is it possible to land on mun with nuclear engines?

[senoiurkabls]

i made a 3 man lander and the landing stage uses 4 nuclear engines but i was unable to slow down enough and failed my 3rd attempt at a mun landing

can i use nuclear engines or are they to much weight?

Thanks so much for the input now I have to solve lateral movement and tipping over

Edited May 6, 2013 by senoiurkabls

[Semni]

Nuclear engines are going to give you a poor thrust to weight ratio but it is quite possible as long as you are conservative about your fuel loads. With their high efficiency they don't need all that much fuel to put you on the mun and lift back off again.

Now, for the mun they are a bit overkill but say if you are using the same engines to get you to another planet they can be quite worth your while. You may also be not giving yourself enough time to slow down, with the poor power to weight ratio you will have trouble decelerating in short distances even when you are capable of lifting off from the munar surface.

[rryy]

Yes, you can. Almost all of my Mun landers use nuclear engines. Try reducing the amount of fuel you are carrying, or stage the lander so you decouple empty tanks to increase the TWR.

[sgt_flyer]

i once made a mün lander with 8 nuclear engines which worked pretty well (it was for kethane mod) it was able to deorbit, land, fill it's Kethane tanks, refill it's fuel tanks (i added a kethane fuel converter on it) and go into orbit:) (net result after topping my fuel tanks once in orbit was almost 2/3rd of my kethane tanks still full, ready to be transfered to my tanker waiting to take it back to earth ) (basically, the lander was in cross configuration, 1 3man capsule, 1 rockomax /16 tank, and 4 of the second largest kethane tanks extending horizontally around it. (+ 2 nuclear engines strapped onto the sides of the kethane tanks) + all you would need on a lander (RCS, landing legs, etc)

one advantage it had, is it also worked as the transfer stage to mün with it's 8 nuclear engines (i just added a decoupler and a rockomax /32 under the lander for having enough fuel for the transfer) and jetisonning the extra tank when i just started my descent to mün

Edited May 5, 2013 by sgt_flyer

[Akalaël]

Of course you can !

Actually, it is on Minmus, but this kind of ships lands on the Mün for sure ! Begin your break burn early, the thrust is poor in fact !

[sal_vager]

i was unable to slow down enough and failed my 3rd attempt at a mun landing

Maybe how you are landing is the issue here?

If you are making a direct descent, i.e. you are dropping straight (or mostly straight) down to the Mun's surface from a Munar impact trajectory, then yeah you are going to have trouble stopping in time.

What you need to do is get into a trajectory that takes you past the Mun, but with a reasonable periapsis, 50km maybe.

Then at periapsis you burn retrograde to get into a Munar orbit, then lower your periapsis again to about 5 to 10km.

When at the new low periapsis, you burn retrograde once more to stop your horizontal movement, then lower yourself to the surface on the LV-N engine thrust nice and carefully.

If you do it this way you won't find yourself slamming into the Mun at high speed any more

[EndlessWaves]

Nuclear engines are going to give you a poor thrust to weight ratio

No they're not, because your weight is so much lower on the Mun.

An LV-N on the mun has a TWR of 16.6. For comparison an LV-T45 has a TWR on Kerbin of 13.5, and an LV-T30 17.5

The overall acceleration is lower yes, so you do need a few kilometres to slow down but unless you're in a low orbit they will manage it and are easily the best choice.

[Shrike42]

If you're having a lot of trouble, you might have an action group with some Rockomax radial engines to give you an extra bit of thrust. But I've certainly managed to safely land on the mun and return using 2 LV-Ns.

[CalculusWarrior]

WARNING: The following contains a lot of math. For those who are math-adverse, I suggest you skip down to the bottom where I summarize the whole thing.

Nuclear engines have a thrust of 60 kN each, so four of them would give you a total thrust of 240 kN. Gravity on the Mun is 1.63 m/s^2, so in order for your ship to have a TWR of exactly 1, it needs to be 147 tonnes. However, you want a signifigantly higher TWR, something like 2 or 3, so your actual weight will be decidedly smaller, something in the range of 50 tonnes. However, the nuclear engines weigh 2.25 tonnes each, so you will have at least 9 tonnes of mass initially. The three-man capsule weighs a lot as well, 4 tonnes.

So our total mass so far is 13 tons. That still gives us a good amount of space left for fuel. To find out the amount of fuel we need, we can use the Tsiolkovsky Rocket Equation:

delta-v = (specific impulse)(9.81 m/s^2) ln[(initial fuel mass/final fuel mass)]

The amount of delta-v required for a landing on the Mun is ~640 m/s. Giving you a bit of a margin of error, let's assume 700. Doubling that, you want to take off afterwards, right? Our final delta-v required for a lander is going to be about 1400 m/s.

Nuclear engines have a fantastic specific impulse, 800 s in vacuum! Therefore, plugging in our values and solving for the fuel ratio gives:

(1400 m/s)/(800 s)(9.81 m/s^2) = ln(fuel ratio)

0.1784 = ln(fuel ratio)

fuel ratio = 1.195

Therefore, the final fuel level will be equal to the initial level divided by 1.195, or 0.8366 times the initial.

Fuel appears to weigh 1 ton per 90 units, so we can calculate how much we need:

(mass of fuel + dry mass) = initial mass of fuel tank

initial mass + weight of parts = fuel + dry + 13

fuel ratio = initial/final

[fuel + dry + 13]/[dry + 13] = 1.195

We can simplify this to:

[fuel/(dry + 13)] + 1 = 1.195, or fuel/(dry + 13) = 0.195

We want to solve for the mass of fuel, while assuming a dry mass of half a ton (the assumes we require the fuel tank with stripes on the side; if we obtain a value for the fuel that fills a smaller tank, then we can use this)

fuel = (0.195)(13+0.5) = 2.633 tons

Since fuel weighs 1 t / 90 L:

2.633t * 90L/1t = 237 L of fuel are required.

Analysis of the weights and capacities of the fuel tanks, I would say that this is vastly under capacity for our stripy tank, which holds 360 L. While we could use it, as it would give us a final weight of 17.5 t, well within our 50 t weight limit, we want to save costs. Adding the smallest fuel tank gives us a fuel capacity of 270 L, which is a little more than what we calculated, but will be useful for extra delta-v.

So now, using the awesome power of math, we have determined that the following rocket can land on the Mun:

Notice my helper, the Kerbal Engineer in the corner saying that this rocket has a TWR of 9, and almost 1.6 km/s of delta-v. Yay! Our math worked, and we have a bit of extra delta-v left over as a result of using the larger tank!

So yes, it is possible to land on the Mun with nuclear engines, which answers your initial question.

HOWEVER:

This craft has a total weight of 16.4 tons, most of which comes from the nuclear engines (a whopping 55%). If we redesign the rocket slightly by using different engines:

Note how this has less delta-v, but actually a higher TWR? Redesigning even further:

This craft has the worst TWR of all, but still more than enough to land, and its delta-v requirements are amazing. This ship would require a boost to travel out to the Mun, but once there, it could put itself into a capturing orbit, land, and take off. It would be able to return to Kerbin as well!

Basically, while yes, it is possible, it is not in your best interests to do so. The total mass of the final design was simply 9 tons, which is within a normal-sized rocket's ability to lift. The ln(inital/final) term in the Rocket Equation essentially states that the more mass you need to lift, the larger the rocket must be, which in turn requires a larger rocket, and so on and so forth.

To summarize: Yes, you could land on the Mun with nuclear engines, but it would be very inefficient to do so.

(I ignored ladders, lights, etc in my lander designs, as generally they don't weigh all that much and not every Mun lander requires them. Just slap on some more fuel if you need to; or you could adjust my calculations to take that into account)

[foamyesque]

No they're not, because your weight is so much lower on the Mun.

An LV-N on the mun has a TWR of 16.6. For comparison an LV-T45 has a TWR on Kerbin of 13.5, and an LV-T30 17.5

The overall acceleration is lower yes, so you do need a few kilometres to slow down but unless you're in a low orbit they will manage it and are easily the best choice.

There are a couple of counter-vailing disadvantages. They're just so bleedin' heavy and long that they bork up your boost stages and are awkward to position for a lander.

[Semni]

An LV-N on the mun has a TWR of 16.6. For comparison an LV-T45 has a TWR on Kerbin of 13.5, and an LV-T30 17.5

Kinda like comparing apples to oranges there. Everything is going to have a way better thrust to weight ratio on the mun compared to Kerbin. As Calculus calculated sure, nuclear engines are quite capable of doing so they do take up a huge portion of your lander's final weight. Now where they do come in handy is if your lander craft is also the main engine cluster on a much larger interplanetary craft where you leave your large fuelcans in orbit before descending to a rocky moon and fuel efficiency is king. As demonstrated for short hops out to the nearby moons you can probably get much more utility out of a regular engine.

[senoiurkabls]

How do I delete replies?

[sal_vager]

You don't delete other peoples comments, sorry

[Semni]

It's a little bit rude to do so if you could.

[Van Disaster]

They're pretty good. Even with all the excess weight of a plane ( and entering orbit with 3500+dV in the tanks! ), was easy to gently make my way down. Final moments were with orange radials, but that's only because it has to land on the wheels.

TBH though the orange radials are also ok for medium weight stuff if you don't want to lug heavy 1m rockets around.

Pics are from that "get to Mun & back" mission challenge thing in General, which has only gone halfway so far. Goes against instincts to go back...

[EndlessWaves]

To summarize: Yes, you could land on the Mun with nuclear engines, but it would be very inefficient to do so.

Not trying the LV-N after your re-design wasn't very scientific. If you do so then you get this result:

It's actually 50% more efficient than the LV-909 in this configuration, and thanks to the heavier fuel tank there's even less difference in the TWR than before.

On anything much over 1 ton it rarely makes sense not to use an LV-N. Either the vessel is heavy enough that you can split up the fuel tanks to form landing legs or it's light enough that the joints between parts won't be stressed much and you can land on the engine nozzle. Generally the exception is either that you're deliberately selecting a sloped landing site or your vessel is launched from another and carried very little fuel.

Kinda like comparing apples to oranges there. Everything is going to have a way better thrust to weight ratio on the mun compared to Kerbin.

I was pointing out that it would handle the same as the other engines do on Kerbin and would in now way feel sluggish when it came to maneuvering near the surface.

Edited May 6, 2013 by EndlessWaves

[GusTurbo]

There are a couple of counter-vailing disadvantages. They're just so bleedin' heavy and long that they bork up your boost stages and are awkward to position for a lander.

Not if you get...creative...with where the game will allow you to place them (note: no debug menu was used for this).

[foamyesque]

Like I said: Awkward.

[Tristavius]

Excellent explanation CalculusWarrior. And just to add my less mathematic opinion, yes, nuclear engines can be used to make landing easily enough... my standard rover landing frame weighs about 9 tons before fuel and engines and uses 3 nuclear engines to descend on a variety of worlds. This is not the most efficient method I suspect but it save any extra engines for the landing phase as opposed to the flight phase.

[Nao]

To summarize: Yes, you could land on the Mun with nuclear engines, but it would be very inefficient to do so.

Actually it's not the LV-N's that are inefficient, its the design.

The LV-N's work best at low TWR's. If you stack a lot of them on small lander it would be inefficient by principle.

Here i compared LV-N, LV-909 and rockomax 24-77 at different TWR's for Mun ascent. Descent would have almost the same results. (requiring a little less TWR even)

The "engine and fuel mass fraction" shows what percentage of whole craft's mass is used for propulsion system. The lower the better.

The problem is that nobody creates landers that heavy to use proper TWR for LV-N's

The case above is also a worst case scenario for LV-N's since it only covers one maneuver worth around 750m/s Dv. If we would include descent as well as transfer burns in one stage the LV-N would outclass other engines even more.

For example:

This craft's mass efficiency from ground to orbit is 91% and the engines+fuel tanks that are used to lift the ship are only 18% of the total craft mass.

Of course i won't argue that LV-N's are not good for landers as it's really hard to make a design that would use it well. But if we consider the whole mission, they may be the engine to go for.

For example if we use a 3kerbal pod with one LV-N and a kerbin launch stage with only 3500m/s Dv so the craft will use the LV-N to finish orbit, transfer to mun, land, ascent and return to kerbin in one stage (up to 4000m/s of Dv). It would be quite efficient use of the engine. And probably lighter design than one based on different engines and more staging.

[Merinsan]

I've got a drop ship I use to drop off parts for my bases on the Mun. It uses 4 nukes and carries it's payload underneath. It easily lands it's payload using it's own fuel supply, with plenty of fuel to get back to the orbiting station. It consists of a lander can plus fuel and engines. The engines are not even pointing straight down.

If it had landing legs, it could land itself but that's not it's job.

[numerobis]

IIRC you can land the cockpit and return to Kerbin on a single tonne of bipropellant with a single LV-N. it's not the most efficient landing system, but if you use the LV-N to get to the Mun and to reach Kerbin orbit (burning from drop tanks), it edges out the LV-909.

You can use cube struts for landing gear. They are weightless, so use as many as you want (though they do boost part count, and thus hurt frame rate).

Edited May 6, 2013 by numerobis

[Tw1]

I would not recommend the LV-N as landing gear. You'll need legs, definitely. .LV-Ns have a tendency to come of, even with a lot of struts.(I was trying to land with my transfer stage.)

Fine for a one way trip though, if there's nothing to worry about leaving radioactive debris

Edited May 6, 2013 by Tw1