Is the coast to apoapsis on launch realistic/necessary?

[photogineer]

Hey all,

Another question for you, I am just full of them lately. I am getting pretty decent at getting my rockets in to orbit. I can more or less hit the orbit I want every time now, with my light and heavy launch vehicles, My ascent profile is pretty straightforward:

1. Launch, burn out first stage boosters and liquid engines, stage them

2. Around 8km to 10km, depending on payload, pitch over to 45 degree gravity turn on launch azimuth.

3. Hold that attitude until second stage burnout, then move to track prograde marker as soon as Navball switches to orbit

4. Follow prograde burn until apoapsis is just about 1-2km above target and kill engines, if that stage isn't enough for insertion burn (almost always the case), ditch it while it's still suborbital.

5. Wait until coast brings me high enough that apoapsis stops dropping, usually settles right on my target, jump to map view and plan a maneuver node for my final orbit (circular, intercept phasing, etc, whatever the mission needs)

6. Coast to apoapsis and fire insertion burn

Also, on recent launches I've been experimenting with modulating the throttle to keep atmospheric efficiency (as measured by Kerbal Engineer) near 100%, until I'm high enough that I saturate at full thrust then I stop fiddling with it. It doesn't seem to be helping my dV budget that much, though it does make early launch a bit less tedious

This is working pretty well, but it makes me wonder, is the coast really necessary? Do real spacecraft do that? I was always under the impression that most craft burned solid from launch to parking orbit. I realize many craft will restart the engines later from their parking orbit for target orbits, and other operations, but is there usually a coast between liftoff and parking? If not, why is it that way in KSP? Is it just easier? Would a single burn to parking be more efficient or more challenging?

Thanks,

Matt

[numerobis]

It's that way in KSP because it's easier to design with too much thrust. You would save on your mass budget by having less thrust (i.e. smaller / fewer engines) and burning all the way up, staying prograde the whole way except when you do your pitchover maneuver, which should only be a few degrees.

[Tarrow]

Do real spacecraft do that?

Yes they do

The launch profile for the shuttle (as an example) has a coast phase of about two minutes, between the main engine shutdown / external tank dump and the firing of the orbital maneuvering system to circularize the orbit.

[maccollo]

Doing the ultimately efficient launch is very hard even when you have a launcher that is very well optimized for the payload.

The reason being that if you're burning at max thrust throughout the launch it becomes very hard to rescue the launch if your profile was just a little bit off.

[UmbralRaptor]

Yeah, coast phases happen. In real life it's more common with solid fuel stages (eg: Pegasus) or very late in the flight. (eg: the Centaur in an Atlas V or DCSS in a Delta IV will do multiple burns) Or for that matter, the Shuttle would do burns after MECO to reach a stable orbit.

[Taki117]

It's completely realistic because of the Oberth effect. Basically you want to coast to near apoapsis so that when you burn to circularize all you are doing is raising your periapsis, not anything else.

[Sirrobert]

I believe most rockets are designed with JUST the correct amount of thrust that the initial accent ends just as the apoaps begins. So they merge into 1 long burn.

If you use Mechjeb with a very shallow lauch profile this already happends a little bit.

Set a shallow profile, launch to a 200km orbit, and you might already have a 50km periaps by the time your apoaps is where you want it

[photogineer]

It's that way in KSP because it's easier to design with too much thrust. You would save on your mass budget by having less thrust (i.e. smaller / fewer engines) and burning all the way up, staying prograde the whole way except when you do your pitchover maneuver, which should only be a few degrees.

Everytime I've tried to do that I usually end up pitched all the way over burning without getting a higher apoapsis, and then scrubbing the launch, I am not sure what I'm doing wrong?

Yes they do

The launch profile for the shuttle (as an example) has a coast phase of about two minutes, between the main engine shutdown / external tank dump and the firing of the orbital maneuvering system to circularize the orbit.

Very cool, I didn't know about that, thanks!

Yeah, coast phases happen. In real life it's more common with solid fuel stages (eg: Pegasus) or very late in the flight. (eg: the Centaur in an Atlas V or DCSS in a Delta IV will do multiple burns) Or for that matter, the Shuttle would do burns after MECO to reach a stable orbit.

Again, cool, thanks.

Doing the ultimately efficient launch is very hard even when you have a launcher that is very well optimized for the payload.

The reason being that if you're burning at max thrust throughout the launch it becomes very hard to rescue the launch if your profile was just a little bit off.

I believe most rockets are designed with JUST the correct amount of thrust that the initial accent ends just as the apoaps begins. So they merge into 1 long burn.

If you use Mechjeb with a very shallow lauch profile this already happends a little bit.

Set a shallow profile, launch to a 200km orbit, and you might already have a 50km periaps by the time your apoaps is where you want it

Is this really that super difficult to do? Is it more difficult when your rocket isn't designed for one payload to one specific orbit? Is it worth the trouble? It sounds like a fun challenge, but not if it won't really net me anything

Thanks,

Matt

[EtherDragon]

Check out my ascent profiles at the end of this response - it will help answer some of your questions in KSP, during ascent.

As others pointed out, yes IRL there are often coasting phases for real launches, depending on the lifter and the cargo. But you aptly note that most rocket motors can be throttled, for good reason.

As far as coasting to Apoapsis being most efficient? Absolutely, yes. Simply put, in orbital mechanics - the most efficient way to affect your Periapsis is to do so at your Apoapsis and vice versa. This really has to do with conservation of energy, actually - Given a starting vector that passes close to a point, the further you are from that point, the less energy it takes to affect your course relative to that point. This is why people will do mid-course corrections for an encounter when they are reasonably far away, often times having not yet reached the SOI.

Likewise, changing your Periapsis is best done at the most distance you can - which happens to be the Apoapsis.

All that said, the most efficient flight profiles tend to be ones where the circularization burn and ascent burn appear to be a single long burn - but the Dv savings you get from doing so might not be noticeable in KSP.

I promised a video - this one is about Ascent Profiles, and why certain maneuvers are done at certain places... Some of this video is really basic stuff - but not everyone knows it all going in, so it's in there.

[arq]

There is nothing wrong with a coast phase. In many cases it may be more efficient than a direct injection.

For example, from about 40km up, terminal velocity is well-above orbital velocity. The gist of this is that it is possible to insert into a 40x<apoapsis> orbit from here without massive drag losses at periapsis. Let us assume for a moment that the drag losses are actually nonexistent at this point. Then the most efficient way to get into a higher orbit is via a Hohmann transfer. This means that you should pick up as much speed at your current 40km orbit as desired to get to your target altitude (plus a little spare to deal with the small drag losses). Then, at apoapsis you should circularize. Of course, this is half an orbit later because you just left periapsis. This is a 'coast phase' and is part of the most efficient way to get from 40km into stable orbit.

So yes, it is reasonable and efficient to use a coast phase to achieve orbit.

[Motokid600]

For everyone of my launches I burn until almost circular. I ALWAYS shoot for a 71km Apo. So as I near that number I start to tilt the rocket down. And by time I get to 71km I tilt the rocket down into the brown on the navball until my apoapsis stops rising, but at the same time im gaining horizontal speed. I have manually circularized without a coast phase before at 80km. Its hard, but it can be done.

It also has to do with Kerbins small radius. As you ascend the ground falls away under you much faster then it would if Kerbin were earth sized. This is why your apo will shoot up VERY fast as you get above 50km. Now.. I haven't tried the RSS mod, but I bet with that you burn all the way to orbit. ( more or less )

[maccollo]

If your rocket isn't designed specifically for the payload you're trying to lift you will either fail to make orbit if it's to heavy, or you will have to turn of the engines/reduce thrust during portions of the launch to make sure the orbit doesn't end up being to large.

The initial boost stages need to have the right amount of thrust and delta V to give the orbital insertion stage enough time to increase the velocity to orbital speed.

Your gravity turn needs to be just right for the same reason. If you want the rocket to be running at full thrust for for the entire launch while following the the prograde marker then miscalculating the gravity turn means you will overshoot the apoapsis before you make orbit.

I should note that as far as orbital mechanics go it's less efficient to accelerate during this phase, but if done correctly it is still a net gain because you loose the heavy booster engines earlier and performing the last part of the orbital insertion phase with more efficient engines like the NERVAs.

This is probably the best such launch that I've performed.

As for weather it's worth it, it's like everything else in KSP. It depends on whether it is part of the challenge you set for yourself.

It's sort of like space planes. They don't hold a candle to rockets in terms of lifting capacity but they are fun to build.

Edited December 14, 2013 by maccollo

[blizzy78]

It's completely realistic because of the Oberth effect. Basically you want to coast to near apoapsis so that when you burn to circularize all you are doing is raising your periapsis, not anything else.

While your point is correct, the Oberth effect has nothing to do with it.

[longhornchris]

Yeah, coast phases happen. In real life it's more common with solid fuel stages (eg: Pegasus) or very late in the flight. (eg: the Centaur in an Atlas V or DCSS in a Delta IV will do multiple burns) Or for that matter, the Shuttle would do burns after MECO to reach a stable orbit.

The Shuttle had a very good reason to have the coast phase. At ET separation the tank and shuttle aren't in orbit yet, so w/o the OMS burn the shuttle would follow the tank back in. During the coast, the shuttle would run a last systems check to ensure the vehicle was safe to enter orbit. Key to that was ensuring the bay doors opened so the vehicle was thermally stable (heat rejection is a major concern in real spacecraft... not so much in KSP).

As for other vehicles, part of the concern is having enough thrust for the circularization burn to allow a coast phase. Most upper stages are low thrust, so the burn has to start early enough that for a low orbit there really isn't much time for a coast phase.