Escape burns with realistic ion engines

[NFunky]

Hey all, so I've been trying to build a mars tranfer vehicle somewhat in the vein of Hermes (from The Martian).  For me, it's an 80 ton, crewed, ion propelled spacecraft.  Unfortunately, when I got it assembled and fueled in orbit, I discovered that my mod install had reduced the thrust of the engines from 2 kN to 0.25 N which is 8000 times less thrust, and KER told me a 10 m/s burn would take 4 hours and change (8 engines).  I am obviously going to have to rethink my plan, but that's actually not my main point of interest.

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All this got me wondering, how would a real world ion rocket leave earth orbit for an interplanetary transfer?  I am very experienced with using multiple perigee kicks to make low thrust escapes, but the thrust of real life ions is SO low that this isn't possible.  Even if I halved the mass of my rocket to 40t and tripled the number of engines, it'd still be more than half an hour for 10 m/s.  At that rate, it'd take more than 300 kicks to reach escape velocity.

I know that all real world ion spacecraft to date have been lauched into heliocentric orbits using chemical rockets, so I'm not sure it's actually possible to use such a low thrust system for earth escape.  There are however, several hypothetical vehicles, both fictional and real concept designs, that do appear to exclusively use ion propulsion once in orbit.

Does anyone know what kind of trajectory would be used?  The only one I can think of is a very long spiral up to very high orbit, and from there on to deep space.  However, I imagine this would eat up an absurd amount of dV, so it doesn't seem like a good option for anything except maybe a solar sail vehicle.

Ion drives in stock are time consuming, but you can basically treat them the same way you would small chemical rocket engines like the LV-1.  With realistic ion drives though, I don't have the first clue how to use them.  Thoughts?  Ideas?

[bewing]

35 minutes ago, NFunky said:

The only one I can think of is a very long spiral up to very high orbit, and from there on to deep space.  However, I imagine this would eat up an absurd amount of dV,

It's not absurd -- it's only something like a factor of two. Beyond that, it becomes much more important to use gravity assists. Since you are spiraling up,  you are basically always at the orbital velocity for that altitude. So any CB you encounter on the way up is already traveling at the same speed as you. (So you want the fastest moving one, which is always the first one you encounter, so: the Mun.) So when you create an SOI transition your relative speed is nearly 0, and you have lots of time to create a low-Pe encounter and generate all the Oberth that you can. Which means it becomes simply a matter of timing so that you encounter it at the proper point in your journey to get slingshotted in the right direction, and maybe saving some chemical dV (because you will want 250 m/s suddenly, at just the right moment).

 

 

 

[DAL59]

A slingshot around the Mun is the way to go.  Also, you can engage physical timewarp by pressing alt period

Edited November 5, 2017 by DAL59

[NFunky]

That's very interesting!  I guess I forgot to mention I was playing with RSS, but a lunar slingshot would work just the same.  A factor of two doesn't sound too bad.  Only thing is, an Earth escape burn is usually minimum 3210 m/s, so doubling that is a lot, but again, doable.

So, the flip side of this whole issue is, what do I do when I reach Mars?  The vehicle is not thermally shielded, so I can only use very minimal aerobraking.  Should I aim for a high periareon and try to spiral down from there?  Or, should I put my periareon in the atmosphere and try to use a combination of light aerobraking and ion thrust to at least capture into a very eccentric orbit?

Another question I have is approximately how much dV should I pack for a Mars orbit and return to LEO?  Sounds like I'll need quite a bit more than I would if I was doing high impulse burns (about 12 km/s).

 

 

[Streetwind]

This might be relevant to your interest: 

 

TL;DR: a worst-case constant thrust trajectory consumes as much as 2.41 times the dV of an ideal Hohmann transfer. Based on this, you should be able to make provisions for enough fuel.

[NFunky]

Amazing, thank you!  This link and info is very interesting and helpful.  I've been thinking about eventually doing some sort of RSS torchship in the near future, so this is very applicable to that as well.

As far as ion rockets are concerned.  I just remembered that they do mention Hermes' acceleration in The Martian as being 2 mm/s.  This is toward the end of the book, so I suspect it is quite a bit lower when the ship is fully fueled in LEO.  I seem to remember the book saying Hermes has four engines powered by a nuclear reactor.  Anyone remember if they mention the mass of the ship at any point?  I'd love to calculate how powerful those engines are.  I have a feeling they have WAY more thrust than the current generation of ion engines (like the one in the game).

Anyway, no matter what, I'm going to need a lot more engines.  I needto be able to leave Earth orbit in days or weeks, not years.

[wumpus]

Putting crew onboard an ion-powered ship is likely a bad idea (unless it is *just* about to use the Moon to escape, and then don't count on using ions to get back).

Ion propulsion is incredibly efficient to get from LEO to L2 (Earth-Moon Lagrange point), although current solar panel tech might not be up to the job (mainly the Van Allen belts).  Once you get to L2, you can pretty much go anywhere via the "Interplanetary Superhighway" (https://www.jpl.nasa.gov/releases/2002/release_2002_147.html ) [don't try this in KSP without Principia installed, and I have no idea how many CPU cycles it takes to find the routes].

I'd expect that once you use gravity tricks to get to escape velocity you will spend the rest of the journey doing a capture burn (matching Mar's velocity), and still doing a final aerocapture (you might still need a chemical burn to raise PE after an aerocapture).  Of course, getting back from moonless planets is an exercise for the reader (presumably boring old spirals all the way back to Earth).

[NFunky]

Wow, that's some serious food for thought.  Thank you.  Sounds possible, but now you've got me thinking it would be worth taking the loss and changing propulsion systems.  The habitation, life support, power and attitude control, so I can just slap a different kind of propulsion system on it.

[winged]

On 6.11.2017 at 2:49 PM, NFunky said:

Anyone remember if they mention the mass of the ship at any point?

110 t of inert mass before earth departure: