How to orbit without stopping ?

[Charlie the Kerbal]

I use MechJeb2 and a big rocket to land into a 200km orbit (repeatedly due to ongoing space station build). All good, first burn takes me to 65-70km, with 2300m/s horizontal velocity. Then I wait until 200km and make a small burn to circularise. Nothing to complain really but I am wondering if all can be done at one go ? I mean how can I adjust MechJeb to fly to 200km at once and keep firing to complete circularising ? I feel like there must be an ascent profile such that correct vertical velocity is achieved at the desired altitude.

Hope it is not a silly question.

Thanks,

Charles

[paul_c]

I don't know about MechJob but certainly yes, you could just fly it up to 200km then circularise. And also, due to the "reversibility of orbital changes" I think it would be the same energy required to do so.

[FloppyRocket]

Yes, it can be done in one continuous burn. Early rockets did this primarily because they did not have re-startable engines.

The basic trick is that you fly to a higher Ap than your target orbit, then fly "downhill" to raise the Pe  while lowering the Ap. If you balance everything just right, you get a circular orbit at the desired altitude.

It's not easy. The math is very complicated.

[bewing]

It can certainly be done, but you gain nothing by doing it. The energy required to get to LKO and then raise your orbit to 200km is pretty much exactly the same as to go to 200km directly. But by going to LKO first, you get the opportunity to time your second burn, and achieve a perfect rendezvous.

 

[paul_c]

AAAaaah if you mean one burn without relight, then yes BUT you need to either throttle down towards the end, or do some funky maths to work out what the TWR ought to be all the way up.

Its relevant in real life because engines don't always have a restart capability; and also the range of throttle control in real world is less.

[Reactordrone]

To do it in a single burn you basically want to keep raising your apoapsis so that it stays just in front of your current position. This will generally require very low thrust and you won't be able to time warp up to apoapse.

[Zhetaan]

18 hours ago, Charlie the Kerbal said:

I use MechJeb2 and a big rocket to land into a 200km orbit (repeatedly due to ongoing space station build). All good, first burn takes me to 65-70km, with 2300m/s horizontal velocity. Then I wait until 200km and make a small burn to circularise. Nothing to complain really but I am wondering if all can be done at one go ?

As others have said, yes, it can be done.  One way is to throttle down and use a low simmer to keep your apoapsis in front of you until you circularise, but in fairness, shutting off the engines and restarting is just a special case of this where low burn becomes no burn.

The way that real rockets do it, at least occasionally, is to burn full blast but to turn radial inward to adjust both apoapsis and periapsis so as to achieve a circular orbit with one burn.  This necessitates a bit of planning and a willingness to use more fuel.

18 hours ago, Charlie the Kerbal said:

I mean how can I adjust MechJeb to fly to 200km at once and keep firing to complete circularising ?

I don't think that anyone has answered this part yet.  You can adjust the ascent profile in MechJeb in a couple of ways.  One is to adjust the gravity turn profile, but you probably don't want to do that.  The other thing that you can do is to adjust the orbit altitude setting, along with the thrust and turn limiters (and the other settings) in the appropriate panel.  I believe that MechJeb more or less assumes that you'll need to coast to a circularisation burn (it's a consequence of Kerbin being a toy-sized planet; real rocket ascents take twenty minutes or more), but with a bit of tweaking, I think that you can get it to put you in orbit with a continuous burn.  Note that as @bewing said, the exercise is academic in terms of practical use; it saves neither time nor fuel.  That's not a recommendation against trying it--by all means, do so; it's your game--but a caution that most people here chase efficiency and so there probably isn't much material to guide you:  you'll need to figure out the appropriate settings for yourself.

That being said, if you do manage it, then please do post the settings here; you can help to establish the guidance material for anyone else who wants to do what you're attempting.

[vv3k70r]

21 hours ago, Charlie the Kerbal said:

Nothing to complain really but I am wondering if all can be done at one go ?

Not with reasonable fuel use. You have to get up over the atmosphere, then circularize in AP.

Of course You can get high horizontal speed during launch to get kind of circular orbit but heat and drag will cost You few galons.

21 hours ago, Charlie the Kerbal said:

I mean how can I adjust MechJeb to fly to 200km at once and keep firing to complete circularising ? I feel like there must be an ascent profile such that correct vertical velocity is achieved at the desired altitude.

By manipulating TWR - result will be exactly same.

19 hours ago, FloppyRocket said:

Yes, it can be done in one continuous burn. Early rockets did this primarily because they did not have re-startable engines.

And because they have to weak engines for achieving it in AP. It was just a very long manouver.

19 hours ago, FloppyRocket said:

The basic trick is that you fly to a higher Ap than your target orbit, then fly "downhill" to raise the Pe  while lowering the Ap. If you balance everything just right, you get a circular orbit at the desired altitude.

It's not easy. The math is very complicated.

It is almost compuslory early in career if vac engines are to weak. Like jumping on boosters and then flying horizon for minutes on vac engine to avoid falling back. Reasonable for efecting vac flights untill You realised that long flight efficency is not a biggest issue.

2 hours ago, Zhetaan said:

The way that real rockets do it, at least occasionally, is to burn full blast but to turn radial inward to adjust both apoapsis and periapsis so as to achieve a circular orbit with one burn.

Reality is special case. There are more issues with engines than in game.

 

[Charlie the Kerbal]

Thanks for all the answers !!! Learned new things today

[Graknils]

Turn on the MechJeb2 Ascent Guidance window. You can set any altitude and enable automatic circularization there. As long as you have enough fuel, you can do it in one maneuver.