Sattelite scanning and orbits

[Esme]

Hello!

During my second attempt at sending probes to the Joolian msystem, I encountered a problem I;ve not had before, and I'm wondering whether there's a clear answer as to what is the best thing to do under the circumstances, or whether it's very much one of those that depends on the precise circumstances.

I had a probe equipped with Scansat, which will work up to 500km from the surface of a body, in an orbit about Laythe inclined at about 20 degrees, with a periapsis of 60km and an apoapsis of something like 2,500km. Fuel wsa in short supply, as it'd taken quite a bit of the remaining monoprop just to lift the trajectory from impacting Laythe at a low angle up to 60km. So I knew I had no hope of getting the probe into a near-polar orbit entirely within 500km. With the benefit of hindsight, I probably had just enough fuel to drop the apoapsis to 500km, leaving the incloination unchanged. What I actually did was to change the inclination to about 34 degrees (I was trying for 45, saw it wouldnt happen, so stopped and decided to use teh rest to drop the apopapsis), and then could only drop the apoapsis to about 600km. This seems to be giving me mapping coverage up to about 40 degrees N and S of the equator.

Is it likely that I could have done better, gotten greater coverage of the surface, and if so, how? Is there some general principle to work this out, or is it a case of using best judgment from experience and crossing ones fingers?

Edited May 25, 2015 by Esme

[Snark]

The basic principles for conserving dV:

1. If you're burning prograde/retrograde, do it at the lowest altitude possible, when you're going really fast. (Oberth effect.)

2. If you're burning normal/antinormal, do it at the highest altitude possible, when you're going really slow.

3. Never burn radial/anti-radial.

...I haven't sat down to work out the math, but I'd guess that probably what would have been your best option would have been to wait until you were up at your 2500km apoapsis, adjust your inclination there (it would have been relatively cheap for dV). Then, once that's done, spend all your remaining fuel doing a retrograde burn at periapsis, to lower the apoapsis as much as possible.

That said: As a matter of practicality, I find that going for the completely-polar orbit isn't all that useful (to me, anyway), since my reason for SCANsat'ing is to locate resources, and I'd prefer to do my resource mining close to the equator; the only time I ever go to the poles is to pick up science points, which I don't need SCANSat for. But it's all a matter of what your priorities are.

[Esme]

Thanks Snark. Hmm. Thanks, I see what you're saying with 2, hadn't really thought about that. As for the why - I'm in it for the science points, generally. As I've only just got adept enough to be able to get probes to the Joolian system, I wanted to get as much science as possibel out of it so I could unlock more stuff (I'm playing v0.9, science game). I've set up bases using KAS before, and am using TAC LS on mannd missions, but haven;t yet got to grips with MKS. I'm looking forward to when I get a new machien so I can run 1.0 sensibly, and try the stock game in 1.0 before trying to emulate Mars Direct. :-)

[Snark]

Here's a tip for going to the Joolian system: Do your aerobraking on Laythe, not Jool (even if Laythe isn't your planned first stop--i.e. it's a valid option to use Laythe to aerobrake into Jool capture without actually capturing to Laythe itself).

Rationale: If you aerobrake on Jool, you're going to end up in an orbit that has periapsis right down on the surface of Jool. It means you're going to have to do a lot of dV to match with any of its moons, since your orbit will be highly eccentric and have very low angular momentum.

On the other hand, if you aerobrake on Laythe, then you're in an orbit whose periapsis is up at Laythe's orbit, rather than down at the surface of Jool, so it's a much lighter dV load to match any of the various moons.

Plus, of course, there's always the option of aerobraking all the way to Laythe capture, if you want to make that the first stop on your Joolian tour. But the dV savings is worth it even if you're not doing Laythe first.

Timing your approach so that Laythe is in the right spot when you need it to be might sound hard, but actually it's not that much of a hassle as long as you set it up long before you get to the Jool system. I was expecting it to be hard, and the simplicity surprised me the first time I did it:

Do your usual maneuvering so you've got a Jool intercept, then plop a maneuver node down long-before-Jool and switch your map focus to Jool. Play with the node handles so that your trajectory is tangent to Laythe's orbit at the right spot. If you're lucky, it'll show a Laythe intercept already. If you're not, play around with the prograde/retrograde versus radial/antiradial handles to adjust your Jool periapsis forward/backward in time while keeping the location pretty much the same. Pretty soon, Laythe will end up doing an intercept. Then you can click on Laythe and focus there, and it'll show your trajectory there, and then you can fine-tune the intercept to exactly where you want it.