Best Engine For A Reusable Lander

[WaitingCynicism]

Hi all,

Currently I've been working on a reusable lander (think Falcon 9, but just the lander instead of the booster). What engine would you recommend for it?

[ajburges]

Depends on role. Terrier and Spark engines are my goto. Sometimes I use Aerospikes.

Considerations:

ISP: more ISP means less fuel used per landing while there are corner cases where engine mass itself would eat into ISP savings, craft with more efficient engines almost always get more dV per unit fuel. (see Nerv, Poodle, Terrier)

Length: shorter engines allow for shorter stacks which are easier to make stable on the ground. (see Terrier, Aerospike, Spark, Ant) Alternatively, surface mounts trade efficiency to remove engines from the stack length (see Twitch, Thud)

TWR: more TWR means you need less engine mass to achieve the TWR you desire for landing. Not commonly considered, but this consideration is why you don't see many landers with Nerv or Ion engines.

[WaitingCynicism]

I'm thinking of mostly using it for a mission to the moon and back.

[Aegolius13]

I'm a little confused about the mission profile - does it land on the Mun and just go back to Mun orbit to refuel?  Or does it come back to Kerbin?  

If you're only going down from Mun orbit and back, you don't need a lot of range so convenience may play a role as well as efficiency.  For an ordinary-sized lander, the Terrier is usually a good all-round choice.  As mentioned above, they're short so you don't need a ton of landing leg and can stay close to the ground.  If one doesn't provide enough thrust, you can always cluster several.  Sparks can serve a similar purpose the same thing for smaller landers, and again you can use more than one if you want a little more thrust.  Poodles are good for larger craft or higher gravity, but are probably overkill on the Mun for anything short of mining ship.

If you have Making History, the Cub is a nice lander engine.  Best thrust and ISP of any of the radials.  Only catch is they only gimbal in one axis each, but reaction wheels usually suffice for attitude control.  

Nuclear engines can be useful for big mining rigs and the like, but for a typical lander, I think it'd be more trouble than it's worth to accommodate their height.

 

[ajburges]

Moon and back to where?

Assuming 1-3 seats and a science suite

Mid/Low Munar orbit: Ant array or Spark. Ants are more efficient on dV, but 1 Spark gives plenty of thrust and spamming 6-8 Ants just looks silly if you know anything about rocket thrust physics.

Kerbin orbit: Spark(s) or Terrier. You need much more dV to go from LKO to Mun and back. More care in design if you wish to aerobrake. This might push you out of the "weight class" where sparks dominate.

Mun is about the biggest body you can consider Nerv engines. Even then you need a massive vessel for them to make sense.

[WaitingCynicism]

It is going back kerbin from the mun.

[ajburges]

Wait, do you want a reusable lander or a SSTO capable of a Mun landing?

[WaitingCynicism]

It's not completely an SSTO - there is a booster stage that will get it first into orbit. But from orbit, I want it to be able to have enough dV to be able to sort of coast to the Mun, land, and then come back, though I realize it's probably borderline impossible to do that.

 

[Ultimate Steve]

14 minutes ago, WaitingCynicism said:

It's not completely an SSTO - there is a booster stage that will get it first into orbit. But from orbit, I want it to be able to have enough dV to be able to sort of coast to the Mun, land, and then come back, though I realize it's probably borderline impossible to do that.

 

That should be possible. I'd recommend the nuclear engine or the terrier if you haven't unlocked the nuclear engine yet.

[steuben]

Well with in the realm of the doable. It is approximately 3700 m/s for a round trip. My go-to would a Nerv if you have them, since you are using a lifter to get it to orbit.

Borderline impossible is an Eeloo return with only 90 science tech.

[WaitingCynicism]

Thanks guys! It's in sandbox mode so I'm mostly just screwing around, haha - thanks!

[ajburges]

16 minutes ago, WaitingCynicism said:

It's not completely an SSTO - there is a booster stage that will get it first into orbit. But from orbit, I want it to be able to have enough dV to be able to sort of coast to the Mun, land, and then come back, though I realize it's probably borderline impossible to do that.

 

That's possible (2700 m/s from LKO and only need to land with 900 m/s), but it seems like an odd mission profile. The only advantage of bringing your lander back to Kerbin's surface is the science for a vessel that has been to the Mun. It's essentially just a direct ascent lander without staging. Terriers or Sparks are the best for this as I mentioned due to mass considerations.

What I find more interesting is a lander that makes multiple Mun landings. It can dock with a mother-ship in LMO for refueling or return to a station in LKO after an Aerobrake. The Aerobrake dV requirements are similar, but you don't need parachutes!