ION engine question 2

[grooble]

Could you have some capsule or container with an ION engine bolted on break free from 200km LEO and get to an escape velocity and on a course for mars?<br /><br />No time restrictions.

 

[mrmorris]

Certainly. This is similar to what SMART-1 did. That trajectory was a bit easier, as they shifted from Earth-capture to Lunar capture (not quite escape velocity) -- but given sufficient time and propellant it could have achieved escape velocity as well. <br /><br />However -- it's unlikely that the container would be placed into a 200km orbit (the one figure implying a circular orbit). More likely it would be placed into a highly elliptical orbit. This would make reaching escape velocity much easier for the craft.

 

[grooble]

Well it wouldn't have people in, just water.<br /><br />Say i have 5000kg of water to shift, is 1000kg enough for the capsule, ion engine + fuel and anything else needed to get it safely to mars, do you think?<br /><br />I get that number from the 6000kg the Falcon 5 is said to be able to put into 200km leo.

 

[henryhallam]

I think you would struggle with that low a mass fraction unless your ion engine is extremely lightweight with a high ISP. Remember it needs a power supply too.<br />You need ~4 km/s from LEO to get on a trans-Mars trajectory, a bit less if it's a "good" launch window: the launch windows come every 26 months but they follow a longer cycle, 15 years I think, and the dV required changes a bit over that period.<br /><br />dV may also be somewhat higher for a spiral trajectory; I have heard this in a couple of places but I'd really like to know for sure about that if anyone is clued in?<br /><br />Given an ion engine Isp of about 2500 secs, the mass fraction must be e^(4000/(9.8 * 2500)) = 1.18. This is the ratio of initial mass including propellant to final mass. Therefore for 6000kg initial mass you have 6000/1.18 = 5080kg final mass, which includes your water and also solar cells, comm. gear (or just a computer if you're feeling brave) and the engines. I think you could probably get about 4000kg of water to Mars, or a bit more with some clever engineering - perhaps you ship it as ice instead, and just have a thin reflective metal layer instead of a tank? You can also get improvements if you increase the Isp.<br /><br />Of course this only puts it on a Mars-intercept course, if you want to brake into Mars orbit and descent to the surface you won't get a payload nearly that high.<br />If you want to move lots of water around it might be easier (and faster!) to set up a few robust robotic tractors to cut ice from the pole and drag it to where you need it, than to import it from Earth.

 

[spacester]

grooble, here's how I would do it. <br /><br />If I'm constrained to Falcon V and the objective is to put water in Martian orbit, I find it impractical to do all the deltaV in one shot. Well I find it impractical overall, that is just too many flights. Be that as it may, what you want to do is deliver as much of that LEO-delivered high cost stuff - the water in this case - with reusable hardware.<br /><br />With ion drive, what you're looking for is a spiral trajectory to reduce the mars arrival deltaV to near zero, IOW aerocapture. This can be done. What you need to do is get that 6000 pounds started off on the correct trajectory. Unlike traditional burn - coast - burn trajectories, if you stage your vessel after TMI (Trans Mars Injection), the booster is not headed all the way to Mars before turning around. <br /><br />IOW I would use a chemical-powered booster (Kerosene / LOX perhaps) to push my ion driven, RTG powered second stage in the right direction to have the ion drive get me to Mars such that aerocapture becomes a near certainty. The chemical booster would return to LEO for another mission, keeping the pipeline flowing.<br /><br />The second stage would be reusable as well, the return trip for it could be a long spiral, maybe Venus-assisted.<br /><br />I've read many times (IIRC lol) that the Hohmann transfer is the <i>lowest possible deltaV</i> trajectory. BUT . . .<br /><br />Hohmann assumes circular orbits. Mars is a long ways from circular. When you plot out a Hohmann transfer in the real solar system, you find that, unlike circular Hohmann, your arrival velocity tangent vector is NOT parallel to the velocity vector for Mars in its orbit. This leads to gravity losses to be added to your Hohmann calculation.<br /><br />The result is that it seems entirely possible to me that a modified Hohmann, a biconic, a GRABER trajectory would come in with less dV than Hohmann.<br /><br />I'm pretty sure that any spiral trajectory is going to need more total dV than Hohmann. If fo <div class="Discussion_UserSignature"> </div>