk. The effects were usually something like 'Well, I shaved a couple hundred m/s from the Gravity Losses, but gained it all back with interest in Drag and Steering losses.' Or vice-versa. Of course, like I said in an earlier post, I am still new to using MechJeb so there are quite possibly things I overlooked or failed to account for. For example, it wouldn\'t give the stats for ascents flown manually. I really wanted to know how my own method fared.

Anyway, I highly recommend using the Ascent Stats to help figure out where to try and shave off those precious m/s. It was an invaluable tool in my process.

I certainly agree! But there is just one thing that i can\'t deal with. If i tell my Mechjeb to start gravity turn at 10km it sometimes starts it at 10 but sometimes starts doing it at 20 or 15 or some random number. I just couldn\'t figure out if this is about the aerodynamics of my craft or a bug?

[Tarmenius]

Well, in my limited experience with MechJeb I did notice that if I had a steeper ascent with a Turn Start Altitude above 10km, it would make a small pitch adjustment at first and then the bulk of the gravity turn significantly later. So far, the timing of its maneuvers has been consistent if the settings have remained unchanged. I haven\'t yet used MechJeb on the larger, multi-stage designs, so I can\'t say how it handles them. My next set of trials will use my standard Mun-capable rocket to see whether its ideal profile is much different. It\'ll be interesting to see how different a (relatively) more massive rocket performs.

[jqhullekes]

My new personal record: 87.2kg - no MechJeb

75.2 x 75.1 km

I\'m now starting to turn at around 8000m (to 60 degrees) and when I reach 45 degrees I start slowly turning towards the horizon.

That seems to work a little better for me.

[jqhullekes]

Getting closer: new personal record: 87.9kg - no MechJeb

75.1 x 75.1 km

Just finetuning. Experimenting a little bit here and there.

[jqhullekes]

New record 89.5kg! - no MechJeb

75.0 x 75.2 km

Did it much more on feeling this time,

[closette]

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).