Non-stop slingshot mothership

[aluc24]

Hey guys,

Some time after watching The Martian, I came up with this admittedly strange idea to make manned Earth - Mars trips cheaper. The idea expands on the multiple slingshot theory presented in the movie. It goes like this:

A mothership, housing living quarters, gravity rings, heavy radiation shielding and whatnot, launches on a escape trajectory from Earth to Mars. Several hours before periapsis at Mars, a lander (or multiple landers) with people and cargo detaches, makes a minor burn to intercept the atmosphere, and performs an aerobraking maneuver to land at the target site. Mothership, however, does not decelerate, and makes a slingshot past Mars to make it back to Earth. Upon reaching Earth, it slingshots again, towards Mars, while a rocket with new crew or cargo launches from Earth and intercepts with the mothership as it passes past Earth. As the mothership approaches Mars again, it ditches the crew/cargo landers into the atmosphere for aerobrake, just like before, while the old crew ascends to intercept with the mothership, which is bound to Earth again. Upon arrival to Earth, the crew capsule detaches for aerobrake, while the mothership continues the slingshot cycle.

The idea is that after the initial transfer burn, the mothership doesn't perform any major burns anymore, except for corrections, and that means a lot less fuel to carry. Of course, crew/cargo that launches from Earth and Mars still needs to accelerate to escape velocity to intercept with the mothership, but they can be a lot lighter, since they don't need to carry extensive life support, supplies, or radiation shielding. Just enough to survive for a few hours or so. So all the heavy stuff that is needed for long interplanetary travel stays in motion forever.

To put it in layman's terms, the mothership is like a train that never stops, while people jump on and off it at each station.

Now, I haven't run the math, and I'm not sure if it's possible to perform regular slingshots between Earth and Mars. I know that launch window opens every 1.5 years, but I don't know how if slingshot windows are regular too. Even in the best case scenario, the crew on Mars would have to stay many years before the mothership made the round trip back. However, there could be several motherships - one is headed to Mars, while the other is returning to Earth. That would cut the wait time roughly in half. Again, I hope someone with a better understanding of orbital mechanics could say if slingshot windows are reasonably regular.

Even if they are, it is more than likely that mothership would sometimes have to slingshot pretty far away from either planet, so the lander would require quite a lot of ΔV to change trajectory to intercept the atmosphere. However, the mothership could pass closer to the atmosphere than it actually needs, and perform a correction burn to make up for the sub-optimal slingshot altitude. That would require having somewhat more fuel onboard, but still a lot less than it would need for capture and escape burns on each planet.

Apart from that, I'm not seeing any inherent flaws with this idea. I'm just not sure if unlimited number of slingshots between two planets is at all possible, and what would the transit times be.

I would be happy if some greater minds than mine could comment on the feasibility of this idea. Most likely I'm missing some practical consideration here. Or maybe someone before me has already considered and scrubbed this idea. Anyway, I'm interested in what you have to say

[Tex_NL]

And that's just the first example I found. Dozens more examples are easily found on the forum.

Edited July 3, 2017 by Tex_NL

[aluc24]

1 minute ago, Tex_NL said:

And that's just the first example I found. Dozens more examples are easily found on the forum.

Oh, so that's what it's called. I used the wrong terms before writing this topic. My bad

[Tex_NL]

1 minute ago, aluc24 said:

Oh, so that's what it's called. I used the wrong terms before writing this topic. My bad

You can also google the term Aldrin Cycler.

[Bill Phil]

No harm no fowl. 

The idea is alright, but it only really helps if you're sticking to low isp engines, which we shouldn't do. Minimag Orion, I'm looking at you.

[PLAD]

I tried a cycler ship in this thread a while back. Ultimately I didn't have the tools needed to plan the cycler path more than a couple of flybys ahead, and without that sooner or later the cycler drifts way off course. The fact that the transfer ship has to have more than enough dv to get to the destination planet just to catch the cycler makes a cycler unnecessary if you're not using life support mods. The Kerbin-Duna run is a quick trip anyway. Maybe a Kerbin-Dres run...

[andrewas]

Cyclers are for luxury travel. Build them big, and you can travel to mars with a ship thats only big enough to get to the moon. But the investment needed to create a fleet of Cyclers allowing for regular travel is immense, so this is a far future thing. 

Edited July 4, 2017 by andrewas

[The Space Dino]

This is definitely an interesting concept, although if anything went wrong with something like the refuelling mission to keep the craft in its Earth-Mars slingshot trajectory, it would basically be done for unless you went crash course, as in The Martian. 

Only attempt this if you are human. Kerbals only make it work once or twice and soon it will find itself in solar orbit.

[Bill Phil]

On 7/3/2017 at 8:25 PM, PLAD said:

I tried a cycler ship in this thread a while back. Ultimately I didn't have the tools needed to plan the cycler path more than a couple of flybys ahead, and without that sooner or later the cycler drifts way off course. The fact that the transfer ship has to have more than enough dv to get to the destination planet just to catch the cycler makes a cycler unnecessary if you're not using life support mods. The Kerbin-Duna run is a quick trip anyway. Maybe a Kerbin-Dres run...

Well we could use a momentum exchange tether, slinging a smaller ship to the cycler would let us use a smaller (and cheaper) tether, and then we can get the benefits... but then we could just use momentum tethers without the cycler...

[aluc24]

On 7/4/2017 at 4:25 AM, PLAD said:

I tried a cycler ship in this thread a while back. Ultimately I didn't have the tools needed to plan the cycler path more than a couple of flybys ahead, and without that sooner or later the cycler drifts way off course. The fact that the transfer ship has to have more than enough dv to get to the destination planet just to catch the cycler makes a cycler unnecessary if you're not using life support mods. The Kerbin-Duna run is a quick trip anyway. Maybe a Kerbin-Dres run...

I wonder why that didn't work for you. I mean, from what I've read, Aldrin's cycler has a regular interval between each encounter, right? So, put an empty maneuver node at X days after the previous slingshot, and then set up a middle-course-correction burn before this dummy node, using it only as a reference to get an encounter at the right time. Afterwards, create another empty reference node for the next encounter after pre-determined X days, and use MCC's to make sure that dummy node happens to be at the periapsis of next encounter planet... Or is it more complicated than I'm assuming?

[sevenperforce]

The Aldrin Cycler is one particular solution to the overall Mars Cycler problem. Here are a few different possible cycler periods.

One thing you can do is have a cycler with a long transit time (e.g., five months) on one leg and a short transit time (e.g., 18 months) on the other leg. This allows you to send cargo on the long transit leg and crew on the short transit leg. With two or three cyclers running in parallel, you can have reasonably regular transits for crew or for cargo.

Just now, aluc24 said:

I wonder why that didn't work for you. I mean, from what I've read, Aldrin's cycler has a regular interval between each encounter, right? So, put an empty maneuver node at X days after the previous slingshot, and then set up a middle-course-correction burn before this dummy node, using it only as a reference to get an encounter at the right time. Afterwards, create another empty reference node for the next encounter after pre-determined X days, and use MCC's to make sure that dummy node happens to be at the periapsis of next encounter planet... Or is it more complicated than I'm assuming?

An Aldrin cycler repeats transit periods every seven years without requiring mid-course corrections, but in KSP, you'd need a mid-course correction at least every other cycle.

[aluc24]

11 minutes ago, sevenperforce said:

The Aldrin Cycler is one particular solution to the overall Mars Cycler problem. Here are a few different possible cycler periods.

One thing you can do is have a cycler with a long transit time (e.g., five months) on one leg and a short transit time (e.g., 18 months) on the other leg. This allows you to send cargo on the long transit leg and crew on the short transit leg. With two or three cyclers running in parallel, you can have reasonably regular transits for crew or for cargo.

An Aldrin cycler repeats transit periods every seven years without requiring mid-course corrections, but in KSP, you'd need a mid-course correction at least every other cycle.

7 years? This Aldrin Cycler demonstration on YouTube makes it look like 3 years and a month or two (from Earth to Earth, which counts as one cycle, as far as I understand).

[sevenperforce]

Just now, aluc24 said:

7 years? This Aldrin Cycler demonstration on YouTube makes it look like 3 years and a month or two (from Earth to Earth, which counts as one cycle, as far as I understand).

No, no, sorry for the confusion. It's a 7-year recycle period for the overall transit cycle set. So there are varying cycle lengths, but the placement of Mars and Earth relative to the cycler repeats every seven years.

[aluc24]

2 minutes ago, sevenperforce said:

No, no, sorry for the confusion. It's a 7-year recycle period for the overall transit cycle set. So there are varying cycle lengths, but the placement of Mars and Earth relative to the cycler repeats every seven years.

I'm not sure I understand. The videos says that 1 synodic year for Earth-Mars Aldrin Cycle is 783 days, which is just a little over 2 years (not 3, I made a mistake in my previous post). Correct me if I'm wrong, but that video certainly makes it look like the Earth-Mars placement relative to the cycler also repeats after 783 days (0:20 in the video).

[mikegarrison]

In the Red Mars Green Mars Blue Mars series (and follow-on books) this idea is what makes travel throughout the solar system practical. They set up a whole fleet of these "continuous shuttles", most of which are made out of hollowed, spun asteroids.

The only real advantage to this is that you get lots of room in the ship that you spend the most time in. So if they are big enough to contain an actual functioning ecosystem inside, they can produce at least some of their own food, water, and air. But otherwise, since you have to accelerate all your passengers and expendable provisions up beyond escape velocity just in order to dock, you don't really save a lot of energy.

[sevenperforce]

3 minutes ago, mikegarrison said:

In the Red Mars Green Mars Blue Mars series (and follow-on books) this idea is what makes travel throughout the solar system practical. They set up a whole fleet of these "continuous shuttles", most of which are made out of hollowed, spun asteroids.

The only real advantage to this is that you get lots of room in the ship that you spend the most time in. So if they are big enough to contain an actual functioning ecosystem inside, they can produce at least some of their own food, water, and air. But otherwise, since you have to accelerate all your passengers and expendable provisions up beyond escape velocity just in order to dock, you don't really save a lot of energy.

Which is precisely why you can't "hitch a ride on a comet".

[Thor Wotansen]

The other problem with cyclers is that the second we crack fusion they're obsolete.  A fusion engine would allow trajectories similar to what you see in The Expanse (albeit with a coast period).

[PLAD]

2 hours ago, aluc24 said:

I wonder why that didn't work for you. I mean, from what I've read, Aldrin's cycler has a regular interval between each encounter, right? So, put an empty maneuver node at X days after the previous slingshot, and then set up a middle-course-correction burn before this dummy node, using it only as a reference to get an encounter at the right time. Afterwards, create another empty reference node for the next encounter after pre-determined X days, and use MCC's to make sure that dummy node happens to be at the periapsis of next encounter planet... Or is it more complicated than I'm assuming?

The Youtube video, and the basic Aldrin cycler concept, assume the orbits of Earth and Mars are both circular and coplanar. (They also start with the assumption that the orbital periods are exactly in the ratio of 1.875:1 but that is easy to compensate for.) When the Aldrin cycler, or any cycler, is simulated in an accurate solar system then course adjustments are needed to compensate for the varying motions of the planets. For instance Mars' orbit is quite eccentric so if you encounter it near it's periapsis Mars will be way ahead of its mean position. So the time between encounters has to vary back and forth from orbit to orbit. You can greatly reduce the size and frequencies of the adjustments if you can calculate the ship's motion several orbits ahead, but that takes some serious calculating chops.

[Bill Phil]

3 hours ago, Thor Wotansen said:

The other problem with cyclers is that the second we crack fusion they're obsolete.  A fusion engine would allow trajectories similar to what you see in The Expanse (albeit with a coast period).

Yeah, sure, but we'd also see huge radiators. So maybe something more like a few months to Mars.

And even then, fusion may not be scalable down to that size.

Not to mention that we could use Orion, although people may not like nuke powered rocket ships.

And even if it's possible to scale fusion down like that, it would likely take decades. It won't be the second we crack fusion.

[wumpus]

On 7/5/2017 at 5:22 PM, sevenperforce said:

One thing you can do is have a cycler with a long transit time (e.g., five months) on one leg and a short transit time (e.g., 18 months) on the other leg. This allows you to send cargo on the long transit leg and crew on the short transit leg. With two or three cyclers running in parallel, you can have reasonably regular transits for crew or for cargo.

Cyclers have a fairly limited advantage for cargo, unless you have enough of them so that the your tugs from you gravity well (orbit, surface, whatever) to the cycler are always in motion.  For high-Isp/low thrust, the trip to Mars (or wherever) may be shorter than the trip in and out of the gravity wells.  The main advantage is for people who want more room than an Orion or Dragon to stay for several months.  I suppose you could try some sort of "fishing reel" approach (occasionally suggested as a cheap orbital elevator), but I have yet to see a workable approach.

On 7/5/2017 at 11:37 PM, Bill Phil said:

Yeah, sure, but we'd also see huge radiators. So maybe something more like a few months to Mars.
And even then, fusion may not be scalable down to that size.
Not to mention that we could use Orion, although people may not like nuke powered rocket ships.
And even if it's possible to scale fusion down like that, it would likely take decades. It won't be the second we crack fusion.

Fusion has some serious problems, and many of the benefits could be had with fission.  If you go the fusion->electricity->VASIMR route, you wind up with roughly as much radiators as the solar panels you replace (although this may be ideal for going through the Van Allan belts).

Your other means is to use the same [magnetic] apparatus that contains the plasma as the nozzle of a rocket.  This gets rid of all the "Ve melts your rocket" issues, and provides direct cooling by removing heat out the rocket.  You still need all kinds of heat to get rid of the heat from the energy you needed to start fusion, and the heat radiating out of the main fusing section.

[sevenperforce]

Just cut out the middleman and go with a Z-pinch.

[wumpus]

On 7/7/2017 at 11:30 AM, sevenperforce said:

Just cut out the middleman and go with a Z-pinch.

Just pay no attention to the fission-based power supply needed (not a complaint, unless you are convinced that fission rockets can generate >1000Isp with reaction mass that can be stored long-term).  Also please ignore the shear size of the radiators.

[sevenperforce]

19 minutes ago, wumpus said:

Just pay no attention to the fission-based power supply needed (not a complaint, unless you are convinced that fission rockets can generate >1000Isp with reaction mass that can be stored long-term).  Also please ignore the shear size of the radiators.

A Z-pinch's capacitor bank can charge on solar power if the pulse rate is lower, and still outperform NERVA. But a Rankine-cycle small nuclear reactor would likely be a good idea anyway for the ship's power systems.