Xenon Engine use?

[CubertFarnsworth]

Trying to find a reason as to why I would use the Xenon engine/fuel on a build-out. Aside from VERY small probes any other reason? 

[ForScience6686]

No other reason really.  And to be honest I never use them.  Chemical engines do just fine for anything I've sent out.  And if you need that kind of dv, your thrust is so miserable that you end up losing dv due to immensely long burn times.  My patience just isn't big enough for them.

[Starman4308]

It really shines for expanded-scale systems, like 6.4x, where delta-V requirements can be absurdly higher than stock (such as roughly 7-10 km/sec to capture into elliptical Moho orbit). It helps to have loooots of patience (and a second screen to do other things like post on the KSP forums), though, as even at 4x physical timewarp, burns can take a long time.

[Palaceviking]

Just bought a lab back from pol on ions, took eons, don't do it if your craft is anything but tiny.

[DD_bwest]

i only  tend to use them when going to moho as the proximity to the sun massively increases your power output by your panels, this allows you to use more ion drives.    my last trip to moho had a command vessel with 14000 delta v.

4x physics warp is sortof a must tho,  brings hour long burns to 15minutes or so.

and you need to pay attention to your approach location. your  trip can be ruined if your not paying attention and have that approach  on the dark side.. running outta juice and having to wait until you  re amerge from the other side really sucks..

Edited December 17, 2016 by DD_bwest

[CubertFarnsworth]

2 hours ago, DD_bwest said:

i only  tend to use them when going to moho as the proximity to the sun massively increases your power output by your panels, this allows you to use more ion drives.    my last trip to moho had a command vessel with 14000 delta v

I understand you have a ton of Delta, but that is also limited to the amount of xenon you have, correct?

[DD_bwest]

Just now, CubertFarnsworth said:

I understand you have a ton of Delta, but that is also limited to the amount of xenon you have, correct?

yea,  Your deltaV is directly related to your amount of Xenon.  just like any other propellant.   For Moho, the capture burn is rather expensive, so my methods usually include over doing it on the fuel so that i have more then enough deltaV to get everything done and get home.  For me to make a ship capable of that using nuclear engines on my current laptop drastically slows it down to a point where its frustrating to play. 

[bewing]

I like ions for a reusable biome hopper for medium-G airless CBs. With a nuke engine on the Mun, you can only hit about 3 biomes before you have to refuel any reasonable biome hopping lander. With an ion hopper you can visit them all (provided you stay on the sunny sides of things, or have RTGs).

 

[surge]

realism? duh.

[foamyesque]

For a lot of stuff that can get away with a Kerbin TWR of 0.2 or less they're very effective, but they can skyrocket part count with all the fiddly bits they need; nukes or chemical engines are much simpler solutions as craft weight climbs.

[Snark]

They're good for when you want scads of dV in a relatively compact package.  The Isp of 4200 is insanely higher than any other stock engine in the game.  If you need really high dV and don't want a ship massing thousands of tons, this is your go-to solution.

The tradeoff is that they have super low TWR, which mean you have to be extremely patient about your burn times.  They're also not super practical in the outer solar system, where solar power is scarce.

In short:  they're a highly specialized engine.  Within their specialty, they're fantastic (and I love them).  Outside that narrow specialty, they're not super useful.  So it's a matter of your use case.

[Aegolius13]

In addition to the expected tiny probe missions, they're quite good if you need to rescue a Kerbal in a low solar orbit.  They provide the high delta v needed,  are easy to power with solar, and TWR is not very important in solar orbits.

[Blaarkies]

On 12/17/2016 at 10:33 PM, CubertFarnsworth said:

Aside from VERY small probes any other reason? 

If one Ion engine can decently propel a 1t probe, then it stands to reason that 10 Ion engines can do the same for a 10t ship, right? Don't get caught up in the "ion are only for small probes" argument, it's false for all engines. Maybe for a 50t ship this becomes a part count problem, since every Ion engine needs fuels tanks, RTG/solar, batteries.

At the end of the day, its all about engine TWR. But to compare TWR, you need to measure your engine mass as (engine + fuel source + electrics) vs the thrust produced. That TWR there is higher than the max acceleration you could expect after adding the payload. Want more TWR? add more of the engine+fuel+electrics packages, to essentially divide the payload burden among more engines.

Doing the same comparison for nukes shows that they have a much better TWR, but still limited. You can always just add more, but you will never beat that calculated max TWR. For practicality, remember that everything other than Xenon can be mined almost everywhere, so using engines other than Ion drives means you don't carry all the return fuel with you, decreasing mass and engine requirements.

[Guest]

Xenon is one of the most dense practical propellants... in reality, and I think that carries over to the game. Smaller craft are easier to launch/transport on a lifter/mothership.

 

1 hour ago, Blaarkies said:

If one Ion engine can decently propel a 1t probe, then it stands to reason that 10 Ion engines can do the same for a 10t ship, right? Don't get caught up in the "ion are only for small probes" argument, it's false for all engines.

The trouble with large ships is the square cube law: their mass scales with volume, while the available space for mounting engines and solar panels scales much less dramatically with surface area... and to conserve mass there is an incentive to keep surface area as low as possible.

Edited December 20, 2016 by Guest

[Blaarkies]

3 hours ago, String Witch said:

Xenon is one of the most dense practical propellants... in reality, and I think that carries over to the game. Smaller craft are easier to launch/transport on a lifter/mothership.

 

The trouble with large ships is the square cube law: their mass scales with volume, while the available space for mounting engines and solar panels scales much less dramatically with surface area... and to conserve mass there is an incentive to keep surface area as low as possible.

Not sure about the most dense, though it is used by a type of engine that has a very high exhaust velocity. The big xenon tank holds about 0.5t xenon fuel, thats a bad wet to dry mass ratio. The small 1.25m LFO tank holds close to the same mass within the same volume...but dv wise, the xenon wins by far.

The square cube law is very very true. I did however see a "Ion launch from Laythe to orbit" challenge on the forums today...apparently ion engines can thrust while inside a fairing? Anyway besides clipping parts for aesthetic purposes, I would suggest engine mountings all over the ships surface...thus only battling the "square square law" ?

[Streetwind]

5 hours ago, Blaarkies said:

Not sure about the most dense, though it is used by a type of engine that has a very high exhaust velocity. The big xenon tank holds about 0.5t xenon fuel, thats a bad wet to dry mass ratio. The small 1.25m LFO tank holds close to the same mass within the same volume...but dv wise, the xenon wins by far.

That's not density you're talking about though. You're talking about mass fractions of tanks. It is completely independent of density.

IRL, density is a measurement of how much mass fits into a specific volume. In KSP, "density" is the name of a field in the resource config file that says how much mass one 'unit' of this resource represents. At first glance this may look like it's the same thing, but since KSP does not have any volume units, there is no such thing as an actual quantifiable density. Instead, what KSP calls "density" is actually a unit of mass, not a unit of the relationship between mass and volume. The 1.25m xenon tank holds 5,250 'units' of xenon, which means nothing other than that it holds 5,250 times a mass of 100 grams. Which is the "density" - or rather the mass - of xenon.

And the only reason why it has to be 5,250, and not perhaps 15,000 or 42, is because that's the design target for the mass fraction of a xenon tank with a dry mass of 413 kg. Its hypothetical volume has nothing to do with it

Edited December 20, 2016 by Streetwind

[Guest]

7 hours ago, Blaarkies said:

The square cube law is very very true. I did however see a "Ion launch from Laythe to orbit" challenge on the forums today...apparently ion engines can thrust while inside a fairing? Anyway besides clipping parts for aesthetic purposes, I would suggest engine mountings all over the ships surface...thus only battling the "square square law" ?

Ions firing from within a fairing might be because they don't do exhaust damage, so maybe that means they can't be occluded either, or it's to do with fairings not occluding things properly.

Yeah, you totally could pull off a Corellian Corvette type design for a larger ion ship. The stock structural girders and such are very heavy but even with those the extra thrust from more engines probably negates the supporting structure and then some. I made a ship with a ring of ion engines around the centre and it works reasonably well but the partcount is a bit too intense for my system.

[Blaarkies]

15 hours ago, Streetwind said:

That's not density you're talking about though. You're talking about mass fractions of tanks. It is completely independent of density.

Thats completely true, the part config files show how twisted these things become in KSP. To clear it out I meant literal mass divided by volume, where volume is an eye estimation of the 1.25m tank sizes.

I have an idea of using the tiny cube attachment part for this, to connect 5 engines onto one part(offset them a bit for aesthetics) and use that part as a sub assembly. This can cut down on the weight and part count for ion powered craft. Luckily Ion engines don't need much structural support strength since they are so weak.

Maybe for larger ships it is fair to build multi-layer ion engine stacks(the upper clipped engines firing directly into the bottom engines). Even though that is horrible clipping, it's the closest we get to building a stock 100kN ion engine.

...but they do cost a lot of funds.