Could you use a Gravity Assist to get to Mars?

[ace.1991]

I've been wondering about this for some time for use in a writing project of mine:

With time not being a constraint, could you use a gravity assist to get a manned mission to Mars?

My more specific question: Would a Luna gravity assist, followed by an Earth gravity assist yield significant enough velocity to be a plausible manoeuvre for a Mars mission?

[baggers]

Nozomi, a little japanese probe, used that trick:

https://en.wikipedia.org/wiki/Nozomi_(spacecraft)

It still have to burn some fuel. But it certainly was an advantage.

[UmbralRaptor]

Almost certainly. But if you're throwing humans around, you're dealing with time-sensitive cargo and giant rockets anyway...

[K^2]

Yeah, not for manned missions, because Earth gravity-assist alone takes an extra year. Rosetta used a similar maneuver, though. So it's certainly doable. If you don't care when you get there, you can get to anywhere in the Solar System with the fuel it takes you to get to a Trans-Lunar orbit, which is about the same as getting into GEO.

[ace.1991]

Nozomi, a little japanese probe, used that trick:

It still have to burn some fuel. But it certainly was an advantage.

I had never heard of that probe until now. Gonna have to look it up. but yeah, I knew a mission using this type of maneuver would require an extra burn to make up the Delta-V to reach Mars. I'm mainly looking at it as a means of conserving fuel, and by extension, weight.

Yeah, not for manned missions, because Earth gravity-assist alone takes an extra year. Rosetta used a similar maneuver, though. So it's certainly doable. If you don't care when you get there, you can get to anywhere in the Solar System with the fuel it takes you to get to a Trans-Lunar orbit, which is about the same as getting into GEO.

I was actually thinking more along the lines of a grav assist around the moon then, during the 'free-fall' back to Earth, use Earths gravity to sling the vehicle out towards Mars, after that it would be a final burn to get to Mars. But then i don't know if that would yield any significant delta-V with that maneuver.

In either case, mentioned above, I'm thinking more towards fuel/weight conservation, rather than time scale. although that is obviously a factor too.

[SargeRho]

That's not how gravity assists work. At most, it'd allow you to use less fuel for the trans-mars insertion burn due to the oberth effect.

[UmbralRaptor]

I was actually thinking more along the lines of a grav assist around the moon then, during the 'free-fall' back to Earth, use Earths gravity to sling the vehicle out towards Mars, after that it would be a final burn to get to Mars. But then i don't know if that would yield any significant delta-V with that maneuver.

I'm not sure such a flyby is possible. Most assists would kick you out into a solar orbit after moving away from the Moon.

In either case, mentioned above, I'm thinking more towards fuel/weight conservation, rather than time scale. although that is obviously a factor too.

May I suggest an NTR, aerocapture, or especially ISRU. It's the payloads (and timescales) that make humans on Mars so intractable. (My back of the envelope guess is that this bit of gravity fanciness saves up to 1 km/s.) Edited October 29, 2015 by UmbralRaptor

[K^2]

I was actually thinking more along the lines of a grav assist around the moon then, during the 'free-fall' back to Earth, use Earths gravity to sling the vehicle out towards Mars, after that it would be a final burn to get to Mars. But then i don't know if that would yield any significant delta-V with that maneuver.

Can't do that. To get gravity assist from Earth, you need a speed differential with Earth in Sun's orbit. The only practical way to do that is to get into a slightly elipitical orbit around the Sun that lets Earth catch up with you almost exactly a year later. That gives you enough of a boost to lower perhilion to bellow Venus' orbit, and aphelion to above Mars'. You then have an option to get a proper assist from Venus or Mars to go to wherever. Or, go for capture if these are your final destinations.

Moon is a different matter. When you enter LTO from Earth, you are already on a highly eliptical orbit, so you get a great boost from the Moon. Sufficient to eject you from Earth's gravity all together. In fact, if you time everything just right, it's just enough to get you to the aforementioned Earth gravity assist maneuver, but you still have to spend a year in Sun's orbit do do that.

[prophet_01]

Well... the thing with manned missions is, that time actually is a concern. Humans need supplies and protection against radiation and usually don't like to be locked away in a tin can... Longer duration leads to additional payload required just to keep the astronauts alive.

It might pay off though to send the ship unmanned on the described lunar+earth gravity assist maneuver and pick up the crew during your final flyby near earth. Kinda like proposed with the cycler concept.

Safety concerns aside, it could realy help to keep the fuel requirements down.

You don't want to screw up the crew transfer though. A rescue mission would be very tricky at that point to say the least :/

[ace.1991]

My back of the envelope guess is that this bit of gravity fanciness saves up to 1 km/s.

Kind of the answer I was expecting, to be honest.

Moon is a different matter. When you enter LTO from Earth, you are already on a highly eliptical orbit, so you get a great boost from the Moon. Sufficient to eject you from Earth's gravity all together. In fact, if you time everything just right, it's just enough to get you to the aforementioned Earth gravity assist maneuver, but you still have to spend a year in Sun's orbit do do that.

It might pay off though to send the ship unmanned on the described lunar+earth gravity assist maneuver and pick up the crew during your final flyby near earth. Kinda like proposed with the cycler concept.

That's actually a much better for what I have in mind. Yes, the intercept with the ship would need to be unforgivingly precise, but assuming you could, then that more or less solves my fuel problem and mission time for the crew.

Thanks guys

[Silver_Swift]

It might pay off though to send the ship unmanned on the described lunar+earth gravity assist maneuver and pick up the crew during your final flyby near earth. Kinda like proposed with the cycler concept.

Safety concerns aside, it could realy help to keep the fuel requirements down.

You don't want to screw up the crew transfer though. A rescue mission would be very tricky at that point to say the least :/

How would you do the transfer in the first place, if you have enough dV to rendezvous with the craft don't you also have enough dV to get to the desired speed without gravity assists?

[KerikBalm]

Well, to do the rendevous.. you only need to rendevous a capsule with people and limited supplies in it.

To get back to Earth, you need a much large life support system+ supplies, a heat shield... all that stuff.

A lunar grav assist makes sense for sending supplies and hardware to Mars, so that everything is alreayd there ready for the humans when they get there.

[Silver_Swift]

Ah ok, so basically the advantage is that a lot of the weight you have to move to Mars is already at the right speed (the second craft has enough dV to do a Mars flyby on its own, but it doesn't have the additional stuff you need to survive out there, allowing it to be much lighter)?

[fredinno]

Better yet, ION drives for the Cargo.

[fredinno]

To "intercept" your ship is one thing. To dock with it and transfer crew without splattering them across the windshield means matching the orbit exactly. You could save some expense and crew time by doing the initial part unmanned, and just launching them on a do-or-die intercept to go on to Mars, but that's insanely risky. If they miss they die.

Wouldn't it be possible to do a direct abort back to earth under the "Cycler" concept?

[Stoney3K]

Wouldn't it be possible to do a direct abort back to earth under the "Cycler" concept?

To dock with a cycler you would also need to match its orbit.

The big advantage here is that you would only need to launch a cycler *once*, with all the big hardware, unmanned, and you can send supplies and crew in tiny pods which smaller launch vehicles to and from the cycler.

A direct crew vehicle could be sent on such a cycler trajectory but it would not be useful for abort: If the crew wanted to abort mid-flight, it would mean waiting over a year until their next encounter with Earth, which is not beneficial if you're in a life-and-death scenario that warrants an abort.

[cantab]

Based on this delta-V map: http://i.imgur.com/SqdzxzF.png a gravity assist from the Moon will save about 500 m/s or so at best. Not negligible, but small fry compared to the overall requirements of the mission. It's the same problem as in KSP, the delta-V to go from LEO to intercept the Moon is almost as much as it takes to go from LEO straight to a Mars intercept.

[prophet_01]

True, it's not all that much. But there is also the idea of doing multiple short burns on earth's periapsis with low thrust high ISP engines in order to max out efficency.

The indian mars probe did a similar thing. It could also be used for this case with a crew transfer on the last flyby.

Edited November 1, 2015 by prophet_01

[Guest]

The mass you'd save by putting less fuel would be regained by extended life support.

[prophet_01]

Not if you pick up the crew on your final flyby

[B787_300]

as I am working on a Senior design Project that involves sending stuff to and getting back from Mars, the biggest issue is Time. If you start waiting for things to line up to get gravity assists you are generally in space for a whole heck of a lot longer than you need to be. and every extra minute you are in space increases the chance that your electronics get fried by a higher energy particle from the sun. The only time it becomes super useful is when you are trying to get into one off orbits (non-ecliptic generally) or you need some stupid fast way out of the solar system like voyagers.

Also it helps if you are going out beyond the asteroid belt, but anything closer fall in the slightly more fuel > time spent travelling