Dart vs Terrier for this science-harvesting moon lander

[THX1138]

For a lander intended to collect science around the Jool moons without atmosphere, should the engine be a Terrier or Dart?

I think the Dart might be over powered and the Terrier under powered. If I take off with an under powered engine, I'm wasting energy "fighting gravity", right? All the science I don't understand; it's just my job five days week. I could use the Dart and double the size of the fuel tank? 

[Kertech]

The way I answer this question is with kerbal engineer (KER). It gives you a thrust to weight ratio and you can change which body you're around! I aim for about 1.5 take off thrust to weight but that's cause that's what I'm used to more than any technical reasoning. 

 

Side note terrier might be over kill out there, apart from tylo and laythe, jools moons have pretty pathetic gravity!

[Sereneti]

how much mass can you bring to your target?


the question is: how much dv do you need?

 

Edited July 12, 2016 by Sereneti

[foamyesque]

Your limiting factor is Tylo. Is this a one-way lander? If so, you want to budget probably 2.5km/s of deltaV, and probably carry a Tylo TWR of over 1.2 (approximately 1.0 on Kerbin). You could cheat it and start your deorbit process with under 1.0 Tylo TWR as long as it climbs above that by the time you're going vertical, but that's taking some nasty chances. Checking it, I'd go with the Dart. You could use the Terrier, but it'd be risky.

 

EDIT:

 

Another option would be to radial-mount a pair of Twitch engines to pair with the Terrier, which would give you much more control than the Dart, and a lighter lander with marginally higher delta-V.

Edited July 12, 2016 by foamyesque

[juanml82]

For Bop and Pol, Terrier. Those moons have ridiculous gravity.

For Vall, Dart. You could use KER or MJ to check your TWR, but I don't think Terriers will give you enough thrust.

For Tylo and Laythe, I don't think that lander is adecuate

[Snark]

Moving to Gameplay Questions.

For Bop and Pol, definitely Terrier.  Anything bigger is massive overkill there.

As for Vall:  I think you'd be okay there.  It has a surface gravity of 2.3 m/s², so assuming your lander isn't much more than 9 tons, a Terrier would give you a local TWR of nearly 3, which is plenty.

For Tylo, in general you'd definitely want the Dart, because a Terrier simply wouldn't have the oomph unless the lander is ultra-light.  You need TWR on Tylo.

However, in your case it's moot, because it's mathematically impossible for the lander you've pictured to land on Tylo; it doesn't have anywhere near enough dV.  Just visually estimating, I'd guess your lander is around 9 tons or so, fully loaded with fuel?  And it looks like it's packing 4.4 tons of fuel, if you include the Oscars on the rovers.  So that would be about 2200 m/s of dV for your lander.  That's not even enough to land on Tylo, let alone take off again.  You'd need more than twice that much dV to go from low Tylo orbit to land and back to orbit.  (Or do you have more stages for it, or something?)

 

[Norcalplanner]

Tylo needs something in addition to whatever you're using on other moons. It could be drop tanks, asparagus stages, or a separate landing stage similar to the Apollo LM.

[Aegolius13]

I'd go Dart.  I like overpowered vacuum landers, in part because I tend to misjudge the suicide burn timing and need the TWR to stop in time.  But the gravity drag savings also helps.  

You can also tweak down the thrust limiter if it seems excessive in some situations, like docking maneuvers.

[Zhetaan]

I'd go Terrier.  The Terrier and Dart have nearly equal efficiency in vacuum, but the Dart is twice the mass, ten times the cost, has no vectoring, and while it is true that you can tweak the thrust down to Terrier levels for low-gravity moons, there's a much easier way to get Terrier-like thrust (guess what it is!).  If you think you need extra thrust, I'd suggest that you bring two Terriers.  The Dart only starts to carry its rather heavy weight when you need three Terriers' worth of thrust or you want a single-engine solution for an atmospheric lander.

[Corona688]

6 minutes ago, Zhetaan said:

I'd go Terrier.  The Terrier and Dart have nearly equal efficiency in vacuum, but the Dart is twice the mass, ten times the cost, has no vectoring, and while it is true that you can tweak the thrust down to Terrier levels for low-gravity moons, there's a much easier way to get Terrier-like thrust (guess what it is!).  If you think you need extra thrust, I'd suggest that you bring two Terriers.  The Dart only starts to carry its rather heavy weight when you need three Terriers' worth of thrust or you want a single-engine solution for an atmospheric lander.

Agreed.  I managed one of those annoying 'make crew reports landed at these 5 nearby spots' mun missions with droptanks, a pair of terriers, and 4 suborbital hops.  And I suspect thrust vectoring will be needed if you ditch one of those rovers and take off with the other.

Edited July 13, 2016 by Corona688

[Firemetal]

I'd say terrier. The Mun is such a simple moon to land on so you don't really need a dart. Now lets say you were landing on Tylo (In this case you would need a bigger lander) you'd want to use the dart due to high TWR. Darts also work nicely for Eve ascent vehicles. But I always find that darts make my rocket incredibly hard to maneuver in denser atmosphere.