Gravitational slingshots... any advice?

[MaverickSawyer]

I'm currently just before the heart of my first Duna transfer window of my current save, and I'm eyeballing a crewed flyby mission. I've never attempted a flyby of anything other than the Mun, and even then it was more of "let's see what happens" than out of any real need, so... I'm not entirely sure how this is going to shake out.

Does anyone have any experience with these kinds of missions, or better yet, with doing a flyby during that first Duna window? Any tips or advice would be much appreciated.

[Starhawk]

A crewed flyby of Duna is not a mission profile I would recommend.

This is because the transfer window from Duna to Kerbin does not occur anywhere near the time when a craft arrives at Duna from Kerbin on the first transfer window.

My recommendation is to plan to get into Duna orbit and stay for a bit.

Happy landings!

Edit: Upon seeing the thread title, it occurs to me that my assessment may have been incorrect.  Good luck.

[bewing]

You are doing a slingshot past Duna to where? Dres? Jool? Eeloo?

 

[MaverickSawyer]

My plan is to do a flyby of Duna, then return home, akin to a real-world opposition-class mission to Mars. But, as I've never done such a mission before, I'm not even sure it's possible. (Just got home from class and KSP is loading now, so checking to see if it's possible at all is high on my priority list.) I've also got a Cassini-esque orbiter headed to Jool right now, and the plan with that is to run a bunch of slingshots and/or assists to reduce my dV expenditures to allow me to make as many flybys as possible, gathering data the entire time.

[James Kerman]

I do a Duna flyby once per career save and as Starhawk mentions it's a little different to a flyby in Kerbins SOI in that it takes some maneuvering to get back home.  Usually I do the flyby, set kerbin as a target and wait a full solar orbit before making a node that intercepts kerbin.  This method is fairly cheap on Dv but usually means a high speed encounter so I try to plan my burn without using any prograde component (radial in/out) and try to encounter kerbin as far from solar Pe as is reasonable.

Another method I have read about but not tried is to burn for Eve after the Duna encounter and use the assist to lower your solar Ap closer to kerbins orbit.

Edit: I just read your reply and it seems as if you need a resonant orbit.

Edited February 20, 2019 by James Kerman
Edited for more information.

[MaverickSawyer]

3 minutes ago, James Kerman said:

Another method I have read about but not tried is to burn for Eve after the Duna encounter and use the assist to lower your solar Ap closer to kerbins orbit.

That's kinda what I want to try for.

[OHara]

23 minutes ago, MaverickSawyer said:

My plan is to do a flyby of Duna, then return home, akin to a real-world opposition-class mission to Mars.

We talked about that a while ago here: 

 

[MaverickSawyer]

4 minutes ago, OHara said:

We talked about that a while ago here: 

 

Aahhhh. Okay, looks like I'm too late for an opposition mission. Darn. Looks like the Duna mission will have to be a full-stay mission.

[OHara]

3 minutes ago, MaverickSawyer said:

too late for an opposition mission

Well too late for a close to lowest fuel opposition mission. 

If you are still before the transfer window, you could plot a course that gets you to Duna during Kerbin-Duna opposition, and see if the required craft seems interesting to you.  The Hohmann transfer has you about midway between planets during opposition, so you'll need to go very roughly twice as fast to get there in roughly half the time --- and then capture in high orbit so you can time your escape from Duna to aim properly to get back.

[Foxster]

Are you going for the science or for the hell of it?

[MaverickSawyer]

29 minutes ago, Foxster said:

Are you going for the science or for the hell of it?

For science, of course. If I was in sandbox, I'd be running a torchdrive. 

[Foxster]

3 minutes ago, MaverickSawyer said:

For science, of course. If I was in sandbox, I'd be running a torchdrive. 

In that case I recommend a high and low pass/orbit of Duna (+/-140km) and same for Ike (+/-50km) to maximise the science collection opportunities. Have a scientist onboard to reset the experiments and use the ForScience! mod to automate the experiments. 

[MaverickSawyer]

6 hours ago, Foxster said:

In that case I recommend a high and low pass/orbit of Duna (+/-140km) and same for Ike (+/-50km) to maximise the science collection opportunities. Have a scientist onboard to reset the experiments and use the ForScience! mod to automate the experiments. 

Nah, got a few unmanned orbiters and landers en route as well. Just need a small crew for collecting goo, materials exposure, and EVA reports.

Or, as it's going to turn out, to run down to the surface, board a rover, and go exploring while I wait for the return window.

[Aeroboi]

@MaverickSawyer

Usually the greater gravitational wells that are Eve, Kerbin and Jool (includes Tylo and Laythe) are really benefitial when doing slingshots.

There are also patterns that are orbit resonances whereby you can slingshot using the same planet in a 2/3 resonance using Kerbin or Eve. That means you use Kerbin as a slingshot after visiting Eve and then wait 2 or 3 orbits until it slingshots back again. Usually you only have to calculate the orbital period. If it's 1.5 times that of Kerbin after a Eve gravity assists it means you need to wait 3 orbits while Kerbin goes around 2 times to meet Kerbin without doing much of any course correction after slingshotting at Eve.

Duna and dres are no visitors for slinghots because Eve and Kerbin are better to slinghot outside the solar system as they have more gravity and are closer to the Sun so there's more oberth effect when doing burns. It doesn't mean you could never use a dres or duna slinghot. But it is only viable if dress or duna happen to cross target orbit like Jool for instance. That would be considered luck and won't turn out to gratitude much of a assist anyway.
If you want to get into the inner solar system you would only use slinghots to get to Moho. For this you will only use Eve mostly.

 

[Aegolius13]

Re: the possible subsequent Eve flyby, you'll likely run into the same problem where Eve is not in the right phase angle when you hit its orbit.  One option to deal with this is to:

1. Burn from Duna such that your perhelion matches Eve's orbit (i.e.,  your orbit is tangent to its orbit). 
2. Set a maneuver node at perhelion, which will likely be in deep space if the phase angles don't match.
3. Set up a retrograde burn to bring your aphelion in.  But mess around with the size of the burn until, on your next pass (or two), Eve is in the right place.  This is similar to a regular vessel rendezvous, and to the the "node" method to get to Moho.

Of course, you lose some Oberth efficiency on the retrograde burn to bring aphelion down, but it's not that big of a deal, and you can use really low TWR engines.   

I suppose you could use the same approach to get back to Kerbin, but this will likely take more in-game time.  (Since Eve revolves faster, it'll be in the right place sooner).  

 

[Gogi]

Just ti be clear. You can go from anywhere to everywhere at anytime. It's just a matter of  deltaV

[herbal space program]

On 2/20/2019 at 8:24 AM, MaverickSawyer said:

Nah, got a few unmanned orbiters and landers en route as well. Just need a small crew for collecting goo, materials exposure, and EVA reports.

Or, as it's going to turn out, to run down to the surface, board a rover, and go exploring while I wait for the return window.

So I did some noodling around in my transfer window and resonant orbit worksheets, and here's the breakdown: The usual way, doing a Hohman transfer during the first K-D window (day 225), waiting for the next D-K window (day 1062), and then heading home (~318 days), will get you back to the KSC around 1,155 days after launch. Interestingly, the Hohman transfer orbit to Duna is also almost exactly 3:2 Kerbin resonant, so if you did this as a flyby you would end up back at Kerbin 3 years after launch, or 1,278 days later.  You could however shave almost a year off of this by launching to a 2:1-resonant orbit that encounters Duna either on the way out or on the way back in. Not sure how close to the window your game is now, but you would probably have to do this at least a few weeks beforehand,   actually you do it after!, h/t @Kryxal). Anyway, that would get you back to Kerbin in 852 days, which is roughly the fastest you can do it without shredding the pork chop with excessive dV. Hope that helps!

PS: just for due diligence, I'll say that you could shave a few more weeks off of this by doing a retrograde DSM near your AP, that would lower your PE and cause you to re-encounter Kerbin somewhat sooner. But the price for those 4-5 weeks would be a very high-speed Kerbin re-entry, which may not be worth building for if you're trying to save money.

Edited February 27, 2019 by herbal space program

[Kryxal]

Thinking it through, I'd expect the 2:1 resonant path to launch later than the Hohmann transfer.  You'll be reaching the target sooner and with a "flatter" path, which counter each other somewhat, but I think the net result will be launching later for the flyby.  Also, be careful if you do a low flyby, ANY flyby is going to change your path and a low one even more so.

[herbal space program]

4 minutes ago, Kryxal said:

Thinking it through, I'd expect the 2:1 resonant path to launch later than the Hohmann transfer.  You'll be reaching the target sooner and with a "flatter" path, which counter each other somewhat, but I think the net result will be launching later for the flyby.  Also, be careful if you do a low flyby, ANY flyby is going to change your path and a low one even more so.

Right you are! I was turned around on that .

[MaverickSawyer]

It's kinda irrelevant now... I've committed to a full-stop mission to Duna. Habs, labs, rovers, cargo pods, the whole works. 

[herbal space program]

8 minutes ago, MaverickSawyer said:

It's kinda irrelevant now... I've committed to a full-stop mission to Duna. Habs, labs, rovers, cargo pods, the whole works. 

Probably the best approach in terms of overall ROI anyway.

[MrSystems]

I just ran a Duna flyby-and-return mission early in my new career. There might be lower delta-V was to do this, and there are certainly faster ways than what I did, but my method is very simple to plan and did not require much delta-V. Here are the mission parameters:

• Starting from circular Kerbin orbit at 73km altitude, ...
• Ejection burn of 1050 m/s on Year 1 Day 227
• Minor burn adjustments in Kerbol orbit (~20 m/s) to get Duna periapsis of 80km
• Arrive at Duna Year 2 Day 47
• At Duna periapsis, burn prograde to lower Kerbol periapsis to 13.3 Gm, 483 m/s dV
• In Kerbol orbit at ascending/descending node relative to Kerbin, burn 32 m/s dV to yield 0 relative inclination
• In Kerbol orbit at periapsis, burn -190 m/s to get encounter with Kerbin one orbit later
• Aerocapture at Kerbin at approximately the same (slightly less) relative velocity as an ordinary transfer back from Duna.

Total delta-V budget, including the initial transfer burn to Duna, was 1,800 m/s. Mission elapsed time was 3 Years 36 Days.