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help i need to land on moho with only an ion engine


pshimko27
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okay so a bit of contex, so i got to moho using a main flyby rocket and a mini probe with an ion engine, we got to low orbit but we want to achieve landing, the rocket is only 2.2 T, we cannot send another mission, and it has a twr of only 0.32, it has 5722 deltaV leftover

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You need to shed weight to land, which in this case probably means fuel. Point normal or antinormal (:normal: or :antinormal:) and burn until your TWR goes above 1 or you go below 1000m/s of delta-V; if your TWR goes above 1 first you have a chance of landing, but if you have less than 1km/s then you don’t have enough left to attempt a landing. A single ion engine may not be enough for a Moho landing since Moho has relatively high gravity and ion engines are pretty weak.

If you get a high enough TWR and have the fuel left to attempt a landing, lower your periapsis to just skim over the surface at periapsis, then brake hard at periapsis to slow down for landing. Try to control your vertical speed so you don’t crash, but remember that pointing away from retrograde will cost you more delta-V and hovering is just wasting fuel.

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3 hours ago, deltadouchnozzle said:

okay so a bit of contex, so i got to moho using a main flyby rocket and a mini probe with an ion engine, we got to low orbit but we want to achieve landing, the rocket is only 2.2 T, we cannot send another mission, and it has a twr of only 0.32, it has 5722 deltaV leftover

A TWR of .32 at Moho sounds about right for a single Ion engine and a ship that weighs about 2.2T.  If that is the case, you will never get your TWR above 1.0 at Moho with your single Ion engine.  So a safe landing will not be possible.

I estimate that a small probe would need at least 3 Ion engines to have a TWR > 1.0 at Moho.  You'd also need enough solar panels and batteries to keep them running full blast for several minutes.

 

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Joking aside, to land on Moho the bare minimum from the DV map is 870, so yes 1000 is probably a good number to go to as @jimmymcgoochie states. If you want to get back off of the ground into orbit, you'll need BARE MINIMUM 870*2 = 1740 m/s before you start your descent, and it's much better to have at least 2000.

Note if you're daring you can start to descend before you're under a local TWR of 1 just so long as you are over 1 before you get close to landing. So you could note how much dV you have currently, quick save, burn enough to to get to TWR 1, and then restore from the quick save and plan ahead so you have that much TWR with a few hundred m/s left in your descent. It'll be a harrowing landing but it will be possible with care. I did it on Tylo once. Once.

You could also use the rocket equation to do all this but I've never used it myself and don't plan to in the future.

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29 minutes ago, 18Watt said:

A TWR of .32 at Moho sounds about right for a single Ion engine and a ship that weighs about 2.2T.  If that is the case, you will never get your TWR above 1.0 at Moho with your single Ion engine.  So a safe landing will not be possible.

I estimate that a small probe would need at least 3 Ion engines to have a TWR > 1.0 at Moho.  You'd also need enough solar panels and batteries to keep them running full blast for several minutes.

 

i already have 2 solar panels and a 1.25M powerbank, i have 3 lt2 landing struts can those handle the stress?, also since i will probably have to send another mission anyways would it be better to land a probe on eve or try again with moho?

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1 hour ago, jimmymcgoochie said:

You need to shed weight to land, which in this case probably means fuel. Point normal or antinormal (:normal: or :antinormal:) and burn until your TWR goes above 1 or you go below 1000m/s of delta-V; if your TWR goes above 1 first you have a chance of landing, but if you have less than 1km/s then you don’t have enough left to attempt a landing. A single ion engine may not be enough for a Moho landing since Moho has relatively high gravity and ion engines are pretty weak.

If you get a high enough TWR and have the fuel left to attempt a landing, lower your periapsis to just skim over the surface at periapsis, then brake hard at periapsis to slow down for landing. Try to control your vertical speed so you don’t crash, but remember that pointing away from retrograde will cost you more delta-V and hovering is just wasting fuel.

thanks but lowest weight i can get to is not much better, only 1.9 Ts

1 hour ago, Gargamel said:

Moved to Gameplay Questions. 

thank you, i dont know where everything is

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3 hours ago, pshimko27 said:

i already have 2 solar panels and a 1.25M powerbank, i have 3 lt2 landing struts can those handle the stress?, also since i will probably have to send another mission anyways would it be better to land a probe on eve or try again with moho?

The short answer is No.  Your landing struts will not be able to absorb the stress of impact, no matter how well you fly the descent.  At some point in your descent you will need a TWR > 1.0.  And with a single Ion engine, you will never achieve that at Moho.   If you really want to land on Moho, you will need a vessel which has the thrust to accomplish that.  A probe with a single Ion engine is not going to work.

Or, as @Superfluous J mentioned, you WILL land, guaranteed!  Just not in one piece.  But then again, that's the Kerbal way!  In other words, you've gotten as much use out of that probe as you can- Why not attempt a landing?  Yeah, us folks on the forums are telling you it's going to crash, but so what?  It's just a game.  You now know you need a better TWR to land at Moho, and your probe is at the end of it's usefulness anyway.  So why not make some landing attempts?  

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I'd just like to point out - landing a probe with a single ion engine on Moho absolutely can work. I've done it before. It just can't be massing two tons.

You can get about 1.5 terminal TWR out of a single ion engine while still doing some useful science. It's not fun or easy to land with that, but it's possible.

Sample probe:

1x Dawn engine, 1x HECS probe core, 1x radial xenon tank, balanced on the other side by 1x DTS-M1 antenna, 2x OX-4L panels, 2x Z200 batteries, 1x thermometer, 1x barometer, 1x seismometer, 1x gravioli detector.

Wet mass 529 kg, dry mass 489 kg, wet TWR = 1.400, dry TWR = 1,515, wet dv = 3238 m/s.

You could shave off a battery but I packed two for the sake of the antenna which is pretty hungry when transmitting. Solar panel output should be above 21 EC/s before heat losses, so the engine is going to be fine even with a slightly unfavorable sun angle.

The descent trajectory should still be chosen so that the solar panels remain in sunlight - i.e. ideally a polar orbit around Moho aligned with its solar orbit. This will also prevent you from losing your commnet link. The probe has no landing legs, so it must balance on its engine. The HECS' reaction wheels should help keep it upright as long as it has power. If the probe did not spend too much dV maneuvering prior to descent, and you're worried about falling over and/or losing power while on the ground, you might have enough dV to get back to orbit before starting to transmit your surface science.

Edited by Streetwind
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