Xenon fuel and Probes

[Tandy45]

I'm heading off to work shortly so I'll not be able to reply for a while, however I have a couple of questions that I haven't been able to find any answer too. My first question regarding the Xenon fuel, Is there at this moment in time anyway to re-fuel a probe or are they generally one way tickets to where ever I wish to send them? I've built my first one and I'm looking at launching it to Duna, however I'm concerned with the 700 litres of Xenon fuel I may not be able to get there and orbit sufficiently with the fuel I have. Secondly I was wondering at which stage do you tend to separate the launch stage and the probe? would this be done on an orbit of Kerbin or during an escape trajectory?. Many thanks for any advice.

Edited August 7, 2014 by Tandy45

[Kasuha]

You can refuel xenon as easily as any other fuel - just dock or claw the refueler with your ship, right-click the two containers and do the transfer.

Whether or not your probe can get to Duna depends on how much dv it has. That depends on how heavy it is compared to the mass of fuel you have on it.

The right time to stage off your previous stage is when it runs out of fuel. If it can get your probe all the way to Duna, you save a lot of Xenon.

Don't forget you can aerobrake at Duna to save a lot of fuel on braking.

[tjsnh]

The short answer is : you can re-fuel xenon from a docked ship/tanker/station just like regular fuel.

You can also move xenon around on the same ship by right clicking the fuel tanks (it might be alt click) like Kashua said.

Most people only use xenon/ion on one-way probes.

I've generally found that for a SMALL probe, two of the in-line xenon tanks can get me to jool and at least one of it's moons. This can DRASTICALLY change based on your flight profile, orbital windows, and all that.

I normally don't turn on my ion engines until I'm outside kerbin's SOI and orbiting the sun (I do the same for LV-N engines).

[Kasuha]

I normally don't turn on my ion engines until I'm outside kerbin's SOI and orbiting the sun (I do the same for LV-N engines).

Not a good choice. Ion burns are long so you don't gain as much from Oberth effect as with stronger engines but you can still save a lot of fuel if you make direct burn from low orbit, even with ions. When in interplanetary space, you should only issue corrections such as fine-tuning your intercept or performing inclination change if really, really necessary.

[Streetwind]

I've built my first one and I'm looking at launching it to Duna, however I'm concerned with the 700 litres of Xenon fuel I may not be able to get there and orbit sufficiently with the fuel I have.

If this worries you, you could try solving the rocket equation: dV = Isp * g0 * ln(m1/m2)

First, determine your probe's fully fueled mass. To do this (without mods), remove all stages except for the probe itself, then press launch to put it onto the launchpad. You can now go into map mode and click the little "i" icon on the righthand border of the screen. A little window pops out where yo ucan see the mass. Let's assume your probe weighs something weird like 1.83 tons.

Next up, the probe's empty mass is easily determined through knowing that 700 units of xenon fuel weigh 0.07 tons. Thus the empty weight is 1.76 tons.

You know the Isp from the PB-ION engine's stats, and g0 is a physical constant that is set to 9.82 in KSP. Now all you need is a calculator to do the math for you.

dV = 4200 * 9.82 * ln(1.83/1.76) = 1608,6 m/s

Consulting a dV map or launch planner app will tell you that this made-up example probe should be capable to travel one-way from low Kerbin orbit to Duna (but probably requires aerobraking).

Edited August 7, 2014 by Streetwind

[tjsnh]

Not a good choice. ..snip...really necessary.

I've never been able to do an interplanetary burn from LKO (mostly because I'm far too lazy to wait for launch windows before going up, I usually get to solar orbit then 'fast forward' to the window), I know it's not the most optimally efficient method but it's "quick and easy" for my impatience ;-)

[cantab]

The kethane mod lets you mine kethane and turn it to xenon, but the equipment's heavy so not really suited to light probes. Otherwise you can transfer from a refueller like others have said - but really, with ion engines it's relatively easy to pile on the dV, especially if you use drop tanks.

As for when to release the probe, there's three main options. Release it in LKO, in which case it goes up on a standard launcher. Release it after the departure burn, in which case it needs either a Kerbin departure stage or just an overengineered launcher eg one that normally puts bigger things into LKO. Or release it when the stage before runs out of fuel, which may be most efficient overall but liable to litter Kerbin orbit.

[DavidInAuburn]

I've never been able to do an interplanetary burn from LKO (mostly because I'm far too lazy to wait for launch windows before going up, I usually get to solar orbit then 'fast forward' to the window), I know it's not the most optimally efficient method but it's "quick and easy" for my impatience ;-)

Why not just switch back to the Space Center after you've reached LKO and fast forward to the window from there?

[tjsnh]

Why not just switch back to the Space Center after you've reached LKO and fast forward to the window from there?

No particular reason. I usually over-gas my missions by a good margin (I'm paranoid like that) so squeezing every last ounce of efficiency hasn't really been an issue. I know, academically, that there are far more efficient flight profiles, I just haven't needed to rely on them - yet.

[Kasuha]

I've never been able to do an interplanetary burn from LKO (mostly because I'm far too lazy to wait for launch windows before going up, I usually get to solar orbit then 'fast forward' to the window), I know it's not the most optimally efficient method but it's "quick and easy" for my impatience ;-)

I don't really blame you for anything as I used to fly my missions that way too. But then I found out that I can save a lot of fuel if instead of ejecting I just plan a maneuver that will get me on Kerbin's Solar orbit and then add another maneuver on that to find the intercept ... and then I just fast forward till Kerbin is at that point and eject from LKO.

Then I stopped using even that and started using alexmoon's planner. Not only you can find really optimal transfers, you can also make it to give you immediate optimal transfer if you're impatient.

[Tandy45]

Thanks for the information guys, looks like I'm going to have to start sending out re-fuelling mission's shortly, I always ended up staging off as soon as I began my manouvers which probably explains a lot of my frustration when probes didn't act as planned. Looks like I'm going to be rethinking my entire space strategy.