How do you change your orbit?

[Johno]

Okay, so a the Kerbal Space Program Challenge requires you to fly in a 150km orbit (+/- 250m). Now, I have boosted into circular orbits before; mostly by luck, it should be said. But the process of circularising my orbit has been really long and painful, and I'm obviously doing something wrong here.

So let's put it out there. Simply put, assuming you have a spacecraft with ample propellant, what is a quick and simple way from going from an initial, highly eccentric orbit to a circular orbit of a specific altitude (in this case 150km)?

I know the theory of a Hoffman transfer, I can even draw one on a whiteboard; so I'm not looking for theory. I'm more interested in 'Now turn so your nose is 45 degrees above the horizon, and burn until your speed is . . .' type of instructions.

Thanks in advance!

[Ordosis]

I'm no rocket scientist, but if you understand the Hoffman transfer and read the wiki on orbiting it says it all IMHO.

If you're going from high to low you need to burn retrograde/opposite direction when you're at the apokee of the high orbit, you can use the calc or look up the wiki chart for the target speed. Then see if your perikee hits the target hight and make adjustment prograde/retrograde if necessary. If all goes well you have a transferred to a lower circular orbit.

So in sum, high circular orbit --> burn to get elliptical orbit with target perikee --> burn perikee to adjust for circular.

Do a prograde/forward burn if you're transferring to higher orbit.

Did this newbie explain well?

EDIT: Oh an check the video by Artreides below, he shoes how to transfer orbits as well.

EDIT2: Apologies to the rocket scientists, where I said 'Hoffman' I meant 'Hohmann'.

[Xivios]

First off, download the KSP Orbital Calculator

Then, haul ass into space and get into an orbit. Doesn't matter how it looks. Bring fuel. And liquid fueled engines. SRB's won't do you much good up here.

Wait until apogee - the highest point of the orbit, and - quickly now before you descend too much (maybe even pause) plug your altitude into the circular orbit tab of the calculator. It will spit out an orbital velocity for that altitude.

Point craft directly into TVV - the green circle thinger on the nav ball, and accelerate to the velocity given by the calculator.

Congratulations! You've now intentionally circularized an orbit, more or less.

Second step, open the Hohman Transfer Orbit tab. Enter your current orbit, and then your desired orbit. It'll spit out 6 different velocities, for now, focus on the final velocity of the Transfer Orbit Injection. You may wish to time this maneuver to perigee or apogee to create more stable numbers to input into the calc. In any case, accelerate to the new Final Velocity. Again, looking into the little green TVV

Congratulations, you're now hurtling through space on an elliptical orbit. I suggest grabbing a snack, stretching, nap, piss, etc. Now you wait.

Until new apogee. If all goes well it should be at the altitude of the desired new altitude. Pointed still at the TVV, accelerate to the Final Velocity for Target Orbit Injection. In fact, if your altitude is a bit off, but you wish to circularize here anyway, you might update your 'desired orbit' number to match reality and use the velocity provided by that altitude.

Congratulations! You've changed orbit!

[rdfox]

What Xivios said. The basic thing to remember is that whatever burn you make won't affect your position when you make the burn, but rather will affect the position 180 degrees later. So you burn at perikee to adjust apokee, and burn at apokee to adjust perikee. (Good lord, it's almost getting more natural to type those with the K instead of the G...)

If you follow that rule, you can usually get a successful *de*orbit burn with a delta-vee of under 50 m/sec by burning at apokee to put you on an elliptical orbit with apokee of about 34km. When I'm doing an Apollo 4 emulation with the Kerbal Katapult V (available at http://kerbalspaceprogram.com/forum/index.php?topic=1530.0, shill shill shill!), I usually end up with about a 45km perikee and roughly 200km apokee; from that, I can do an apokee deorbit with a mere 10m/sec delta-vee. (This leaves me with gobs of extra fuel to perform downward vertical burns to increase entry speed, just like Apollo 4 did.)

The other rule, of course--which I'm sure you understand, but just to make sure newbies who might stumble across this know it--is that posigrade burns (ones aimed for the TVV, like Xivios suggested) will result in a higher altitude (and, perversely, lower speed at that altitude!), while retrograde burns, aimed 180 degrees from the TVV (at the green circle with an X in it), will result in a lower altitude. And unless you're trying to shift your orbital plane (which has no point in KSP as of this writing), burns from a stable orbit in any direction other than posigrade or retrograde serve no purpose and just waste fuel...

[foamyesque]

What Xivios said. The basic thing to remember is that whatever burn you make won't affect your position when you make the burn, but rather will affect the position 180 degrees later. So you burn at perikee to adjust apokee, and burn at apokee to adjust perikee. (Good lord, it's almost getting more natural to type those with the K instead of the G...)

If you follow that rule, you can usually get a successful *de*orbit burn with a delta-vee of under 50 m/sec by burning at apokee to put you on an elliptical orbit with apokee of about 34km. When I'm doing an Apollo 4 emulation with the Kerbal Katapult V (available at http://kerbalspaceprogram.com/forum/index.php?topic=1530.0, shill shill shill!), I usually end up with about a 45km perikee and roughly 200km apokee; from that, I can do an apokee deorbit with a mere 10m/sec delta-vee. (This leaves me with gobs of extra fuel to perform downward vertical burns to increase entry speed, just like Apollo 4 did.)

The other rule, of course--which I'm sure you understand, but just to make sure newbies who might stumble across this know it--is that posigrade burns (ones aimed for the TVV, like Xivios suggested) will result in a higher altitude (and, perversely, lower speed at that altitude!), while retrograde burns, aimed 180 degrees from the TVV (at the green circle with an X in it), will result in a lower altitude. And unless you're trying to shift your orbital plane (which has no point in KSP as of this writing), burns from a stable orbit in any direction other than posigrade or retrograde serve no purpose and just waste fuel...

They do, however, allow for more precision in your final orbital speed adjustments. Hurrah for cosines!

In addition, I would point out that there's no need to circularise your orbit before beginning a Hohmann transfer. So long as you know you're at apogee, you can transition directly to the Hohmann transfer injection velocity.

[rdfox]

They do, however, allow for more precision in your final orbital speed adjustments. Hurrah for cosines!

Stupid trigonometry, always screwing me up, first in math classes and now here in KSP... ;D

In addition, I would point out that there's no need to circularise your orbit before beginning a Hohmann transfer. So long as you know you're at apogee, you can transition directly to the Hohmann transfer injection velocity.

You could *probably* even do it from perikee, couldn't you? After all, an elliptical orbit is basically a Hohmann transfer already. Granted, firing from apokee would be *faster* if you're trying to get on your final orbit without a stopoff in a parking orbit, but if you've already directly injected into a stable elliptical orbit, you'd be able to use either point for a transfer to the new higher orbit. I think it'd probably use the same amount of fuel, whether you raise perikee first, or raise apokee first...

(Yes, I know, most spacecraft do require a burn at apokee to stabilize the orbit. But the Kerbal Katapult V, for example, at least with Sunday Punch's 0.9 pack, would inject directly to a stable 160x45 orbit even with loss of one engine at SRB sep, or about a 320x60 orbit if nothing broke. I believe the Saturn V would also directly inject an Apollo lunar mission into a stable parking orbit instead of requiring a burn to raise perigee high enough to prevent radical decay...)

[foamyesque]

Certainly you could burn from perigee. However, that entails waiting for perigee, and you'll always hit apogee first.

(also, you have a broken quote tag)

[rdfox]

Fixed the tag. And valid point. The only time you'd really need to wait for perikee would be if A) you miss apokee because you're distracted (it happens!), or you want a lower orbit than your current apokee.

[DoctorEvo]

Seems like you two've got it pretty well-covered, so I'll only add one thing:

Just 'cause you're at the right altitude and speed doesn't mean your orbit is circular. You should also make sure your descent rate is zero (or close to it) at the end of your circularization burn, or else you'll end up with an elliptical orbit of the same semimajor axis as your target orbit, but higher (nonzero) eccentricity. (However, this is a lower priority than getting on-speed for your altitude, as its impact is less pronounced.)