approaching Jool real fast, slow down options?

[justDerrick]

I used 3/4ths of my fuel just getting to Jool, and I dont have enough left I think to get into orbit..

[ulzgoroth]

Well, you could try:
-Aerobraking. Fly into Jool's atmosphere. Hope you didn't fly in so far that you burn up or fall in.

-Slingshot off a moon. If you're approaching the moon from behind and getting swung around its front, you should lose some speed. Of course, you'll also change direction in ways that may be unhelpful.

-Make sure you really don't have enough for the orbital insertion burn. If you don't on your current trajectory, see if you could cheaply tweak your approach so that your periapsis is lower, which will make the insertion burn more efficient.

[GunnyErmy]

Gravity brakes from Tylo and/or Laythe, and aerobraking at Jool. You can get into a Jool orbit with no fuel at all. Almost the same with leaving the Jool system, for that matter.

[Quizzical]

If your trajectory comes close to Jool, then a modest retrograde burn at periapsis will be enough to get you into a stable orbit. Jool's atmosphere extends to 200 km, so you want to stay above that, but not too far above it. So long as you do it right, it only takes a few hundred m/s of delta-v to get to a stable orbit about Jool.

Alternatively, you could use a gravity assist off of Laythe. Tylo can work, too, but Laythe is easier. What you should do really depends on the purpose of your mission, however. Are you just trying to get to a stable orbit about Jool, or are you targeting one of its moons?

Aerobraking at Jool really isn't viable. You'll enter Jool's atmosphere at about 10 km/s and then promptly explode. At that speed, a periapsis more than about 5 km into Jool's atmosphere is enough that you'll explode, and a periapsis higher than that will barely slow you at all.

While it is possible to aerobrake at Jool, it takes some very complex vehicle design to make that work. It's hard enough that it's a really bad idea to try unless your goal is to go deep into Jool's atmosphere. Diving down to Jool's lower atmosphere and then making it back out alive is one of the game's ultimate challenges, and shouldn't be attempted until you've done nearly everything else.

[justDerrick]

Originally posted by ulzgoroth:

-Slingshot off a moon. If you're approaching the moon from behind and getting swung around its front, you should lose some speed. .

thanks, very nice

[justDerrick]

Originally posted by Quizzical:

If your trajectory comes close to Jool, then a modest retrograde burn at periapsis will be enough to get you into a stable orbit. Jool's atmosphere extends to 200 km, so you want to stay above that, but not too far above it. So long as you do it right, it only takes a few hundred m/s of delta-v to get to a stable orbit about Jool.

good point, might be best option.. im kinda afraid of aerobraking

[Quizzical]

The only sane reasons to attempt aerobraking on Jool is if you're trying to reach Jool's lower atmosphere or if you're looking for an artificial challenge. Anything else, including a low orbit about Jool, can be done more efficiently by means of a retrograde burn and possibly a gravity assist.

[ulzgoroth]

I'm puzzled as to how there can be no space between 'negligible drag' and 'you explode', but given that I've never been in Jool's SOI I can't bring any authority on the matter. (Of course, many space probes aren't designed to handle atmospheres at all.)

...If you do in fact have enough delta-V, I definitely recommend just doing the insertion burn. Getting fancy is for when you really have no choice or when you're doing it for its own sake.

[Quizzical]

Originally posted by ulzgoroth:

I'm puzzled as to how there can be no space between 'negligible drag' and 'you explode', but given that I've never been in Jool's SOI I can't bring any authority on the matter. (Of course, many space probes aren't designed to handle atmospheres at all.)

...If you do in fact have enough delta-V, I definitely recommend just doing the insertion burn. Getting fancy is for when you really have no choice or when you're doing it for its own sake.

Drag is proportional to the square of your velocity. Heating is proportional to the cube of your velocity. Thus, the ratio of heating to drag is proportional to your velocity. When entering Jool's atmosphere at about 10 km/sec, that ratio is going to be quite high.

You can demonstrate that there's a difference pretty easily, even on Kerbin. Going 300 m/s at sea level on Kerbin will slow you down from drag a whole lot faster than going 3000 m/s at an altitude of 50 km. Yet the latter will cause serious heat problems and likely explosions, while the former doesn't register as overheating at all.

[ulzgoroth]

Originally posted by Quizzical:

Drag is proportional to the square of your velocity. Heating is proportional to the cube of your velocity. Thus, the ratio of heating to drag is proportional to your velocity. When entering Jool's atmosphere at about 10 km/sec, that ratio is going to be quite high.

You can demonstrate that there's a difference pretty easily, even on Kerbin. Going 300 m/s at sea level on Kerbin will slow you down from drag a whole lot faster than going 3000 m/s at an altitude of 50 km. Yet the latter will cause serious heat problems and likely explosions, while the former doesn't register as overheating at all.

Thanks, that makes sense. It's not a property of Jool's atmosphere, but rather of the high speeds resulting from descending Jool's gravity well.

(The wiki claims it works well if you've brought along a heat shield, but presumably the OP's ship doesn't have one.)

[Quizzical]

Originally posted by ulzgoroth:

Originally posted by Quizzical:

Drag is proportional to the square of your velocity. Heating is proportional to the cube of your velocity. Thus, the ratio of heating to drag is proportional to your velocity. When entering Jool's atmosphere at about 10 km/sec, that ratio is going to be quite high.

You can demonstrate that there's a difference pretty easily, even on Kerbin. Going 300 m/s at sea level on Kerbin will slow you down from drag a whole lot faster than going 3000 m/s at an altitude of 50 km. Yet the latter will cause serious heat problems and likely explosions, while the former doesn't register as overheating at all.

Thanks, that makes sense. It's not a property of Jool's atmosphere, but rather of the high speeds resulting from descending Jool's gravity well.

(The wiki claims it works well if you've brought along a heat shield, but presumably the OP's ship doesn't have one.)

Whatever statement you're referring to from the wiki is probably just leftover from before version 1.0 completely redid aerodynamics and should be deleted. Now, the difference between using a heat shield or not at high speeds is usually just the difference between burning up versus flipping over and then burning up.

While you can use a heat shield for aerobraking without flipping over and burning up, it takes peculiar and unnatural designs to do so. Just sticking a heat shield on the top of your vehicle won't do it.

[ulzgoroth]

Originally posted by Quizzical:

Whatever statement you're referring to from the wiki is probably just leftover from before version 1.0 completely redid aerodynamics and should be deleted. Now, the difference between using a heat shield or not at high speeds is usually just the difference between burning up versus flipping over and then burning up.

While you can use a heat shield for aerobraking without flipping over and burning up, it takes peculiar and unnatural designs to do so. Just sticking a heat shield on the top of your vehicle won't do it.

It is not, it specifically says "since 1.0.5". It might still be obsolete information perhaps, but it's not from before version 1.0.

Obviously a heat shield is only going to help if you can keep it forward, either by natural stability (center of mass close to the desired front usually does it) or sufficiently strong SAS to force it (probably not a good prospect at such high speeds).

[Bomoo]

Lithobraking if all else fails.

[GunsForBucks]

One other option is to Alt-F12 to open the debug menu and click on the cheat to enable infinite fuel.. then use that to align your orbit a bit better before you get in its Sphere Of Influence.

[Nivlache]

Originally posted by ulzgoroth:

Originally posted by Quizzical:

Whatever statement you're referring to from the wiki is probably just leftover from before version 1.0 completely redid aerodynamics and should be deleted. Now, the difference between using a heat shield or not at high speeds is usually just the difference between burning up versus flipping over and then burning up.

While you can use a heat shield for aerobraking without flipping over and burning up, it takes peculiar and unnatural designs to do so. Just sticking a heat shield on the top of your vehicle won't do it.

It is not, it specifically says "since 1.0.5". It might still be obsolete information perhaps, but it's not from before version 1.0.

Obviously a heat shield is only going to help if you can keep it forward, either by natural stability (center of mass close to the desired front usually does it) or sufficiently strong SAS to force it (probably not a good prospect at such high speeds).

Can confirm that with proper ship design and heat shield it's possible to safely aerobrake with Jool in the recent versions too, though it's way too easy to slow down too much and fall into Jool's atmosphere (is actually a bigger threat than exploding assuming your heatshield has enough ablator)