What the best vacuum engines

[Elon Tusk]

I need to save 5 kerbals from Mun. How? SSTO or rocket? And which engine the best for it?

[Vanamonde]

Once out of atmosphere, thrust becomes less important and efficiency is the primary consideration. However, too little thrust can make burns difficult to time. So it depends on the size of the rocket. What does yours look like? 

[Maria Sirona]

The Terrier is one of the best vacuum engines. It is very efficient, and it has considerable thrust. And you can of course slap many of them on the ship if need to.

[king of nowhere]

you can do it in a dozen different ways, really.

if you have 5 kerbals on mun, i assume you already landed on mun. if you can land on mun, then you can keep using the same rockets you already used. just leaving room for the stranded kerbonauts

[GoSlash27]

I'd recommend a staged rocket for going to the Mun. For a lander using a hitchhiker can and a single seat lander can, that's going to work out to around 3 1/2 tonnes or so. A single 48-7S with 6 FL-T100 tanks will get you from Munar orbit to the surface and back comfortably.

Best,

-Slashy

 

[Elon Tusk]

I need for long distances.

[GoSlash27]

5 minutes ago, Elon Tusk said:

I need for long distances.

How long?

 

[Davi SDF]

If you dont have or dont want to use nuclear engines (Nerv) and you have the Making History DLC, then the Wolfhound is very balanced (perhaps too balanced) between thrust and efficiency (ISP)

[jimmymcgoochie]

Are your five stranded on the surface of the Mun or in orbit?

If in orbit then things are a bit easier and just building something efficient is good enough. Engines like the Terrier (1.25m), Cheetah (1.875m, Making History DLC) and Poodle (2.5m) are the best options as they provide high ISP, with each step up in size coming with twice the thrust and twice the mass, more or less, so it’s pretty easy to scale up. I would stick a Mk1-3 pod (3 crew) under a passenger cabin (2 crew) to hold the five of them- Hitchhikers are sensitive to heat much more than crew pods and the crew pods are better protected and more stable during re-entry- with a probe core inside a service bay on top so it’s controllable with nobody aboard, then a heatshield with 20% ablator (or even less if you have the re-entry heating set below 100%) and then your upper stage powered by either a Terrier or two or a Poodle (or Cheetah if you have MH). It doesn’t take a huge amount of fuel to get good range out of them and by using a powerful first stage engine combined with some chunky boosters you should have oodles of delta-V to get to the Mun, meet up with the crew and then bring them back.

For a crew stuck on the surface things are a bit more difficult- the increased delta-V needed is a factor but you also need a high enough TWR to land (this isn’t too difficult for the Mun) and to land vaguely near them. You might find it easier to send out a small rover that can carry crew so that if your landing is a bit inaccurate you don’t have to walk them all the way over individually.

The alternative method is to just use external seats to carry the crew, making the whole thing much lighter but also increasing the risk of accidentally cooking your crew on re-entry. Putting the seats inside a service bay should protect them and making everything smaller and lighter will also make it easier.

 

The NERV has the best ISP of any rocket engine in KSP, but it’s also heavy and you can only use a specific set of fuel tanks as it uses just liquid fuel. For anything below about 4500m/s, the Terrier is the better option due to its vastly lower weight, not to mention cost. The MH Wolfhound is the best bipropellant engine in KSP in terms of ISP and one of the best vacuum engines in terms of thrust, but it’s pretty heavy for the thrust you get out of it compared to the Poodle; use with caution. The Rhino is massive and while its ISP is respectable and its thrust is the highest of any single engine in space (a whopping 2MN!)  it’s just too big to be used for something like this.

Edited July 29, 2021 by jimmymcgoochie

[Elon Tusk]

On 7/28/2021 at 9:08 PM, GoSlash27 said:

How long?

 

Duna for example

On 7/29/2021 at 2:54 PM, jimmymcgoochie said:

Are your five stranded on the surface of the Mun or in orbit?

If in orbit then things are a bit easier and just building something efficient is good enough. Engines like the Terrier (1.25m), Cheetah (1.875m, Making History DLC) and Poodle (2.5m) are the best options as they provide high ISP, with each step up in size coming with twice the thrust and twice the mass, more or less, so it’s pretty easy to scale up. I would stick a Mk1-3 pod (3 crew) under a passenger cabin (2 crew) to hold the five of them- Hitchhikers are sensitive to heat much more than crew pods and the crew pods are better protected and more stable during re-entry- with a probe core inside a service bay on top so it’s controllable with nobody aboard, then a heatshield with 20% ablator (or even less if you have the re-entry heating set below 100%) and then your upper stage powered by either a Terrier or two or a Poodle (or Cheetah if you have MH). It doesn’t take a huge amount of fuel to get good range out of them and by using a powerful first stage engine combined with some chunky boosters you should have oodles of delta-V to get to the Mun, meet up with the crew and then bring them back.

For a crew stuck on the surface things are a bit more difficult- the increased delta-V needed is a factor but you also need a high enough TWR to land (this isn’t too difficult for the Mun) and to land vaguely near them. You might find it easier to send out a small rover that can carry crew so that if your landing is a bit inaccurate you don’t have to walk them all the way over individually.

The alternative method is to just use external seats to carry the crew, making the whole thing much lighter but also increasing the risk of accidentally cooking your crew on re-entry. Putting the seats inside a service bay should protect them and making everything smaller and lighter will also make it easier.

 

The NERV has the best ISP of any rocket engine in KSP, but it’s also heavy and you can only use a specific set of fuel tanks as it uses just liquid fuel. For anything below about 4500m/s, the Terrier is the better option due to its vastly lower weight, not to mention cost. The MH Wolfhound is the best bipropellant engine in KSP in terms of ISP and one of the best vacuum engines in terms of thrust, but it’s pretty heavy for the thrust you get out of it compared to the Poodle; use with caution. The Rhino is massive and while its ISP is respectable and its thrust is the highest of any single engine in space (a whopping 2MN!)  it’s just too big to be used for something like this.

I already saved them but thx for tips

[GoSlash27]

Elon Tusk,

 Duna is comfortably reachable using chemical rockets such as Terrier or Poodle. If you're going further, such as to the Jool system, you'll want to use the LV-N Nerv. Here's a chart I put together years ago showing the relative performance of both types:

If your trip is less than 2,500 m/s dv you should use a chemical rocket stage. If it's more, you should use the LV-N.

HTHs,

-Slashy

Edited August 1, 2021 by GoSlash27

[Elon Tusk]

But, is there sense for terrier and poodle, if we have Nerv? It's like better at all. Isn't it?

[Maria Sirona]

14 hours ago, Elon Tusk said:

But, is there sense for terrier and poodle, if we have Nerv?

Thrust isn't the speciality of the Nerv. For landing spacecraft on atmosphereless bodies (especially big ones like Tylo) you need to have thrust because there's gravity to fight against. And also, the lenght of it makes it impractical for landings as there should be some space left between the land and the engine. The Nerv is a long boi and there would be little to no room between the two.

[jimmymcgoochie]

16 hours ago, Elon Tusk said:

But, is there sense for terrier and poodle, if we have Nerv? It's like better at all. Isn't it?

The NERV may be the most efficient engine, but it’s also large (so doesn’t fit under a lander very easily), heavy (3 tons versus the Terrier’s half a ton), requires only liquid fuel (meaning only certain fuel tanks can be used), very expensive (many thousands of funds per engine against under 500 for the Terrier) and right at the far end of the tech tree (Terrier is in tier 4 or 5, available much sooner).

In many situations the much lower mass and smaller size of the Terrier makes it a better option- in fact the Terrier is more than enough to land on virtually every airless planet and moon in the system, with the exception of Tylo where greater thrust is needed, and it also works on Duna as the atmosphere is so thin that vacuum engines work at nearly full efficiency.

If you’re building a large ship and need lots of range the NERV is definitely the best choice, likewise if you’re building a spaceplane with jet engines as they use the same fuel and you don’t to lug around heavy oxidiser for some engines and not others, but outside of those situations you might well be better off using a Terrier or Poodle (or Cheetah if you have Making History DLC)- especially if you’re making a heavier craft and need more thrust to move it around, clustering NERVs makes the weight stack up really quickly and the Poodle has more than four times the thrust of a NERV for less weight than one.

[king of nowhere]

On 7/28/2021 at 8:02 PM, Elon Tusk said:

I need for long distances.

there is no such thing as a long distance in orbital mechanics. only large deltaV. Distance is mostly meaningless. 3 million km to go from Moho to Eve are far more expensive to cover than 30 million km to go from Dres to Jool. And then there are plane changes where you cover no distance, but they are super expensive.

On 8/18/2021 at 11:32 PM, Elon Tusk said:

But, is there sense for terrier and poodle, if we have Nerv? It's like better at all. Isn't it?

besides everything that @jimmymcgoochie said, I want to bring a personal example for how Nervs are not always the best despite great Isp.

I was running a Jool 5 mission. I had the mothership that was supposed to land and refuel on Vall, so it needed enough thrust to land on Vall. And of course since it was big I assumed it was a good idea to use the Nervs. The ship was fairly big, carrying fuel for 2 big landers, so I needed 24 nervs to land it on Vall. I also had issues because the mothership used liquid fuel, one lander used oxidizer, and one used a mix.

Later, when I got more experienced, I made some calculations. Those 24 nervs were 72 tons heavy. I could get more thrust with just 4 wolfhounds, for 13 tons. I made some calculations with the rocket equation, and it turned out, I would have gotten a greater deltaV if I had used 3 wolfhounds. the increased mass was just making me lose so much.

Nervs are great to manuever a large ship in deep space, but they have limitations and drawbacks like everything else.