Thrust Limiting...fuel saving?

[6000 Chipmunks]

If I limit the thrust of an engine...am I saving fuel?
I guess I should ask about Booster Rockets as well. Common sense would dictate that on boosters, limiting thrust, does just that and you get no consevation of fuel. However, one would assume that with liquid engines this would save fuel. I'm not seeing much in the way of fuel savings at any level of thrust limitation atm with liquid engines. *Liquid fuel = Fuel & Oxidizer. I haven't tested liquid fuel only engines yet.

[Migz - DH]

One reason a person would reduce the thrust on a solid booster is in order to keep the TWR at or below a certain level which would be exceeded otherwise. The same amount of fuel is expended by the time the booster has completed its burn, but the duration of the burn is increased. (Example would be if at 100% the vessel's velocity went beyond the terminal velocity after launch increasing the drag, so reducing the thrust would keep the velocity below that point allowing more of the booster's energy to be useful instead of wasted.)


For liquid engines, the only reason I see to use a thrust limiter (vs. simply adjusting the throttle) is if you are using multiple engines at the same time and want the comparative thrusts of the various engines to be certain values as you push the global throttle up and down. (Perhaps someone else may have another good reason to use it for liquid engines.)

[dunbaratu]

The main purpose of thrust limiting is to change how engines behave relative to each other. You have 1 throttle lever, so if you want to have, say, your left engine stronger than your right engine, you sort of can't do it from just that 1 thrust lever. Also, in a solid booster, where it has one thrust setting only and stays with it all the time, you can use the thrust limiter to turn down what that setting is before launch, but once you launch it's locked.

[6000 Chipmunks]

Some great responses. I'm kinda familar with the "why", I'm just not seeing this.."The same amount of fuel is expended by the time the booster has completed its burn, but the duration of the burn is increased". I'm seeing boosters at 50% quit at the same altitude they would at 100%. The same seems to be happening with liquid rocket engines. I thought,(and I've seen video of it), that limiting your engines would not only help with drag, and terminal valocity, but could extend the fuel.

I've got a rocket that will get me into orbit every time...it runs perfectly...almost. The problem I have is that the boosters nessesary to lift it off the pad, or that have sufecient dV, thrust or what have you, are really overpowered, and I end up going 500m/s by 3,000m. Really inefficent use of power, as most of it is being converted to drag/heat. They run out about 10-12000m. Reducing thrust starts me much slower, however I still run out of booster, way before 10-12000m. So, I'm a bit confused. If lowering the thrust conserves fuel,(lengthens burn time), why am I running out sooner at lower thrust? I have plenty of dV, I STILL get to 300-500m/s berfore I've reached 3,300m but the thrusters quit sooner...And it doesn't seem to matter what I set my liquid fuel thrust level at either...as long as I have enough dV I reach the exact same altitude at the same time...IE: I run out at the same altitude either way. ??? Head-scratcher.

KSP version 1.0.4.861
New Career.
Steam Version
Window 7

[Migz - DH]

Originally posted by Otterbear:

Some great responses. I'm kinda familar with the "why", I'm just not seeing this.."The same amount of fuel is expended by the time the booster has completed its burn, but the duration of the burn is increased". I'm seeing boosters at 50% quit at the same altitude they would at 100%.

My comment wasn't related directly to altitude; it was related to time required to complete the burn.

Whether the vessel reaches the same altitude also involves other factors such as the vessels' weight and drag as well as the thrust and burn times.

[Migz - DH]

Originally posted by Otterbear:

The problem I have is that the boosters nessesary to lift it off the pad, or that have sufecient dV, thrust or what have you, are really overpowered, and I end up going 500m/s by 3,000m. Really inefficent use of power, as most of it is being converted to drag/heat. They run out about 10-12000m. Reducing thrust starts me much slower, however I still run out of booster, way before 10-12000m. So, I'm a bit confused. If lowering the thrust conserves fuel,(lengthens burn time), why am I running out sooner at lower thrust? I have plenty of dV, I STILL get to 300-500m/s berfore I've reached 3,300m but the thrusters quit sooner...And it doesn't seem to matter what I set my liquid fuel thrust level at either...as long as I have enough dV I reach the exact same altitude at the same time...IE: I run out at the same altitude either way. ???

As to that, a video showing what you're doing with the TWR shown as well as fuel amounts would be useful.


What I suspect happens when you reduce the thrust is that your TWR is too low at the beginning to reach reasonable speeds early on, so you're burning fuel doing little more than hovering (TWR of 1.6 at least while in atmosphere imo) while later on, you recover some of that wasted effort by having the boosters still active for longer. This seems to be balancing out the situation when you are at 100% thrust and the craft is lifted off easily at the beginning, but you later on encounter massive drag due to being supersonic.


I'll hold off on specific recommendations till I see a video.

[6000 Chipmunks]

Wish I had video capabilities. I think you may have hit the problem. Lowering the thrust in an attempt to reduce drag in the lower altitudes does start me off rather slow, and as I burn off fuel then I gain speed...quickly. I guess, I'm trying to figure out what to do about this. I don't want to be going 500m/s at 3,000m, but any less initial thrust, and I can't leave the launchpad.

There are other options I guess. They would just look rediculas. I mean I could stage 10-20 smaller boosters...but that just goes so far out of the realm of realism that I can't bring myself to do it.

Thanks to both of you for your help. I would love to give you a video, but I don't have that capability, or know how either. I could post a screen shot of the specs if that would help.

Its actually just a MK-2 capsule, and 2 med sized Mk-1 fuel tanks, with a poodle between them, and a larger engine on the bottom, staged to ignite after the boosters detach, and four of the largest boosters strapped around it. I've turned off Steam overlay(causes problems with my game at times), so I can't even get a good screen shot at the moment.

[SievertChaser]

I understand your concern. I had a side mounted shuttle strapped to a modded 2.5 m tank with a Skipper under it, and two SRBs; it flew using the Throttle Controlled Avionics to juggle the throttle limiters mid-flight. At the initial take-off the Skipper would briefly go to full throttle until about 1000 m, where MechJeb intiated the gravturn, at which point the Skipper went pretty much to zero, leaving only the shuttle's Aerospikes burning to nullify the torque; the Skipper then stayed off until the SRBs expired.

[Migz - DH]

Originally posted by Otterbear:

Its actually just a MK-2 capsule, and 2 med sized Mk-1 fuel tanks, with a poodle between them, and a larger engine on the bottom, staged to ignite after the boosters detach, and four of the largest boosters strapped around it. I've turned off Steam overlay(causes problems with my game at times), so I can't even get a good screen shot at the moment.

It seems like you could use a similar solution to what Dennis mentioned above.

Turn the SRB thrust down to something lower and burn the liquid fuel to boost your thrust early on maintaining 1.60-1.70 TWR. You'll need to throttle down your liquid engine periodically as your SRB's lose mass in order to not allow your TWR to increase too much.

Then, after the SRBs run out, continue your flight as normal, but you should be at a higher altitude than previously with a good velocity so that even though you'll lose some liquid fuel, you should still come out ahead if everything goes well.

I suggest trying out a few different SRB thrust %s until you find one you like where the throttling is easy to manage.

(Though, looking at your configuration again, you'll probably need to ditch some of your SRBs in favor of liquid fuel tanks.)

[SievertChaser]

Originally posted by Migz - DH:

Originally posted by Otterbear:

Its actually just a MK-2 capsule, and 2 med sized Mk-1 fuel tanks, with a poodle between them, and a larger engine on the bottom, staged to ignite after the boosters detach, and four of the largest boosters strapped around it. I've turned off Steam overlay(causes problems with my game at times), so I can't even get a good screen shot at the moment.

It seems like you could use a similar solution to what Dennis mentioned above.

Turn the SRB thrust down to something lower and burn the liquid fuel to boost your thrust early on maintaining 1.60-1.70 TWR. You'll need to throttle down your liquid engine periodically as your SRB's lose mass in order to not allow your TWR to increase too much.

Then, after the SRBs run out, continue your flight as normal, but you should be at a higher altitude than previously with a good velocity so that even though you'll lose some liquid fuel, you should still come out ahead if everything goes well.

How about bolting some tanks onto the SRBs? I've pulled that off a few times.

[Migz - DH]

Certainly worth a look.

[Empiro]

I recommend getting Kerbal Engineer to help. It can tell you both the inital TWR and also the TWR at the end of the stage (and in flight of course). Generally speaking, it's OK to accelerate quickly at the end of the stage.

However, one extra thing you can do is to use a combination of thrust limiting and throttle on the liquid fuel engine to maintain a decent TWR.

The ideal set-up is to set the thrust limit on the SRBs and throttle on the Liquid Fuel engines such that the SRBs burn as quickly as they can while maintaining a starting TWR of 1.5 (and never going above 3.0). Then, as soon as the SRBs burn out, you throttle the Liquid Fuel engines to max, and have a TWR of close to 2.0 or so. The reasoning for this is that you want to use up your SRBs fast and ditch them -- they're inefficient and heavy. However, you also want to burn enough Liquid Fuel so that by the time you ditch them, your TWR is still sufficient.

This balancing act can be somewhat tough to do properly, but if you manage it, your rockets will fly more efficiently.

[SievertChaser]

Originally posted by Empiro:

This balancing act can be somewhat tough to do properly, but if you manage it, your rockets will fly more efficiently.

That's why I prefer the Soviet way: "SRBs? What's that?"

[Illyrien]

Personally I use thruster limiting if I am doing some fine maneuvering, where the engine running% just isnt fine enough.. also, when I am landing on low G bodies.

[MAD]

Thrust limiting
In space - no saving
In atmosphere - yes because going faster than the speed of sound takes effort to push air aside and wastes fuel. If you see white streaks back off your thrust untill they go away.
ie < 300m/s in the low atmosphere.
Not sure what it is in the middle atmosphere.
When you in space there is no atmosphere hence no limit.

Remember - you can't thrust limit solid rocket boosters
(They are designed to give a particular trust profile, you cant change it.
Liquid rickets you can adjust by altering pump rates)

[El Rushbo]

I used the thrust limiter when launching asymetric vessels, namely my space shuttles. That way I can maintain balance and keep the craft tracking straight as I ascend. It compensated for shifting COM.