Thrust Limiter?

[Xyphos]

Thrust Limiters.
IDK how long this feature has been in game but I just now noticed it.
I don't understand it tho, with MechJeb's dV window open, adjusting Thrust Limiters on engines only effect TWR but not dV. what gives?

[Snark]

Gameplay question moved to Gameplay Questions.

56 minutes ago, Xyphos said:

adjusting Thrust Limiters on engines only effect TWR but not dV. what gives?

What gives is that it doesn't affect dV because changing the thrust limiter has nothing to do with dV. 

dV only depends on the engine's Isp and on the mass ratio between "full" and "empty" rocket.  It doesn't depend on the thrust.

If you set an SRB's thrust limiter, you're making it burn more slowly:  less thrust, but longer burn time.  The two cancel each other out.  For example, if you set the thrust limiter to 50%, the SRB will burn twice as long.  Half the thrust for twice as long = same dV.

[Xyphos]

So basically, it only effects TWR and burn time, which could / should be used to prolong staging, and best example to use it would be under sub-optimal flight conditions such as atmospheric take-off, where an over-engineered heavy lifter stage may be required.

got it. thanks!

[FancyMouse]

Thrust limiter can affect delta V when you have engines with different Isp. Having a different thrust ratio will result in a different total Isp. But if all engines are the same, then Isp is not going to be affected by thrust limiter.

Thrust limiter also have other applications in addition to SRBs:

1. When building a satellite network (for RT or just for fashion) where you want to match orbital period between satellites as close as possible. Using Ant engine+thrust limiter+throttle gives you the finest control of the orbit.

2. When building asymmetric ships (VTOL or shuttles or anything creative) to align CoT with CoM.

[JohnnyPanzer]

7 hours ago, FancyMouse said:

Thrust limiter can affect delta V when you have engines with different Isp. Having a different thrust ratio will result in a different total Isp. But if all engines are the same, then Isp is not going to be affected by thrust limiter.

Thrust limiter also have other applications in addition to SRBs:

1. When building a satellite network (for RT or just for fashion) where you want to match orbital period between satellites as close as possible. Using Ant engine+thrust limiter+throttle gives you the finest control of the orbit.

2. When building asymmetric ships (VTOL or shuttles or anything creative) to align CoT with CoM.

Point one is by far the main reason for using the thrust limiter as far as I'm concerned. It's also great for making small adjustments and/or minor inclination changes to a planetary intercept. When you need something like 1.2m/s for a maneuver, setting the thrust limiter to a minimum is very, very, very helpful.

[Streetwind]

10 hours ago, Xyphos said:

So basically, it only effects TWR and burn time, which could / should be used to prolong staging, and best example to use it would be under sub-optimal flight conditions such as atmospheric take-off, where an over-engineered heavy lifter stage may be required.

got it. thanks!

I'd like to add some things that you might want to consider:

While the vessel's total dV is not dependant on its thrust, the dV cost of launch very much is. This counts particularly for take-off and initial ascent, where you are burning straight up. The reason for this is that as long as you are fully vertical, you need to expend the local gravity's worth of dV every second, just to maintain your status quo (I call it the "hovering component"). That dV is lost, permanently, filed under the heading of "gravity losses". Only the acceleration (and therefore dV expenditure) that exceeds local gravity will actually move your rocket. Thus a rocket capable of producing 1.5 G's of acceleration will only accelerate at 0.5 G's when flying straight up - expending 100% of the fuel for 33% of the actual gain.

(You could also make that case for aerodynamic losses caused by high thrust increasing the dV cost to orbit, but for typical launch profiles, gravity losses outnumber aerodynamic losses by at least 5 to 1.)

As you tilt over towards the horizon, things change to your advantage. Since your engines are no longer pointing at the ground as much, they are no longer fighting gravity as much. When you are pointing fully at the horizon, gravity losses are zero, and 100% of your expended dV goes towards actually changing your velocity.

Because of this, it's generally a good idea to jump off the pad as fast as you can, and have the shortest possible vertical flight portion (even if you throttle back later). 10 seconds less spent fully vertical will decrease your dV cost to orbit by about 100 m/s!

Now, as I like to say, "overloading the launch stage is free". Basically, if you have a TWR of 2.0, you can launch very efficiently and minimize gravity losses. But you could instead also add fuel to the launch stage, resulting in less TWR, but an overally higher dV for your rocket - and unless you really overdid it with your stage already, adding fuel gives you more extra dV than you lose to a less efficient take-off. This holds true until you reach a TWR of 1.2 (at sea level!), a figure you should never drop below. The reason it is "free" is because if you were to add extra dV to any other stage of your rocket to maybe gain the same effect, that stage becomes a bigger payload for all the stages below it, reducing their dV in turn. Only the launch stage has no other stage below it which could choke on its weight. Therefore, overloading it is free.

So if you are concerned for dV, and your launch stage has a lot of TWR, you have two options:
1.) take the high TWR and launch with it, making your ascent more fuel efficient
2.) stuff more fuel into the launch stage until its TWR is no longer too high for your tastes, but still above 1.2 at sea level

Thrust limiting your launch stage (or launching at less than 100% manual throttle) is an option that's almost always a worse idea than the two above. There are edge cases where it can be useful, such as all-solid launch stages. If you have three SRBs, and set the center one to 100% and the other two to 70%, then the center booster will cut out early while the other two still continue burning. This smoothes out your TWR curve, particularly since SRB stages really tend to ramp up in TWR hard near the end of their burns; which in turn can make your rocket's gravity turn more predictable and controllable. A more predictable and more controllable gravity turn can mean a more efficient ascent in its own right, since you're neither going too steep for too long due to being unable to tilt over enough, nor need to risk being too aggressive and ending up too flat too low down.

Edited March 21, 2016 by Streetwind

[Geschosskopf]

15 hours ago, Xyphos said:

Thrust Limiters.
IDK how long this feature has been in game but I just now noticed it.
I don't understand it tho, with MechJeb's dV window open, adjusting Thrust Limiters on engines only effect TWR but not dV. what gives?

Thrust limiters are useful for tweaking and fine-tuning.  They can even be adjusted during flight, not just at the design stage.  I use them as follows:

• Restraining SRB exuberance.  I always use lifters based on a single core LFO stack 2nd stage with an all-SRB 1st stage.  This number of SRBs usually has a TWR of 3.0 or higher, which is too much for a safe ascent.  So I limit their thrust on down so the 1st stage has a TWR of 1.5 or so.
• Balancing RCS thrusters (using RCS Build Aid):  Sometimes you need to put a thruster in a specific spot but the design of your ship won't let you (like there's a joint between stack parts or a spaceplane wing in the way).  So what you do is put it somewhere nearby and then use the thrust limiter either on it or the other thrusters until they balance.
• Special Effects:  Interesting fireworks, flat-shooting missiles, etc.

[SpacedCowboy]

Thrust limiters for SRB's cannot be adjusted on the pad, only LFO's

[pincushionman]

Keep in mind that if you're consistently setting a thrust limiter on launch stages (much) in the VAB it's a sign that your rocket might have too much legs and might be able to get away with fewer or smaller engines. How much leeway you actually have is in that another story and depends on part count, size- and weight limitations, and the discrete nature of the parts.

[Warzouz]

Thrust doesn't affect dV directly, but a too low thrust will makes gravity loss more important or a too high thrust may create more drag loss. In the end, you'll use more dV to go to LKO.

Thrust doesn't affect dV your ship has, but it may affect the dV it needs to go to orbit.

[Zio]

I wonder if real rocket engines with a limited thrust would maintain their isp relative to 100% thrust output or would they just become too inefficient.
Seems like I have homework to do now

[zarakon]

I also like using the thrust limiter for landings, especially on very low gravity worlds like Minmus.  It makes it a lot easier to find the sweetspot on the throttle to hold a steady speed.

[Streetwind]

19 hours ago, Zio said:

I wonder if real rocket engines with a limited thrust would maintain their isp relative to 100% thrust output or would they just become too inefficient.
Seems like I have homework to do now

Real rocket engines have lower Isp when throttled down, chiefly because the combustion champer pressure and temperature decrease as less fuel is being burned. You'd need a smaller chamber to maintain the same pressure and temperature at lower thrust. Which is the chief reason we purpose-build unthrottleable engines for many different thrust levels instead of just a few throttleable ones.