Isp query

[SR1200 THUNDER]

I don't know exactly where to post this, and I couldn't find my question answered through a search. I apologize if this is the wrong place, or if this question has been asked/answered before.

So, my question is about an engine's Isp. If my stage is such that it requires less than 100% thrust, does the engine's Isp also decrease for the purpose of calculating dV? As a console peasant, I can't use mods, so I have to do things the old fashioned way.

Hypothetically speaking, say my engine has an Isp of 100 (for round number purposes), but the mass of my craft only requires 60% engine output in order to thrust efficiently. Does that mean that the Isp would also be considered 60 for the purpose of calculating dV, or is the Isp a constant regardless of engine thrust percentages?

[Snark]

Moving this question about how to play the game to Gameplay Questions. 

4 hours ago, SR1200 THUNDER said:

So, my question is about an engine's Isp. If my stage is such that it requires less than 100% thrust, does the engine's Isp also decrease for the purpose of calculating dV? As a console peasant, I can't use mods, so I have to do things the old fashioned way.

Hypothetically speaking, say my engine has an Isp of 100 (for round number purposes), but the mass of my craft only requires 60% engine output in order to thrust efficiently. Does that mean that the Isp would also be considered 60 for the purpose of calculating dV, or is the Isp a constant regardless of engine thrust percentages?

No.  Isp is not a function of engine thrust.  An engine with a particular Isp will have that Isp value regardless of whether the throttle is set to 1% or 100%.  Isp is a constant.

That said, on a side note:  Isp does get affected by atmospheric pressure, sometimes radically so-- that is, the Isp that an engine gets at Kerbin sea level is not the same that it will get in the vacuum of space.  Generally, the lower the pressure, the higher the Isp.  Some engines are affected a lot more than others.  Specifically, some engines (like the Terrier, Poodle, and Rhino, among others) are so-called "vacuum engines" that get good vacuum Isp but terrible Isp in atmosphere, so you generally want to use those for upper stages.  Other engines (such as the Reliant, Mainsail, and Mammoth) get Isp that doesn't change all that much with pressure.

4 hours ago, SR1200 THUNDER said:

the mass of my craft only requires 60% engine output in order to thrust efficiently.

...Can you explain what you mean by that?  Generally speaking, there's rarely a reason to want to run your engines at anything less than full thrust.  If it's during ascent on takeoff, you absolutely want them to be at the maximum thrust possible (in order to minimize gravity losses).  And even if gravity losses aren't an issue, higher thrust means shorter burn time, which means more accurate execution of maneuver nodes and such.

The only circumstance I can think of where you conceivably might not want to run at full power is on atmospheric ascent, where you're trying not to go too fast too low so as not to get clobbered by atmospheric drag.  However, in most cases, that's not an issue, because gravity losses almost always far outweigh aero losses, so it's still to your advantage to accelerate as hard as possible.  If your ship is so overpowered (really high TWR) that it does manage to get aero loss to exceed gravity loss... well, yes, that ship perhaps should back off the throttle, but what it really means is that the ship has an inefficient design with way too much engine on it.  (Engines are dead weight, so you don't want to put more engine on the ship than you need.)  The real solution there would be to have less engine on the ship, not to back off the throttle.

(The one exception to the above assertion that I can think of is if the atmosphere is exceptionally thick and deep, so you really do need to spend a long ascent in hideously draggy conditions.  Which basically means Eve, and nowhere else, at least in the stock system.)

So... can you explain the circumstances in which you think you should be putting out only 60% thrust?  I can assure you that it won't hurt your Isp, but it might hurt you for other reasons. 

 

[FleshJeb]

6 hours ago, Snark said:

Generally speaking, there's rarely a reason to want to run your engines at anything less than full thrust. 

There are several exceptions to this, and they occur any time you want your throttle inputs to be more granular:

• Executing small maneuver nodes more precisely.
• Setting VTOL engines to have TWR slightly >1.
• Final landing burns when your TWR is >3.

[Snark]

1 minute ago, FleshJeb said:

There are several exceptions to this, and they occur any time you want your throttle inputs to be more granular:

• Executing small maneuver nodes more precisely.
• Setting VTOL engines to have TWR slightly >1.
• Final landing burns when your TWR is >3.

Sure, but all of your examples have to do with control, and we're not talking about control here at all.  The OP was clearly talking about efficiency, and that was the context of my comment.

[OhioBob]

As Snark has already answered, in KSP specific impulse is not affected by the throttle setting.  However, I'd just like to add that in real life throttling an engine can have a small affect on Isp.  This is because when an engine is throttled, we are reducing the flow rate of mass through the engine.  It therefore takes less force to push the lesser amount of exhaust gas through the nozzle throat.  This causes the pressure inside the combustion chamber to drop, and a reduction in the velocity of the exhaust gas.  We therefore have a small drop in Isp, though in most cases the decrease is probably not more than a few seconds.

[SR1200 THUNDER]

18 hours ago, Snark said:

Moving this question about how to play the game to Gameplay Questions. 

No.  Isp is not a function of engine thrust.  An engine with a particular Isp will have that Isp value regardless of whether the throttle is set to 1% or 100%.  Isp is a constant.

That said, on a side note:  Isp does get affected by atmospheric pressure, sometimes radically so-- that is, the Isp that an engine gets at Kerbin sea level is not the same that it will get in the vacuum of space.  Generally, the lower the pressure, the higher the Isp.  Some engines are affected a lot more than others.  Specifically, some engines (like the Terrier, Poodle, and Rhino, among others) are so-called "vacuum engines" that get good vacuum Isp but terrible Isp in atmosphere, so you generally want to use those for upper stages.  Other engines (such as the Reliant, Mainsail, and Mammoth) get Isp that doesn't change all that much with pressure.

...Can you explain what you mean by that?  Generally speaking, there's rarely a reason to want to run your engines at anything less than full thrust.  If it's during ascent on takeoff, you absolutely want them to be at the maximum thrust possible (in order to minimize gravity losses).  And even if gravity losses aren't an issue, higher thrust means shorter burn time, which means more accurate execution of maneuver nodes and such.

The only circumstance I can think of where you conceivably might not want to run at full power is on atmospheric ascent, where you're trying not to go too fast too low so as not to get clobbered by atmospheric drag.  However, in most cases, that's not an issue, because gravity losses almost always far outweigh aero losses, so it's still to your advantage to accelerate as hard as possible.  If your ship is so overpowered (really high TWR) that it does manage to get aero loss to exceed gravity loss... well, yes, that ship perhaps should back off the throttle, but what it really means is that the ship has an inefficient design with way too much engine on it.  (Engines are dead weight, so you don't want to put more engine on the ship than you need.)  The real solution there would be to have less engine on the ship, not to back off the throttle.

(The one exception to the above assertion that I can think of is if the atmosphere is exceptionally thick and deep, so you really do need to spend a long ascent in hideously draggy conditions.  Which basically means Eve, and nowhere else, at least in the stock system.)

So... can you explain the circumstances in which you think you should be putting out only 60% thrust?  I can assure you that it won't hurt your Isp, but it might hurt you for other reasons. 

 

Thank you for your thorough reply. 

I was specifically talking about the ascent to space stages. Normally, I use a formula of ((mass x 1.3) X 9.81) in order to make sure that I have a good T.W.R.. This rarely requires that I burn the engine(s) at 100% efficiency. With the original KSP for XBONE, I found that the drag caused by over burning the engines could limit the altitude of a rocket by as much as 50%. At least as far as shooting straight up and down for testing purposes. I haven't bothered to re-test that theory in KSP: Enhanced Edition, but the rockets are still stable and orbit is achieved easily using my old formula. Plus, I don't have to worry about burning up the parachutes on ascent.

With the introduction of KSP: Enhanced Edition, I started actually figuring out dV, and have been trying to make the craft as efficient as possible. It seemed logical to me, that the Isp would be affected by the amount of thrust being used. However, by adjusting the Isp in the dV formula, I found myself returning from the missions with entire stages of fuel left over. So, clearly, I am doing something wrong. This led to my question about Isp.

I have other questions about figuring out dV in complex spaghetti staging. But that is a different question for a different thread.

[SR1200 THUNDER]

3 hours ago, OhioBob said:

As Snark has already answered, in KSP specific impulse is not affected by the throttle setting.  However, I'd just like to add that in real life throttling an engine can have a small affect on Isp.  This is because when an engine is throttled, we are reducing the flow rate of mass through the engine.  It therefore takes less force to push the lesser amount of exhaust gas through the nozzle throat.  This causes the pressure inside the combustion chamber to drop, and a reduction in the velocity of the exhaust gas.  We therefore have a small drop in Isp, though in most cases the decrease is probably not more than a few seconds.

Thank you for your reply.

I figured it would have to be affected, but my KSP experience said otherwise.

[SR1200 THUNDER]

12 hours ago, FleshJeb said:

There are several exceptions to this, and they occur any time you want your throttle inputs to be more granular:

• Executing small maneuver nodes more precisely.
• Setting VTOL engines to have TWR slightly >1.
• Final landing burns when your TWR is >3.

Thank you for your reply.

One of the exceptions needs to be "on console" It is just so clumsy trying to change parameters in flight without a mouse.

[Snark]

2 hours ago, SR1200 THUNDER said:

Normally, I use a formula of ((mass x 1.3) X 9.81) in order to make sure that I have a good T.W.R..

If you choose a TWR of 1.3, that's fine-- as long as your rocket's designed for it, meaning you've picked an engine that gives you TWR 1.3 when you're running at 100% throttle.

But if you have an engine that's more powerful than that (i.e. higher than TWR 1.3), and you're deliberately running it at low throttle to give a lower TWR... you're not saving anything.  Quite the contrary.  It means you're carrying dead engine weight that you're not using, which is bad.  It also means that you're incurring extra gravity losses that you don't have to, which is also bad.

To be clear:  you can launch a ship just fine at TWR 1.3 (if it's designed for it).  You can also launch a ship just fine at TWR 2.0 just fine (if it's designed for it).  As long as your TWR is roughly somewhere in that 1.3-to-2.0 range, it's perfectly workable, as long as the design of the ship matches the TWR you've picked out.  Plenty of experienced old hands at KSP at both ends of that spectrum-- it's a matter of preferred play style.  (For example, I like to launch my own rockets at 2.0 these days.)

(You probably don't want to go much outside that range:  much lower than 1.3 and you get absolutely clobbered by gravity losses, and much higher than 2.0 and you'll likely slam into a wall of air and get clobbered by drag.)

However... whether you choose to go with 1.3 or 2.0 or somewhere in between:  the one thing that doesn't change is that you want to be running at max throttle during that time.

 

2 hours ago, SR1200 THUNDER said:

With the original KSP for XBONE, I found that the drag caused by over burning the engines could limit the altitude of a rocket by as much as 50%.

That... makes no sense to me, at all, unless there was some horrible aerodynamics bug with the console port that I'm unaware of.  I'm a PC player myself, but my understanding is that the physics and game balance of the console version are basically a direct port from the PC one, and what you just said is absolutely not the case on a PC, for a well-designed rocket.

That said:  it could be the case, if the rocket itself has design problems-- for example, if the rocket's designed with really hideously awful drag and isn't streamlined at all.  But if that's the case, the solution is usually to streamline the rocket, not to lower the throttle (unless the rocket is trying to loft some especially ungainly payload that simply can't be streamlined).

My advice:  run your throttle at 100% off the pad.  Unless doing so would put your TWR higher than 2.0, in which case you should put less engine on the rocket and still run the throttle at 100% off the pad.

All of this is somewhat vague advice, of course-- could give much better advice if we could see a screenshot of your ship.  (I get that "screenshot" is an awkward thing with console, but even if it's just a photo from a cell phone of a TV screen, it could tell us stuff that you'd likely never think to tell us because you might not even realize that something's an issue with the design.)

[GoSlash27]

6 hours ago, Snark said:

But if you have an engine that's more powerful than that (i.e. higher than TWR 1.3), and you're deliberately running it at low throttle to give a lower TWR... you're not saving anything.  Quite the contrary.  It means you're carrying dead engine weight that you're not using, which is bad.  It also means that you're incurring extra gravity losses that you don't have to, which is also bad.

Snark,
 The dead engine mass is only a penalty if there's a lighter or cheaper alternative that can produce the same acceleration at full throttle. As for the gravity losses, they ultimately manifest as a reduction in the total DV of the stage. Not a concern so long as the stage is designed to waste the DV anyway. Like... A stage with an initial t/w of 1.4 will need about 1600-1700 m/sec DV (depending on diameter) to reach 25 km and 800 m/sec in a nominal gravity turn; the point at which I prefer to ditch my first stage. If I leave it blasting 100% throttle the whole time, I could do it in less... but I wasn't planning on using the extra DV anyway.

 I find it's a lot easier from a practical standpoint to limit my thrust as I go. Less heating problems, better aerodynamic stability, and a rocket that's accelerating too quickly doesn't like to follow the proper gravity turn.

 I realize that none of these are "efficiency" improvements, strictly speaking. from a pure efficiency perspective, I agree with you. But from a practicality point of view, throttling back can be desirable or even preferable.

Best,
-Slashy
 

Edited April 1, 2018 by GoSlash27

[SR1200 THUNDER]

On 4/1/2018 at 9:53 AM, Snark said:

My advice:  run your throttle at 100% off the pad.  Unless doing so would put your TWR higher than 2.0, in which case you should put less engine on the rocket and still run the throttle at 100% off the pad.

As per the "training" scenario in the menus, if my engine is crazy overpowered for the size of the craft, it says to limit the amount of thrust available in the VAB. That is why I go with the 1.3 TWR formula. But once on the pad, I absolutely run the throttle at 100%. Is that what you are talking about?

If not, please explain more. Because sometimes I do get strange rotation variations in flight, even though I am using the exact same craft, with exact same settings and payload. And it is EXTREMELY frustrating to have a ship be stable when it stages, only to immediately be cartwheeling out of control back to the ground. Especially when that craft just made orbit, Minmus, the Mun, and returned without incident. I play on the hardest settings available to me, so no reverting flights, quick loading, or reanimated Kerbals.

As for my tests with the original KSP on console, they might have been terribly flawed by poor rocket design. That version of the game was beyond broken, and I did not put too many hours into trying to figure out what was going wrong since every control and interface was poorly designed and implemented making even vehicle construction an absolute chore.

On 4/1/2018 at 3:19 PM, GoSlash27 said:

Snark,
 The dead engine mass is only a penalty if there's a lighter or cheaper alternative that can produce the same acceleration at full throttle. As for the gravity losses, they ultimately manifest as a reduction in the total DV of the stage. Not a concern so long as the stage is designed to waste the DV anyway. Like... A stage with an initial t/w of 1.4 will need about 1600-1700 m/sec DV (depending on diameter) to reach 25 km and 800 m/sec in a nominal gravity turn; the point at which I prefer to ditch my first stage. If I leave it blasting 100% throttle the whole time, I could do it in less... but I wasn't planning on using the extra DV anyway.

 I find it's a lot easier from a practical standpoint to limit my thrust as I go. Less heating problems, better aerodynamic stability, and a rocket that's accelerating too quickly doesn't like to follow the proper gravity turn.

 I realize that none of these are "efficiency" improvements, strictly speaking. from a pure efficiency perspective, I agree with you. But from a practicality point of view, throttling back can be desirable or even preferable.

Best,
-Slashy
 

I hope that you follow my next series of questions about calculating dV in a more complex rocket. Specifically spaghetti staging, utilizing different engines.

I have so much to learn.

[GoSlash27]

SR1200,

 I look forward to it.

13 minutes ago, SR1200 THUNDER said:

I have so much to learn.

As do I, apparently What the heck is "spaghetti staging"?

Best,
-Slashy

[SR1200 THUNDER]

7 minutes ago, GoSlash27 said:

SR1200,

 I look forward to it.

As do I, apparently What the heck is "spaghetti staging"?

Best,
-Slashy

lol, I meant asparagus staging. I'm an idiot.

[GoSlash27]

Ah. Okay. Don't sweat it. You made it this far, so you're clearly not an idiot.

Best,
-Slashy

[Snark]

3 hours ago, SR1200 THUNDER said:

As per the "training" scenario in the menus, if my engine is crazy overpowered for the size of the craft, it says to limit the amount of thrust available in the VAB. That is why I go with the 1.3 TWR formula. But once on the pad, I absolutely run the throttle at 100%. Is that what you are talking about?

No, what I'm saying is "don't make your engine crazy overpowered for the size of the craft".  Because if you do that, then you're lugging a lot more engine mass than you need to.  Redesign the craft to put a smaller engine on it.

Suppose you have two engines to pick from:  a 1-ton engine with 100 kN thrust, or a 2-ton engine with 200 kN thrust, both with the same Isp, and you've worked out that what you need is 100 kN of thrust.  In that case... you put the 1-ton engine on, and run it at 100%.  Don't put the 2-ton engine on there and run it at 50%, because then you're just lugging around an extra ton of dead engine weight for no reason.

It's also worth noting that if you've got a craft that's already lugging around enough engine power to have a TWR significantly higher than 1.3, use the power it has-- 1.3 is on the low side, and you'll get better efficiency with a higher TWR such as 2.  That's because unless your craft is horribly un-aerodynamic, your gravity losses will way outweigh your aero losses, which means higher TWR is better for a given engine mass.

(Note that this is not the same thing as saying "design all your rockets with a high TWR," because that's not what I'm saying.  You can do that-- it's what I do-- but there's a different, also very valid approach, which is to deliberately design the rocket with a lower TWR by putting less engine and more fuel on it.  Bigger gravity losses, yes, but there's less engine weight and more fuel available to offset, so when properly done it all comes out in the wash.  What you do not want to do, however, is build a rocket with enough engine power to have TWR 2, and then run it at less power than it has so that you're at 1.3.  That's just flushing fuel down the toilet, basically.)

The one notable scenario where it's sometimes useful to use less than maximum engine power during liftoff is if you're launching a very small craft with an SRB.  Even the Flea is way overpowered, if all it's pushing is, say, a Mk1 command pod.  So using the thrust limiter in the VAB for an SRB can make some sense.

As with everything in KSP, though, it all comes down to play style and what your personal preferences are.  If you like putting crazy-overpowered engines on your ships just because it looks cool ... nothing wrong with that, and even if you're "wasting" a bunch of dV, so what?  Doesn't matter as long as you can do what you want to in the game, and aren't getting frustrated with rockets that poop out before you get to the destination.

My own bias in the game is having fun with "rockets as an engineering optimization", which means I do care, a lot, about wasting dV, even if it's in the latter stages of a career when I've got money coming out of my ears and I could easily afford to make a rocket twice as big and expensive as it needs to be.  Sure, I could do that, but I don't wanna, because to me that's inelegant.  So that's where I'm coming from, and advice you get from me will tend to be biased in that direction. 

 

[SR1200 THUNDER]

1 hour ago, Snark said:

No, what I'm saying is "don't make your engine crazy overpowered for the size of the craft".  Because if you do that, then you're lugging a lot more engine mass than you need to.  Redesign the craft to put a smaller engine on it...

My own bias in the game is having fun with "rockets as an engineering optimization", which means I do care, a lot, about wasting dV, even if it's in the latter stages of a career when I've got money coming out of my ears and I could easily afford to make a rocket twice as big and expensive as it needs to be.  Sure, I could do that, but I don't wanna, because to me that's inelegant.  So that's where I'm coming from, and advice you get from me will tend to be biased in that direction. 

 

Thank you for the clarification. I do try to build as efficiently as possible.

[Snark]

By the way:  the higher the TWR you design into your rocket, the more you have to care about aerodynamics.  That's because the higher a rocket's TWR, the faster it ends up going when it's still down in the thick part of the atmosphere, and also the more distance it travels through thick atmosphere (because higher TWR means a more aggressive gravity turn and therefore a shallower climb).

So, for example, I like building my rockets with TWR of 2, which is pretty high, so I make sure to make them very streamlined.  On the other hand, someone who designs a rocket to have a lower TWR, like 1.3, could get away with being somewhat less streamlined, because it'll take off a lot more vertically and will be at a higher altitude before it gets going really fast, so aero would be less of an issue.  (It's still relevant-- never make your rocket draggier than it has to be.  It's just a question of what you can get away with.) 

This is one reason I love the conical Soyuz fuel tanks in Making History so much-- they're wicked streamlined, so they're very well suited to a player like me who likes to burn pretty hot off the pad.

[OhioBob]

On ‎4‎/‎1‎/‎2018 at 3:19 PM, GoSlash27 said:

 I find it's a lot easier from a practical standpoint to limit my thrust as I go. Less heating problems, better aerodynamic stability, and a rocket that's accelerating too quickly doesn't like to follow the proper gravity turn.

That last part is something I've been meaning to bring up because I think it's important.  We've been talking much about gravity and drag losses, but the trajectory we fly is extremely important too in how efficient our launch is.  We want to fly as close to horizontal as the atmosphere and rocket design will permit.

Let's consider the two extreme scenarios.  For this I'm going to ignore drag and pretend that Kerbin has no atmosphere.  This will allow us to see just what affect the trajectory has on delta-v.

First, let's say we launch straight up into a trajectory that reaches an apoapsis of 80 km, then when we reach the apex, we accelerate horizontally to orbital velocity.  To simplify the calculations, we'll assume that the delta-v is applied in instantaneous bursts.  From sea level on Kerbin, we need to give our rocket a velocity of 1177 m/s to reach an apoapisis of 80 km.  We when reach the apex, we have to go from zero to orbital velocity, a delta-v of 2279 m/s.  So adding it together, we have a total of 3456 m/s.  That's the worst case.

New let's look at the best case.  The most efficient thing we can do is launch horizontally right off the pad (hoping there's no mountains in the way) and perform a Hohmann transfer to orbit.  This takes a delta-v of 2501 m/s off the launch.  But by the time we coast around to the 80 km apoapsis on the opposite side of the planet, it takes only 72 m/s to circularize the orbit.  That's a total delta-v of only 2573 m/s, which is far more efficient than the first scenario.

In reality we fly a trajectory that is somewhere in between these extremes.  We have to launch our rocket initially upward to get above the thickest part of the atmosphere, and then transition to horizontal.  The faster we can transition to horizontal, the more efficient the trajectory will be.  I tend to measure the efficiency of my trajectory by the magnitude of the apoapsis burn needed to finalize the orbit.  If I can keep the burn down to about 100 m/s or less, I'm happy.  On the other hand, if I have to make an apoapsis burn of, say, 300 m/s or more, then I'm unhappy.  If I have to make that great of a burn, then I know I lofted the rocket into too steep of an arc and reached apoapsis too soon.

Maybe I'm just really bad at flying high TWR rockets, but I find it much more difficult to fly a good trajectory with a high TWR rocket.  We can't pitch over right off the launch pad or else the drag losses will kill us, so we have to gain some altitude first.  While we're gaining that altitude, the high TWR rocket is gaining more speed than the low TWR rocket.  And a high-speed rocket is more difficult to turn because the wind moving over its surface resists the turn.  As a result of this, I just can't flatten out the trajectory of a high TWR rocket enough.  I always end up in highly lofted a trajectory with a large apoapsis burn.  So while I've saved on gravity losses, I've given it back on the poor trajectory and greater drag losses.

This is one of the reasons why I favor low TWR rockets (I typically like about 1.3-1.4).  I find them much more controllable during launch.  I can pitch the rocket over and just slowly push the apoapsis higher and higher.  I try to keep the "time to apoapsis" at around a minute or so while ascending.  By keeping the apoapsis just a short distance ahead, we're flattening out the trajectory.  If we let the apoapsis get away from us, we'll end up in a steep arc (which is what typically happens to me with high TWR rockets).  As we gain altitude and near the end of the burn, the apoapsis will rapidly move far out ahead of us and wrap around the planet, but that's OK.  If we have to travel 1/4 of the way around the planet before performing the apoapsis burn, that's good.  It means we've come a little closer to the ideal Hohmann transfer.
 

Edited April 3, 2018 by OhioBob

[GoSlash27]

1 hour ago, OhioBob said:

Various and sundry good points

OB,

 Yeah, that's pretty much the way I do it as well. I'll let the acceleration creep up towards 2G, but then start tapering it back as a function of 2sin(pitch) until it gets past 450 m/sec or so, then it's "Katie bar the door".
 I also keep my time to Ap around 45 seconds.

The idea is to get a consistent repeatable gravity turn where I don't come up too steeply or too shallow, and am then forced to suffer cosine losses by burning off-axis to correct it.

Lowering the TWR helps to drop me right in on target. Doing it in a systematic manner allows me to do it consistently.

Best,
-Slashy

[TheFlyingKerman]

18 hours ago, Snark said:

The one notable scenario where it's sometimes useful to use less than maximum engine power during liftoff is if you're launching a very small craft with an SRB.  Even the Flea is way overpowered, if all it's pushing is, say, a Mk1 command pod.  So using the thrust limiter in the VAB for an SRB can make some sense.

SRB's are very different from liquid fueled engines. For a liquid fueled rocket, having too high TWR means you may potentially launch more fuel / payload. You cannot add more fuel an SRB engine so the only way to obtain a reasonable burn time / thrust / dV is to use the thrust limiter.