• 0
strider3

Determining most efficient engines?

Question

I am 2/3 of the way through my scientific exploration of Moho. What I have discovered is the tremendous amount of fuel I have expended to keep my science lander fueled and making landings on Moho. I originally sent a scanner, the lander and 3 refueling ships...and the amount of fuel actually available to the lander after the refuelers were in the working orbit was...depressing. I still have 4 landings at biomes to complete and I'm out of available fuel for my lander. I don't know if getting fuel to Moho orbit is always this costly or if it's the UT years I am trying to accomplish this in (11-13). Either way, I may have fallen into the "bigger is better" rabbit hole. I built large refuelers, refueled them from my Kerbin space station and sent them to Moho only to have (comparatively)  little fuel left after they were in my Moho working orbit of 30 km. I feel that part of the problem, besides the size of these refuelers, may be the efficiency of the engines I am using to get from Kerbin orbit to Moho orbit. How does one determine the efficiency of an engine (in vacuum)? What numbers do you look at as far as engine performance to determine which engine will be the most fuel efficient in getting to Moho?

My "off the cuff" calculations for fuel needed to complete 4 more landings, return to a close to equatorial Moho orbit (I'm at 54* inclination now) and return to Kerbin is about 30,000 fuel. I'm basing this number on my lander's Delta V when full of fuel as it will be the return to Kerbin vehicle (barely!). I realize there is way more to this than I am asking but, at this point, I just want to get my biome landings done and get Ferbin and Arbart back home.

Share this post


Link to post
Share on other sites

Recommended Posts

  • 0
Posted (edited)

I think I've worked the problem...but we shall see. Putting a Wolfhound on the bottom of the central stack instead of a mammoth caused issues with getting to Kerbin orbit of 350km due to lack of thrust. What I've done is split the central stack into 2 stages with a Mammoth at the bottom to help get me to 350km Kerbin orbit, and added several tanks and a Wolfhound to the top of the central stack, to get me to Moho. So far I will have no problem reaching my Kerbin refueling station at 350KM with both central stages intact. Now the question is...do I just jettison the lower stage with the Mammoth (which still has fuel) and go for Moho or refuel at my station and burn the Mammoth then the Wolfhound for Moho? I'm going to do the later on this first go as I see no reason to dump the lower stage which will still have fuel even before refueling at the station. Might as well use the lower stage DV and arrive at Moho with more fuel/LoX available, I'm thinking.

Edited by strider3

Share this post


Link to post
Share on other sites
  • 0
Posted (edited)
3 hours ago, strider3 said:

Putting a Wolfhound on the bottom of the central stack instead of a mammoth caused issues with getting to Kerbin orbit of 350km due to lack of thrust.

of course. engines have different efficiencies in vacuum and at sea level. that's why you also have to look for sea level isp. you want that high when launching from kerbin. the wolfhound is good for vacuum, but useless in atmosphere.

Quote

What I've done is split the central stack into 2 stages with a Mammoth at the bottom to help get me to 350km Kerbin orbit, and added several tanks and a Wolfhound to the top of the central stack, to get me to Moho.

yes, very good. that's an additional reason for making multiple stages in rockets: not only you shed dead weight, but you also can use different engines optimized for different conditions. that's true also in real life.

Quote

Now the question is...do I just jettison the lower stage with the Mammoth (which still has fuel) and go for Moho

or refuel at my station and burn the Mammoth then the Wolfhound for Moho?

neither.

you select your decoupler, and select "activate fuel transfer". then you activate the wolfhound, only the wolfhound, you shut down the mammoth, and you start your trajectory to moho. after you have burned some fuel, you transfer fuel from the lower stage to the upper stage, until the lower stage is dry. then you ditch the mammoth.

this way you can save every drop of fuel, but you also get to use the more efficient wolfhound. for an equal amount of fuel, the wolfhound can squeeze 20% more deltaV from it.

also, of course, you have to make sure your fuel shuttles contain as little dead weight as possible. this way, they transfer fuel more efficiently. my fuel transfer vehicle weights 180 tons full, uses two wolfhounds, and has a deltaV over 7000 m/s when full. you need 5000 m/s (EDIT: I misread the deltaV map, it's actually closer to 3000 if you get the right time. so, a return trip is also possible) to get to moho, so my vehicle could get to moho with a decent amount of fuel left (perhaps 15%; remember that fuel gives diminishing returns, and if you burn half your deltaV, you did not burn half your starting fuel, but more than that). my refueler has a couple tons of landing gear you don't need, so you can also save on that, and replace it with two tons of fuel.

but, since you don't need your refueler to be reusable (you don't have enough fuel to get back from moho anyway), you also could add more stages. a single stage for 5000 m/s is feasible, but inefficient. if you were to build your refueler with two stages, one to intercept moho, then you shed that stage and circularize orbit with the second stage, you coould save fuel. if you times things well, you also could get the first stage to return to kerbin (where it can save fuel by aerobraking), and reuse it.

 

anyway, sending fuel to moho is very expensive. it still costs 5000 m/s, on the most optimized trajectory - which i suspect you're not using, since you keep launching new refueling missions, are you waiting the launch windows?

it would really be less expensive to send a small, automated isru complex on moho. even if all it does it refuel your rover, and then you delete it. it's one mission, instead of 4.

Edited by king of nowhere

Share this post


Link to post
Share on other sites
  • 0
10 hours ago, strider3 said:

So far I will have no problem reaching my Kerbin refueling station at 350KM with both central stages intact.

Ideally the tanker you are launching from Kerbin reach the refuelling station with empty tanks, since it goes there to refill before the trip to Moho. 

Which mean that during launch you may use those tanks to carry the fuel for lower stages, eliminating the need of fuel tanks in the lower stage itself. To achieve this just enable crossfeed in the decoupler (need Advanced Tweakables enabled in difficult settings) or put some  External Fuel Ducts.

Don't try to do everything in a single ship because that will bring inefficiencies. Design a optimized ship for each task.

 

Share this post


Link to post
Share on other sites
  • 0
Posted (edited)
20 hours ago, king of nowhere said:

neither.

you select your decoupler, and select "activate fuel transfer". then you activate the wolfhound, only the wolfhound, you shut down the mammoth, and you start your trajectory to moho. after you have burned some fuel, you transfer fuel from the lower stage to the upper stage, until the lower stage is dry. then you ditch the mammoth.

How is it possible to burn the Wolfhound when it is coupled to the top of the Mammoth stack? Won't firing the Wolfhound destroy the lower half of the central stack?

 

 

anyway, sending fuel to moho is very expensive. it still costs 5000 m/s, on the most optimized trajectory - which i suspect you're not using, since you keep launching new refueling missions, are you waiting the launch windows?

I use Transfer Window Planner for all my interplanetary flights and wait for the launch window.

it would really be less expensive to send a small, automated isru complex on moho. even if all it does it refuel your rover, and then you delete it. it's one mission, instead of 4.

If I was going to start the mission to Moho all over, I would consider this but I'm 2/3 of the way through the landings with my lander so I'll just send fuel. It will definitely be a consideration if I return to Moho in "another life" ;)

 

12 hours ago, Spricigo said:

Ideally the tanker you are launching from Kerbin reach the refuelling station with empty tanks, since it goes there to refill before the trip to Moho. 

Which mean that during launch you may use those tanks to carry the fuel for lower stages, eliminating the need of fuel tanks in the lower stage itself. To achieve this just enable crossfeed in the decoupler (need Advanced Tweakables enabled in difficult settings) or put some  External Fuel Ducts.

Don't try to do everything in a single ship because that will bring inefficiencies. Design a optimized ship for each task.

 

I'm using a triple asparagus setup to get to 350km Kerbin orbit and I will not have "a lot" of fuel left in the lower half of the central stack (the tanks feeding the Mammoth) once I arrive at the station.

Edited by strider3

Share this post


Link to post
Share on other sites
  • 0
2 hours ago, strider3 said:

 

How is it possible to burn the Wolfhound when it is coupled to the top of the Mammoth stack? Won't firing the Wolfhound destroy the lower half of the central stack?

ok, i misunderstood the image. i thought the wolfhound was in the bottom of th ecentral stack, and the mammoth were all around it. my bad

in this case, if your station has enough fuel, you should refuel your mammoths, use them as first stage for moho orbital insertion, and drop them when exhausted. as i said, using many stages has advantages over a single stage. even if mammoths will be less fuel-efficient. but the moment you drop them, you still have the wolfhound fully fueled, and you'll be further along the way

Share this post


Link to post
Share on other sites
  • 0
Posted (edited)
8 hours ago, strider3 said:

I'm using a triple asparagus setup to get to 350km Kerbin orbit and I will not have "a lot" of fuel left in the lower half of the central stack (the tanks feeding the Mammoth) once I arrive at the station.

Whatever you brought to the station this time, fill it up and send to Moho. 

But that was not what you wanted, you wanted a tanker that could carry all fuel needed to Moho in one go. Seems to be a lot of fuel, so you need a huge tanker. Which is kind of the idea when designing a tanker. Yes, in this case bigger is better*, go BIG or go home.

I suppose your refueling station get the fuel from somewhere other than Kerbin (Minmus?). If that is the case(and probably even if is not) don't downsize your tanker just because you can't launch and reach the station fully fueled, that is not what this craft is supposed to do**. Just put that huge tanker in orbit,  even if you need to use the last drop of fuel, including any fuel in the tanker itself, to get your periapsis above the atmosphere. Keep in mind that your TWR increases as you burn fuel and the atmosphere above 20 km is almost vacuum, which means is possible you can use the tankers engine for a good chunk of the ascent, saving some engines in the lower stage (or allowing to launch an even bigger tanker)

In the next iterations of the launch vehicle:

1.Make so your lower stages drain from the top central stack, the tanker itself. (enable crossfeed)

2.Remove the now unnecessary fuel tanks from the lower stages

3.Increase the fuel tanks in the top central stack (leave it empty if that fuel is not necessary to the launch)

4. Repeat 2 and 3 as needed, adjusting the engines of the launch vehicle accordingly.

 

The launcher vehicle will look somewhat like this:

Spoiler

bQZNndp.png

Just  a mock up, not a fully designed craft. Notice just above the fairing base, that is where the decoupler with crossfeed is.

The Mock up tanker is a Rhino, 3xS4-512, 2xS3-S4 adapter, 2-3 Adapter and "necessary stuff", readout says 0.2TWR and 7km/s deltaV; 3km/s with a locked S4-512 tank(that's 250t of fuel locked but I'm sure there is people screaming "MOAR FUEL!!" ;))

 

*The tanks, everything else should be kept small to allow MOAR FUEL!!! delivered.

**A craft for each task, if needed design and build a big fuel lifter.

 

Edited by Spricigo

Share this post


Link to post
Share on other sites
  • 0
18 hours ago, king of nowhere said:

ok, i misunderstood the image. i thought the wolfhound was in the bottom of th ecentral stack, and the mammoth were all around it. my bad

in this case, if your station has enough fuel, you should refuel your mammoths, use them as first stage for moho orbital insertion, and drop them when exhausted. as i said, using many stages has advantages over a single stage. even if mammoths will be less fuel-efficient. but the moment you drop them, you still have the wolfhound fully fueled, and you'll be further along the way

Yeah, that was "Plan C"...I'm on D now :cool:. I've split the central stack into 2 stages with a mammoth on the bottom half (to get me into Kerbin orbit with the 6 asparagus Mammoths) and a Wolfhound and added tanks in the top half. I'll refuel all the tanks and use the Mammoth to start my transfer to Moho...no sense in dropping the Mammoth now that I have it in orbit, methinks. I'm going to create a save and then warp to the next Moho launch window and see what the fuel usage is like. I'm wondering if I should move the Wolfhound down one tank in the stack (I'll have to move fuel ducts on the asparagus, if so). At this point I have no idea how much fuel I'll have once I reach my Moho orbit and I haven't figured out a way to mathematically calculate that?

 

screenshot19.jpg

Share this post


Link to post
Share on other sites
  • 0

The Transfer Window Planner doesn't work very well when trying to get to Moho? I copy and paste the window info into my maneuver node but the result gets me nowhere near (in my mind) a Moho encounter?

screenshot20.jpg

Share this post


Link to post
Share on other sites
  • 0
Posted (edited)

Burning the Wolfhound is kinda like watching grass grow :sticktongue:. SOooo, that didn't work. It seems that having part of the burn using the last Mammoth and part using the Wolfhound jacks with the start of burn time? I was down to 240 DV to finish the transfer and the required DV started going back up? If you look at the map image above, I show a "Node Burn Time (1/2)" of 2m 37s but a "Node Burn Time" of 18m 30s. I get the feeling I needed to start the burn at 9m 15s. I was going to totally miss Moho, at that point. I think it's back to the drawing board.

Edited by strider3

Share this post


Link to post
Share on other sites
  • 0
2 hours ago, strider3 said:

Burning the Wolfhound is kinda like watching grass grow :sticktongue:. SOooo, that didn't work. It seems that having part of the burn using the last Mammoth and part using the Wolfhound jacks with the start of burn time? I was down to 240 DV to finish the transfer and the required DV started going back up? If you look at the map image above, I show a "Node Burn Time (1/2)" of 2m 37s but a "Node Burn Time" of 18m 30s. I get the feeling I needed to start the burn at 9m 15s. I was going to totally miss Moho, at that point. I think it's back to the drawing board.

Well, in fairness you did ask for the most efficient engine. The price for bleeding-edge efficiency is, yeah, low TWR. I did a mockup of something similar to what you have with a probe core and 4 S3-14400 tanks plus and ADPT 2-3 adapter on top. (EDIT: Now that I'm looking at your total mass, maybe those are S3-7200s you have? Still, same principle should apply).

c0o229u.jpg

So with a single Wolfhound, you get 7900 m/s. That's pretty extreme for a single stage, but your vacuum TWR is only 0.11.

 

hylsR5e.jpg

You could use an engine plate and stack 3 Wolfhounds. It would almost triple your TWR - so burn time down to 6 minutes or so. The trade off is your dV is down to 7384 m/s.

 

rLwE469.jpg

Or use a Rhino. Still pretty efficient at 340 ISP and gives 2000 kN vs. the Wolfhound's 375. dV is down to 6700, but the TWR is 0.58. That should get you down to about a 3.5 minute burn, which is pretty manageable IMO.

BTW, I've read that a good rule of thumb is that a burn should be less than 1/6 of an orbit to avoid big cosine losses. In your screenshot you're in about a 52 minute orbit so anything less than about 8 minutes should be OK.

Share this post


Link to post
Share on other sites
  • 0

I knew my burn time was going to be extremely long and I think if I had ignored KER's  "Node Burn Time (1/2)" and used half of the 18m 30s number for start time I may have been OK. The very late start because KER's number were incorrect threw the entire burn off. I'm going to add a Wolfhound asparagus as the upper stage and keep Mammoths on the bottom, to get into orbit. The Mammoths will run out of fuel at about 100km so the Wolfhounds will finish the rendezvous, in vacuum, with the space station (without staging off empty couples). I'll refuel the entire Wolfhound asparagus setup (7 stacks) and then stage off each couple as they empty, on the way to Moho. It's not pretty and I agree that, in the future, mining and creating fuel would be the way to go at Moho. As I mentioned I'm well along in my exploration of Moho so I'm just trying to save the mission...as is. I'm hoping 2 of these tankers will allow me to complete the 4 Moho landings I need and get my 2 Kerbals back home. I had no idea how DV intensive Moho would be...which makes me an idiot as the Transfer Window Planner tells all. I finally managed to get a Moho encounter by tweaking the TWP settings...it was a long and aggravating procedure.

If anybody going to Moho reads this thread...listen to the experts! Mine and create your own fuel as Moho is insanely expensive to get fuel to!

This entire Moho mission has been a great learning experience for me. Thanks to all of you, I have a much better understanding of engines and their ISP numbers, and how they affect my exploration plans, construction of ships and general logistics.

Thanks gang!!

(I will let you know here when I get my Kerbals home).

Share this post


Link to post
Share on other sites
  • 0
16 hours ago, strider3 said:

I knew my burn time was going to be extremely long and I think if I had ignored KER's  "Node Burn Time (1/2)" and used half of the 18m 30s number for start time I may have been OK. The very late start because KER's number were incorrect threw the entire burn off.

Part of the issue here is that you're dealing with one of the highest-thrust engines and one of the lowest-thrust engines in the same burn.  The way to solve that is to split the burn according to delta-V rather than relying entirely on time.  For example, if you had a burn of some (manageable) amount of delta-V (let's say 200 m/s for an example) and it worked out that the high-thrust engine had 100 m/s in its tank, then you would want to start the burn at a time early enough that you pass the exact node at the same time as you burn out and need to stage.  However, higher thrust means faster propellant consumption, so in terms of time, the burn would be very lopsided in that you might burn the high-thrust engine for ten seconds and the low-thrust engine for a minute and thirty seconds.

Normally, some error on either side of the node marker will have a negligible effect on the overall manoeuvre.  In this case, the combination of its being an interplanetary trajectory and Moho's near-microscopic sphere of influence makes it difficult to hit precisely, and increasing the burn time (as you would do by using a low-thrust engine) unfortunately increases the potential for that error.  There are techniques to address that (periapsis-kicking is one that you may be interested in knowing more about), and there are some other tricks that you can use, but that's the way with the challenge planets:  you'll learn a lot by making a lot of mistakes.

Do please let us know when you get the kerbals home.  You should consider putting the story in Mission Reports!

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Answer this question...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.