What is an "Ideal Ascent Profile"?

[guitarxe]

I have seen this mentioned a few times on the forums, usually when discussing rocket designs, under the context of something like "assuming an ideal ascent profile..."

What is it? I tried searching, but only got some very old posts discussing when to do a gravity turn.

Edited September 17, 2014 by guitarxe
change to answered

[Red Iron Crown]

It is the ascent profile that minimizes fuel used to orbit, generally speaking. The exact profile varies with rocket design (and whether you use aerodynamic mods).

[Alshain]

It's a little hard to describe so bear with me. This is a generalization, as Red Iron Cross said, each rocket is different. I use Kerbal Engineer to see these values.

1. Launch the rocket and throttle back as needed till your TWR is around 1.5 (better still, don't over build your rocket, TWR will only go up from launch if you never touch the throttle)

2. At roughly 1000 - 2000km, very slowly start to turn the rocket over, and pull the prograde down.

3. Keep adjusting your throttle down to STAY at ~1.5 TWR as shown in engineer.

4. If your doing it right, by about 10-12km you should pass 45 degrees, keep going, but now the prograde will move on it's own. Start to follow it rather than pulling it (keep your craft at the back edge of the marker now)

5. Keep adjusting the throttle but now you want to start watching your time to apoapsis. Your goal now should be to stay 1 minute from Apoapsis. Your trim this two ways, the closer to the artificial horizon your craft is pointed, the slower the time to apoapsis will climb (or decrease even). Once your just slightly above that horizon, use the throttle, keep it at 1 minute till but NEVER power off the engine. It's gonna seem like your burning forever (this method takes a lot longer) but your burning at almost no throttle which means your actually saving fuel.

What does this do? Ultimately the most waste of fuel is your circularization burn. With other methods, you will first go up, then once your up you will circularize. It's like running bases on a baseball diamond, when it's faster to go over the pitchers mound. That's what THIS method does, your raising both Apoapsis and Periapsis simultaneously, this results in a circularization burn that takes seconds not minutes. Sometimes when I launch I will have a 71k Apoapsis and a 65k periapsis, circularization is nothing.

Note, for planes, it's exactly the same after shutting off the air breathers.

This works in both stock and with FAR/NEAR btw. You will find quite a bit more fuel left over.

Edited September 17, 2014 by Alshain

[Pecan]

...1. Launch the rocket and throttle back as needed till your TWR is around 1.5 (better still, don't over build your rocket, TWR will only go up from launch if you never touch the throttle)

...

3. Keep adjusting your throttle down to STAY at ~1.5 TWR as shown in engineer.

...

I take it you're using FAR? Without it anyone will do much better to reach and maintain terminal velocity, which requires a TWR over 2 on average, during the course of each stage. Circularisation stage, being space-only, doesn't need anything that much thrust, of course, but if you've got it you might as well use it ^^.

mhoram did a lot of work on calculating ideal ascent profiles so hopefully he, and Kashua, will pop in soon ...

[Alshain]

I take it you're using FAR? Without it anyone will do much better to reach and maintain terminal velocity, which requires a TWR over 2 on average, during the course of each stage. Circularisation stage, being space-only, doesn't need anything that much thrust, of course, but if you've got it you might as well use it ^^.

mhoram did a lot of work on calculating ideal ascent profiles so hopefully he, and Kashua, will pop in soon ...

I've done it in stock and FAR. In stock Terminal Velocity might be a better indicator, but this method is still better than "go to 10k and turn 45 degrees till you hit 70k" for either aero package.

[Red Iron Crown]

I never reach terminal velocity in most of my ascents, but I chase payload fraction rather than minimizing dV used.

[Pecan]

I've done it in stock and FAR. In stock Terminal Velocity might be a better indicator, but this method is still better than "go to 10k and turn 45 degrees till you hit 70k".

Yep :-) Simplicity is the only advantage of that advice - though it does still work for most ships, of course.

I was just wondering why you kept the thrust so low, since that increases gravity losses.

[capi3101]

In general, an ideal ascent profile is one that A) maintains a even balance between the force of gravity and the force of drag acting on the rocket (i.e. a 100% atmospheric efficiency - a stat KER shows on the SFC data tab) at all times and minimizes the amount of fuel you need for your final orbital insertion burn. Alshain's got the particulars for you, though I generally find that the 100% atmospheric efficiency part is achieved when the TWR is in the 2.2 to 2.3 range; at 1.5 the efficiency is inevitably less than 100%, meaning delta-V losses to gravity. I also try to keep the Apoapsis around 45 seconds ahead past 10,000 meters myself, and I generally don't turn from straight up until I'm at that level (due to how Kerbin's atmosphere works - folks regularly call it "pea soup" down in the troposphere). To pull off an ideal ascent profile takes a steady hand and a lot of practice. Or Mechjeb - I don't use Mechjeb myself but there is definitely something to be said for how it handles launches, and I'd recommend hitting up YouTube for videos of Mechjeb-guided launches.

EDIT: Shinobi'd. And for the record, I use neither NEAR nor FAR.

[Alshain]

Yep :-) Simplicity is the only advantage of that advice - though it does still work for most ships, of course.

I was just wondering why you kept the thrust so low, since that increases gravity losses.

As you suspected, because of FAR. If you over thrust in FAR, you can't turn over fast enough to get to the horizon. More lateral thrust is more valuable than less gravity losses.

[Pecan]

I never reach terminal velocity in most of my ascents, but I chase payload fraction rather than minimizing dV used.

Which is another good point for the OP - the ideal ascent profile also depends on what you're trying to optimise.

If you want to minimise the deltaV used to orbit you'll have as little fuel as possible in the launch vehicle to try to follow the terminal-velocity curve.

If you want maximum deltaV remaining once you've got to orbit you'll pack your launch vehicle with as much additional fuel as it'll carry as well as the payload, giving a lower TWR (my SSTOs are designed that way).

If you're using non-stock aerodynamics (eg; FAR) you just can't go too fast or turn too sharply or drag will tear you apart.

etc.

[O-Doc]

Become familiar with these numbers if you fly stock. That's as fast as you should be going(surface speed) at the listed altitudes. Any faster and you are wasting fuel.

[Alshain]

True, I usually do max dV remaining simply because I rarely do anything at 70k. Most of my stations are between 100k and 300k. If I'm going somewhere else, I do however try to minimize delta V to orbit, but I usually take that fuel I would be using and pack it in a higher stage so I can hop to another biome and for every little fuel I add to the upper stage, I need more to lift it. All in all I end up in the same scenario.

[EdFred]

It's a little hard to describe so bear with me. This is a generalization, as Red Iron Cross said, each rocket is different. I use Kerbal Engineer to see these values.

1. Launch the rocket and throttle back as needed till your TWR is around 1.5 (better still, don't over build your rocket, TWR will only go up from launch if you never touch the throttle)

2. At roughly 1000 - 2000km, very slowly start to turn the rocket over, and pull the prograde down.

3. Keep adjusting your throttle down to STAY at ~1.5 TWR as shown in engineer.

4. If your doing it right, by about 10-12km you should pass 45 degrees, keep going, but now the prograde will move on it's own. Start to follow it rather than pulling it (keep your craft at the back edge of the marker now)

5. Keep adjusting the throttle but now you want to start watching your time to apoapsis. Your goal now should be to stay 1 minute from Apoapsis. Your trim this two ways, the closer to the artificial horizon your craft is pointed, the slower the time to apoapsis will climb (or decrease even). Once your just slightly above that horizon, use the throttle, keep it at 1 minute till but NEVER power off the engine. It's gonna seem like your burning forever (this method takes a lot longer) but your burning at almost no throttle which means your actually saving fuel.

What does this do? Ultimately the most waste of fuel is your circularization burn. With other methods, you will first go up, then once your up you will circularize. It's like running bases on a baseball diamond, when it's faster to go over the pitchers mound. That's what THIS method does, your raising both Apoapsis and Periapsis simultaneously, this results in a circularization burn that takes seconds not minutes. Sometimes when I launch I will have a 71k Apoapsis and a 65k periapsis, circularization is nothing.

Note, for planes, it's exactly the same after shutting off the air breathers.

This works in both stock and with FAR/NEAR btw. You will find quite a bit more fuel left over.

How do I do this when my final stage is only .73TWR?

I hate blanket instructions. Using these instructions on some of my craft they will never make orbit. Some craft you need to make your turn later sometimes waiting until 15km, others earlier.

[Alshain]

How do I do this when my final stage is only .73TWR?

I hate blanket instructions. Using these instructions on some of my craft they will never make orbit. Some craft you need to make your turn later sometimes waiting until 15km, others earlier.

1. I believe you should read the second sentence of that post.

2. By the time you hit your final stage your not watching the TWR anymore, you watching time to apoapsis. Read #5 in that post (this is assuming your talking about a final stage in the ascent). Either that or you really do have a strange rocket design in which case you should read the second sentence of the post. Also be aware, that is referring to your throttle TWR which takes into account the craft's current atmospheric situation, not your TWR on the launch pad.

Edited September 17, 2014 by Alshain

[EdFred]

#5 Does not apply to my rockets, unless I severely overpower them, or want a ridiculously long circularization burn. It's .73 at staging, not on the launch pad.

[capi3101]

#5 Does not apply to my rockets, unless I severely overpower them, or want a ridiculously long circularization burn. It's .73 at staging, not on the launch pad.

Short answer to your question - it depends on how far into the ascent you are. If your TWR is anywhere below 1.0 during most of the ascent, you haven't got sufficient thrust to continue accelerating; you'll hit apoapsis and start coming back down. Now, if you're well into the ascent - say above 50,000 or so and already going about 2000 m/s, you might get away with a TWR that low. A low TWR also works when it comes time for your circularization burn, though by then you're not worried so much about getting up there as you are about staying up there.

[EdFred]

Short answer to your question - it depends on how far into the ascent you are. If your TWR is anywhere below 1.0 during most of the ascent, you haven't got sufficient thrust to continue accelerating; you'll hit apoapsis and start coming back down. Now, if you're well into the ascent - say above 50,000 or so and already going about 2000 m/s, you might get away with a TWR that low. A low TWR also works when it comes time for your circularization burn, though by then you're not worried so much about getting up there as you are about staying up there.

I'm below 1.0TWR starting at around 30km. Not 1.0 on the pad, 1.0 at 30km. MechJeb fails when I let it try with that rocket. It pushes the AP to about 70km, falls back to about 68km, and then pushes the AP on the far side of the orbit. Doing it manually, I move my AP from about 60km at 30km, and slowly keep pushing it higher so that my circularization burn is only about 6dV. My PE has come up to about 50km before I hit AP, and my dV remaining is more than when I let MJ do it's thing.

[Alshain]

I'm below 1.0TWR starting at around 30km. Not 1.0 on the pad, 1.0 at 30km. MechJeb fails when I let it try with that rocket. It pushes the AP to about 70km, falls back to about 68km, and then pushes the AP on the far side of the orbit. Doing it manually, I move my AP from about 60km at 30km, and slowly keep pushing it higher so that my circularization burn is only about 6dV. My PE has come up to about 50km before I hit AP, and my dV remaining is more than when I let MJ do it's thing.

Like I said, at 30KM, you don't need full throttle. #5 says to watch the time to apoapsis, not the TWR.

[EdFred]

Like I said, at 30KM, you don't need full throttle. #5 says to watch the time to apoapsis, not the TWR.

And what if your AP never gets out of the atmosphere? Doesn't really help a whole lot. Some designs you can't keep the AP a minute ahead of you. I've got one design where the AP is less than 30 seconds ahead, and degrades to about 4 seconds. You can't push it past a minute. So, again, #5 doesn't apply.

[Red Iron Crown]

Can we see your rocket EdFred, perhaps with the delta-V window open? That would give us a better idea of what you're working with.

[mhoram]

@Pecan Thanks for inviting me to join the discussion, but I belive that I can not provide any additional knowledge about the OP to what has already been said.

@EdFred

Keeping the Time to Apoapsis small during certain phases of the ascent can be efficient, since this is the most efficient way to rise the Periapsis. So I would not worry about being very near to the apoapsis especially since this means that you don't have too many engines.

And I agree that MJ is bad at bringing certain rocket-TWR-configurations in an efficient way into orbit - especially when low-thrust-stages are involved.

This brings me to the idea that it could be interesting to extend my Ascent research with PSOPT to include a display for "time to apoapsis" but this would require some serious calculating to be done.

Edited September 17, 2014 by mhoram

[Pecan]

I'm below 1.0TWR starting at around 30km...

Build a better rocket.

Or do you have deltaV stats to break existing records?

[EdFred]

There's nothing "wrong" with my rocket. Simply the best ascent profile for it does not fit into a cookie cutter ascent profile.

Feel free to play with it.

Oh, and I was mistaken earlier, I have a TWR of around .8 when the Skipper takes over, but that depends where you drop the Mainsails I suppose. Should have around 300dV remaining in the Skipper upon reaching orbit.

Edited September 18, 2014 by EdFred

[Alshain]

Then why are you arguing here. So you built the impossible rocket, good for you. We are trying to teach the OP on normal rockets and the launch profiles listed work well for those trying to learn.

[Pecan]

Or - to put it the other way around - the "ideal" ascent profiles only fit "ideal" vehicles. Yet another good point, although several of us have stressed that 'ideal' depends on design.

On the other hand, if I want to complain about all the "boring" advice when my vehicle has a TWR of 0.1 at launch I shouldn't be surprised that no-one seems to help me get off the ground. I'm not saying that's your case, but your design is, erm, stressing the boundaries, shall we say?