Mun ISRU how?

[splashboom]

I find that the margins for lifting ore/fuel to the Mun orbit are too thin - I even lose fuel if I mess up. Are there tricks to improve the fuel (or ISRU in general) yield?

Minmus is much more generous due to its low orbital speed. Also have a bunch of asteroids from missions that I could mine, but either requires more planning when it comes to fuel that I would need for the Mun specifically.

Thanks!

[18Watt]

I’d say the basics,

• Don’t lug more weight up and down than is needed.  I’ve made it work taking a bunch of ore and a drill up and down.  Leaving the drill on the surface would help.
• Efficient burns.
• Efficient engines.  Wolfhounds might actually be better than nukes, or at least competitive at Mun.
• Probably most important- don’t try to mine a big, massive moon when there’s a lightweight moon right next door, where you can mine for almost zero dV.  In other words, I’m struggling to find any reason to mine Mun.

[splashboom]

That makes sense - I did not try to leave the drill on the surface, would be interesting to do. I am still getting missions to the Mun surface - including mining missions so I would need to keep a rig around anyway. So that is why I need some fuel at the Mun - otherwise I agree that Mun is not a good spot, certainly not to get fuel and use it somewhere else.

Looks like I would be better off making a Minmus ISRU operation and distributing fuel from there.

[18Watt]

It might be worth asking where you need the fuel, which might change your approach to the problem.

If you need the fuel in orbit of Mun, it might make sense to have an orbiting refinery, and bring ore up to it.  Although it is absolutely possible to ferry the drill up and down each time, it weighs a lot, you’ll be more efficient if you can leave the drill on the surface, and just ferry the ore up to orbit.

If you need fuel on the surface, then obviously you want the entire operation there- drills and refiners.  
 

Again, it is certainly possible to do ISRU at Mun- it’s just much easier and more efficient at Minmus.  

[Blaarkies]

Do you have any images of what you have been attempting?

Without images, it is hard to say if you should try lower TWR, higher wet/dry mass ratios, flight profiles, etc. 

You can test the efficiency of such fuel transporter while still in the VAB:

• It costs ~650 m/s dv from Mun surface to low orbit
• Look at the vacuum dv numbers for you craft in the VAB. Let's say it is 3500 m/s dv
• Now remove fuel from the tanks until the craft dv capability is 3500 - 650 = 2850 m/s
• The fuel left-over in the tanks at this point would be your delivery payload to the low orbit Mun station
• Remember to add some +10% margin of error
• You will need some fuel to land again, but this is much less when near empty

By testing that for different designs, you should be able to get close to the optimal TWR required. Generally, the more fuel tank your lander is made out of, the better the payload delivery ratio will be. On planets/moons without atmosphere, the most efficient flight profile is to immediately tilt sideways as much as possible (obviously without losing altitude or smacking into mountains)

[king of nowhere]

15 hours ago, splashboom said:

I find that the margins for lifting ore/fuel to the Mun orbit are too thin - I even lose fuel if I mess up. Are there tricks to improve the fuel (or ISRU in general) yield?

Minmus is much more generous due to its low orbital speed. Also have a bunch of asteroids from missions that I could mine, but either requires more planning when it comes to fuel that I would need for the Mun specifically.

Thanks!

We can't say what you're doing wrong without knowing details, but you're doing something wrong for sure.

A ship with a large tank can easily take 5000 m/s deltaV. A trip up and down Mun, even considering a large inefficiency margin, is 1500 m/s. It leaves you with 3500 m/s of gained deltaV.

Possible reasons you are spending more fuel than you gain:

- you ship has too little tanks, or too much other stuff; too little deltaV

- you are trying to refuel your ship when it's already mostly full, so that you carry a lot of fuel up and down

- you are being incredibly inefficient with your burn

- you are coming in from low kerbin's orbit, spending the additional cost of going up and down from LKO to Mun.

If you lose fuel, you are probably doing more than one of those, as just one would only make the process less efficient.

Here is a simple design you can use for reference:

https://kerbalx.com/king_of_nowhere/Recycling-Point-Express-RPE

[splashboom]

3 hours ago, king of nowhere said:

without knowing details,

I did get some useful tips(thanks all who posted), and did not think you would get something useful from something I may need to improve . For the record then, I am lifting ore to a converter in orbit, and I am using a single craft for that - in particular carrying the  drill around. This frees me from having to return to the same spot always. I would also prefer to stick to a single craft setup, since doing this at scale is more efficient on Minmus. My burns are just fine, except I use the craft to explore sometimes and if I get too low on fuel I just go back up without ore.

 

3 hours ago, king of nowhere said:

you are coming in from low kerbin's orbit, spending the additional cost of going up and down from LKO to Mun

and .. huh? why would you do that?

[miklkit]

Isn't it more efficient to process the ore on the surface and only bring the fuel up?  Land, fillerup, and take off.

[king of nowhere]

51 minutes ago, splashboom said:

and .. huh? why would you do that?

dunno, was just making hypothesis.

though i did something like that. I made a big fuel depot that I would fill in mun's orbit, and then i sent it back on LKO to refuel all ships that just came to orbit. It allowed for much smaller launch vehicles.

And even then, the fuel mining was super convenient. a good 60% of the fuel mined on Mun could go in the depot; then the depot could go to LKO with only 300 m/s, using aerobraking to circularize. And then it needed 1000 m/s to get back, which is just 10% of the fuel. I could send in LKO as much as half the fuel I mined on Mun.

21 minutes ago, miklkit said:

Isn't it more efficient to process the ore on the surface and only bring the fuel up?  Land, fillerup, and take off.

yes, the most efficient thing to do is to have a land-based facility to mine fuel, and then land a tanker near it, transfer the fuel, and bring the fuel to orbit. this way you don't have to send in orbit and back the drills and convert-o-trons.

But it's a lot more time consuming than just putting the drills and refinery on the tanker, and the efficiency gain is mininal. so, it's most practical to have a mobile tanker/rafinery

 

 

[splashboom]

Mucking around with the rocket equation and fuel accounting I get the fractions of fuel loss below. A is a constant depending on the dV required for a one-way(up or down) and on the ISP -- A=1-exp(-dV/(Isp*g₀)). I threw away some terms that looked small to me

[A/(1-A)]*[1+(2Md/Mw))] when lifting ore

[A/(1-A)]*[1-A+(2Md/Mw))] when lifting fuel

also Md is the dry mass and Mw is the wet mass. So you want the smallest ratio Md/Mw, and even if it were negligible, the smallest possible loss fraction would be A. In practice it is not negligible, since you need decent TWR for a gravity turn and that has implications on what you need on the craft, plus the ISRU equipment. But Md/Mw of around 1/3 should be feasible - at least in KSP.  @king of nowhere I think that is pretty close to what you obtain in your craft. Overall the fraction of loss should be 1.5 to 2 times larger than A.

For Mun I get A=.17, and for Minmus A=.05, which might translate into 30% loss for Mun and 10% loss for Minmus. There is some fine tuning to do since the TWR needed on Minmus is more favorable, but that would be a craft that only works there.

Coming to the real Moon, lifting ore is not a thing(fuel mass is equivalent to ore mass only in KSP) which is fine. Also the dV required is 5 times smaller than Earth or 2 times smaller than Mars. If I look at A and comparing Earth, Mars and the Moon, I get .9, .5 and .3 respectively. This means that the fuel loss would be nearly 100% Earth and even Mars, and the Mun would have 60% loss. So as far as producing fuel, the moon is pretty much the only self-sufficient option. If only there was water ..

 

[magnemoe]

I use Minmus as my fuel station, asteroids also works here. On the mun I mostly use the fuel for local use, having an good size lander with ISRU I have an infinite lasting SSTO. Very nice for doing science and fulfilling contracts. With full ore tanks it can also refuel other crafts in orbit 
However this is flexible not effective you want to leave the mining stuff on the ground. 
I use huge tankers between Minmus and LKO as in  3 long MK3 tanks with extra ore and resources storage. Aerobrake down into LKO and you spend 4-500 m/s to target, back you go empty outside of optional parts. Having another base with an refinery in LKO is nice as you don't know how much liquid fuel to oxidizer you will need.