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Siska

Slowing down at DRES

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Hello, 

 

well, i have been in KSP for some time now.

 

i got to a problem, going to DRES. It is a very small planet, and when i get an encounter, to slow down to get to DRES orbit takes huge amount of Delta V.

 

is there any other way to approach dres to use less delta V for slowing down? Usually i slingshot from Kerbin directly to DRES, should i do it otherway?

 

i do not use mechjab or any mods.

 

thx, best regards

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Nope. No shortcuts. That's just one of the challenges of a mission to Dres. At other planets you could use atmospheric braking or perhaps a moon pass, but Dres is just Dres. 

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Posted (edited)
1 minute ago, Vanamonde said:

At other planets you could use atmospheric braking or perhaps a moon pass, but Dres is just Dres.

Moho btw

Also it is not neccessary to spell Dres with CAPS lock.

Edited by Gapone

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Just now, Gapone said:

Moho btw

And Eeloo. I didn't say ALL other planets. :)

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You can reduce the dV if you get as sweet an encounter as possible.

Get really close to it's plane before your Hohmann and get the Pe close. A decent TWR is handy too so that you don't mess about burning a lot either side of Pe

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6 hours ago, Siska said:

is there any other way to approach dres to use less delta V for slowing down? Usually i slingshot from Kerbin directly to DRES, should i do it otherway?

@Foxster beat me to it... but yes, getting the encounter right will make a big difference.

The other thing is when you get there. Dres's orbit is not circular. Therefore Dres is going faster in lower orbit around the sun (nearer to Kerbin) and slower at its higher orbit.
Since you are approaching from a lower orbit, your ship is going at its slowest when it encounters Dres. Capturing at Dres therefore means speeding up to reduce the difference in velocity (yes, of course you are inside Dres's SOI so therefore your solar orbit is irrelevant... but it comes to the same thing in fact: your Dres-relative velocity on entering Dres's SOI is a function of the difference between your and Dres's orbital velocities around the sun).

So the best time to capture cheaply at Dres is when it is furthest from the sun.

This rule applies the same way for Eeloo (much cheaper to capture far from the sun), and conversely for Moho (it is lower down than you are, so you are going faster than Moho when you meet it, therefore you want to meet it as close to the sun as possible).

If you check Alex Moon's trajectory planner, you'll see that you can leave Dres for a return to Kerbin for about 1300 m/s. It should therefore be perfectly possible to capture at Dres for that exact same amount, at that time.

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46 minutes ago, Plusck said:

If you check Alex Moon's trajectory planner, you'll see that you can leave Dres for a return to Kerbin for about 1300 m/s. It should therefore be perfectly possible to capture at Dres for that exact same amount, at that time.

Indeed, you are correct. Looks like the optimal transfer from years 1-10 gives an insertion dV of 1309 m/s. This is for a 70-km Kerbin orbit to a 30-km Dres orbit.

fz0k664.png

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To close the loop on the "best time to capture cheaply at Dres is when it is furthest from the sun" strategy, where is Dres in its orbit at Year 7 Day 249?

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If you're unable to capture cleanly with a "usual" orbital insertion burn, just start early and probably angle somewhat radial-out ... Dres is small, so Oberth isn't going to be a big deal.

Although why you're going to Dres in the first place, I don't know, it's a boring place.

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10 hours ago, Kryxal said:

Although why you're going to Dres in the first place, I don't know, it's a boring place.

Because it's there?

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11 hours ago, Kryxal said:

Although why you're going to Dres in the first place, I don't know, it's a boring place.

Because you can?

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12 hours ago, Kryxal said:

Dres is small, so Oberth isn't going to be a big deal.

On that note, you can always go high ISP / low TWR (like a single nuke or some ions) and start raising your perihelion before you get to Dres' SOI.  This actually takes more maneuver delta-v, but your craft would have more delta-v available, so it works out fine.

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At the end, i waited proper escape windov via Alex Moon's trajectory planner.

 

If escape window is optimal, you also don't need so much delta V to slow down, as the planet itself further from the sun and it goes slower, as well as my ship because higher apoaphsis.

Now the challenge is to get back to Kerbin, as i enter Kerbin's gravitational infulence with some 6800 m/s.

 

 

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8 hours ago, Siska said:

At the end, i waited proper escape windov via Alex Moon's trajectory planner.

 

If escape window is optimal, you also don't need so much delta V to slow down, as the planet itself further from the sun and it goes slower, as well as my ship because higher apoaphsis.

Now the challenge is to get back to Kerbin, as i enter Kerbin's gravitational infulence with some 6800 m/s.

If your only aim is to return science and a crewman then there is nowhere that you can't make a direct re-entry from (without making orbit first) with a mk1 capsule and a heatshield with 20% ablator. 

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