TWR - a general question.

[paul23]

Those aren't directly comparable, as more SRBs at a lower % will have more propellant to expend (i.e. will add more delta-V). The better comparison is 100% SRBs vs the same number of SRBs set to a lower percentage; generally speaking it is better to leave them at 100% (you are paying the mass for those engines either way) unless it makes your initial TWR so high that drag/overheating become significant.

And stability - the thrust force is inherently unstable. So if thrust is higher you also need more items to increase stability. (= more mass)

[Alshain]

Seems like a weird thing for me to of missed this whole time but I'll check it anyway. Does KER even give the TWR in vacuum in the assembly building?

Yes, and it is the default. You have to click the Atmosphere button to get it at sea level.

[Snark]

Does this apply to SRBs? I frequently reduce the thrust on the SRBs when designing a rocket so my TWR range (according to Kerbal Engineer mod) is reasonable both at the low and high ends. Is it better to use fewer SRBs at 100% than more at a lower %?

The SRB question comes down to this:

• They're really cheap. (Yay!)
  
• They have crappy Isp. (Boo!)
  
• You can't throttle them in flight, so managing TWR is tricky.
  

Because of the Isp issue, that's why SRBs are best when used right off the launchpad-- they're not a good choice for upper stages, it's not worth lofting that much mass. In general, whenever you have a stack of stages of varying Isp, you want to burn your lowest Isp first and your highest last.

The main challenge with SRBs is that you can't adjust their throttle. This can make it difficult to get a good TWR curve-- for one thing, they have so much mass that they tend to be a big fraction of your total mass, which means that your TWR is often a lot higher near the end of their burn than at the start. So if you give them a "reasonable" TWR on the pad, it's tricky not to end up overpowered and over-speed when you're still low in the atmosphere.

I tend to prefer larger numbers of SRBs at a lower thrust level, but a lot of that is just because I happen to like SRBs for no rational reason whatsoever, I just like the show they put on-- the massive gouts of flame, the billowing column of aluminum-oxide dust, gives me a thrill.

One thing I like to do, to smooth out the ride a bit (maintain a somewhat more even TWR during the SRB phase of ascent): I arrange them in what I think of as a "poor man's asparagus". Let's say I work out that I need 8 boosters at 85% thrust. What I do is, instead of adding a single symmetry group of 8 radial boosters (all set at 85% thrust), I add two symmetry groups of 4 each. Group A is set to 100% thrust, group B is set to 70% thrust. All 8 of them fire simultaneously at launch; the thrust is exactly the same as if I had set all eight of them to 85%. But since group A is set to a higher thrust, it burns out (and is jettisoned) first, while group B goes burning for a while longer. This means a large drop in thrust (I'm only running half the boosters, and they're at lower power per booster)-- but on the other hand, the mass of the ship is much lighter, since group A is completely gone and group B has used up most of its fuel, and by now I'm high enough that the boosters' Isp is 20% better, so my TWR is still reasonable. Lower than at launch, but still good.

This technique lets me get about one-and-a-half stages out of my SRBs, instead of just one.

One practical reason I like to use SRBs at less than 100% is that it makes for a friendly repertoire of subassemblies. I like to have about three or four different "booster-stage" subassemblies in different size ranges: for example, one might be "big orange tank, Mainsail, eight radial Kickbacks". That's a pretty good standard booster for lofting payloads of around 35ish tons into LKO, and I'll use it on lots of different payloads. But not all payloads are exactly the same mass, and I like to always have a launchpad TWR of exactly 1.5. So since my standard booster assembly is around 80% average power, that means I can tweak the capacity up and down a bit. If they were all pegged at 100%, then I can't tweak them up and would have to move to a different design.

Another way to smooth out the TWR curve with SRBs is to use them in conjunction with LFO engines. For example, let's say you have a ship that you work out will need 3000 kN of thrust off the launchpad to give you the desired TWR. You could put a ring of eight Thumpers (launchpad thrust: 250 kN each) around a Mainsail (launchpad thrust: 1379 kN). The Thumpers by themselves are not enough (total 2000 kN), but together with the Mainsail they're plenty. So you could set off the Mainsail along with the Thumpers right off the pad, and then gradually throttle the Mainsail down to zero as your TWR gets better (which it will do fairly quickly, both because of all the mass that the Thumpers are dropping, and because their Isp goes up a lot as pressure drops). When the Thumpers burn out, then just throttle the Mainsail up to 100% and be on your way.

[BoilingOil]

Because of the Isp issue, that's why SRBs are best when used right off the launchpad-- they're not a good choice for upper stages, it's not worth lofting that much mass.

I *have* on occasion done just that: just because it was a cheap solution, I added six of the shortest SRBs around a higher up stage, just to get a short but massive boost on a load that the bottom stage had only just managed to lift out of the thickest part of the atmosphere. So I suppose you'd advise me to see what I can replace them with If so, I will indeed!

- - - Updated - - -

One thing I like to do, to smooth out the ride a bit (maintain a somewhat more even TWR during the SRB phase of ascent): I arrange them in what I think of as a "poor man's asparagus". Let's say I work out that I need 8 boosters at 85% thrust. What I do is, instead of adding a single symmetry group of 8 radial boosters (all set at 85% thrust), I add two symmetry groups of 4 each. Group A is set to 100% thrust, group B is set to 70% thrust. All 8 of them fire simultaneously at launch; the thrust is exactly the same as if I had set all eight of them to 85%. But since group A is set to a higher thrust, it burns out (and is jettisoned) first, while group B goes burning for a while longer. This means a large drop in thrust (I'm only running half the boosters, and they're at lower power per booster)-- but on the other hand, the mass of the ship is much lighter, since group A is completely gone and group B has used up most of its fuel, and by now I'm high enough that the boosters' Isp is 20% better, so my TWR is still reasonable. Lower than at launch, but still good.

This technique lets me get about one-and-a-half stages out of my SRBs, instead of just one.

GENIUS!

[Empiro]

I *have* on occasion done just that: just because it was a cheap solution, I added six of the shortest SRBs around a higher up stage, just to get a short but massive boost on a load that the bottom stage had only just managed to lift out of the thickest part of the atmosphere. So I suppose you'd advise me to see what I can replace them with If so, I will indeed!

You'd get more mileage (delta-V) if you replaced those SRBs with something like an LV-909 and an equivalent mass of fuel. In general, the higher you are, the less TWR matters. You don't need that quick acceleration if you follow a good ascent trajectory.

It might seem like those SRBs will get you up to speed better, but it's an illusion -- you're applying maybe 4 times the acceleration but for only 1/10th as long.

Edited November 6, 2015 by Empiro

[BoilingOil]

You'd get more mileage (delta-V) if you replaced those SRBs with something like an LV-909 and an equivalent mass of fuel. In general, the higher you are, the less TWR matters.

Just as I imagined, yes

[Red Iron Crown]

Seems like a weird thing for me to of missed this whole time but I'll check it anyway. Does KER even give the TWR in vacuum in the assembly building?

It has only mattered since 1.0, before that vacuum TWR and sea level TWR were the same.

[Alshain]

You'd get more mileage (delta-V) if you replaced those SRBs with something like an LV-909 and an equivalent mass of fuel. In general, the higher you are, the less TWR matters. You don't need that quick acceleration if you follow a good ascent trajectory.

It might seem like those SRBs will get you up to speed better, but it's an illusion -- you're applying maybe 4 times the acceleration but for only 1/10th as long.

I agree with this. Generally speaking, SRB's should always and forever be first stage. You use them when you just need just a little more TWR. Try to avoid using them for Delta-V. If you need more dV, put more fuel on the main stack and use SRB's if that stack gets too heavy for its engine. By the time you discard the SRB's, the engine will be high enough for it's TWR to be sufficient.

[GoSlash27]

I agree with this. Generally speaking, SRB's should always and forever be first stage. You use them when you just need just a little more TWR. Try to avoid using them for Delta-V. If you need more dV, put more fuel on the main stack and use SRB's if that stack gets too heavy for its engine. By the time you discard the SRB's, the engine will be high enough for it's TWR to be sufficient.

Alshain,

I use SRBs exclusively as a first stage, and for a completely different reason:

SRBs are *cheap!*

Without stage recovery, the booster stage is disposable. No recovery value. Therefore you want to provide that first 1,800 m/sec DV as cheaply as possible. Nothing provides that DV as cheaply as a trash can fulla boom.

If I'm using a recoverable upper stage, I may intentionally reduce or even eliminate the SRBs in order to minimize launch expense (and thus maximize recovered value), but if I just want a cheap disposable launch, I use SRBs for the first stage.

Best,

-Slashy

[Snark]

You'd get more mileage (delta-V) if you replaced those SRBs with something like an LV-909 and an equivalent mass of fuel. In general, the higher you are, the less TWR matters. You don't need that quick acceleration if you follow a good ascent trajectory.

It might seem like those SRBs will get you up to speed better, but it's an illusion -- you're applying maybe 4 times the acceleration but for only 1/10th as long.

+1 to this. The Terrier is a fantastic upper-stage engine: available early in the tech tree, reasonably lightweight, excellent vacuum Isp (second-best non-nuclear vacuum Isp in the game, just a nose behind the Poodle). You'll get a lot more bang for your buck. If you used six Fleas, that's a total of 10.19M kg m/s of impulse, and you had to lug 9 tons up there to get it. If you had just slapped a Terrier on there with 3 tons of LFO in fuel tanks, instead (burning over a longer time), you could get just about exactly the same amount of impulse, but need less than 4 tons to do it!

Whether a Terrier would have done the job or not depends on your exact launch profile. Don't get me wrong, I love SRBs like a brother, but they have abysmal Isp, which means they really don't belong in upper stages. Really. Because math. Anything you build that has an SRB in an upper stage always has a better solution using something else. (By "better" I mean "more efficient", not necessarily "more entertaining.")

[cantab]

Something that hasn't been mentioned yet: now that KSP has (vaguely) realistic aerodynamics, some rockets will have stability problems and be prone to losing control and flipping out. If improving stability isn't an option then by reducing TWR in the ascent you will reduce the aerodynamic forces because you're going slower at any given altitude, and therefore maintain control of the rocket.

[Snark]

Something that hasn't been mentioned yet: now that KSP has (vaguely) realistic aerodynamics, some rockets will have stability problems and be prone to losing control and flipping out. If improving stability isn't an option then by reducing TWR in the ascent you will reduce the aerodynamic forces because you're going slower at any given altitude, and therefore maintain control of the rocket.

And actually, SRBs are good for stability for just this reason, in a somewhat subtle way:

An SRB is a very lightweight "can", full of fuel. It's a cylinder, and its CoM is right in the middle of the cylinder, and stays there the whole time it burns. In other words, its CoM stays stationary as it burns its fuel.

On the other hand, if you have an LFO tank with a powerful liquid-fuel engine below it-- let's say, for example, an FL-T800 with a Swivel underneath-- then its CoM drops lower as fuel is burned, because a big chunk of the mass is tied up in that heavy engine down below, which doesn't get lighter as you burn fuel.

Lowering the CoM makes for worse aerodynamic stability. SRBs become quite light as they burn their fuel, but they still have that long, draggy shape-- it's almost like having a kite tail on your rocket.

Added to which: you can add fins to the very bottom of the SRB. You can't do that with the fuel-tank-plus-liquid-engine, since you can't attach fins to the engine itself. That means you can get the fins positioned much lower, which makes them more effective and adds stability.

[Alshain]

Alshain,

I use SRBs exclusively as a first stage, and for a completely different reason:

SRBs are *cheap!*

Without stage recovery, the booster stage is disposable. No recovery value. Therefore you want to provide that first 1,800 m/sec DV as cheaply as possible. Nothing provides that DV as cheaply as a trash can fulla boom.

If I'm using a recoverable upper stage, I may intentionally reduce or even eliminate the SRBs in order to minimize launch expense (and thus maximize recovered value), but if I just want a cheap disposable launch, I use SRBs for the first stage.

Best,

-Slashy

They aren't cheaper than a fuel tank. You were going to discard the LFO engine anyway. Letting your first stage in the center stack be overpowered just so you can use SRBs for Delta V doesn't scream cost savings to me.

EDIT: well, not on smaller to mid sized rockets anyway, bigger ones this might make more sense. Remember you have to add that decoupler and nose cone cost.

Edited November 6, 2015 by Alshain

[Snark]

They aren't cheaper than a fuel tank. You were going to discard the LFO engine anyway. Letting your first stage in the center stack be overpowered just so you can use SRBs for Delta V doesn't scream cost savings to me.

EDIT: well, not on smaller to mid sized rockets anyway, bigger ones this might make more sense. Remember you have to add that decoupler and nose cone cost.

I think Slashy's point was that SRBs make cheaper disposable boosters than LFO engines do.

A Thumper has a cost-per-impulse of 0.55 funds per Isp-ton of impulse (that is, divide its cost by the product of its Isp and its fuel mass)-- that's using its atmospheric Isp.

By comparison, a Reliant under an FL-T800 costs 1.7 funds per Isp-ton, over three times as much. Yes, that's not quite an apples-to-apples comparison, since the Thumper has somewhat more mass, but it's still a dramatic difference.

(Apologies if I'm misunderstanding either your point or Slashy's, in which case I'll cheerfully bow out.)

[magnemoe]

They aren't cheaper than a fuel tank. You were going to discard the LFO engine anyway. Letting your first stage in the center stack be overpowered just so you can use SRBs for Delta V doesn't scream cost savings to me.

EDIT: well, not on smaller to mid sized rockets anyway, bigger ones this might make more sense. Remember you have to add that decoupler and nose cone cost.

Think he recover core, if so having a too long core will make it harder to land safely.

I always use SRB as boosters mostly for TWR, sometimes I even have two sets where the second has lower trust and burn longer.

Has used SRB first and second stage on some satelite launches as the T30 was overkill and too expensive, I just wanted up so the T45 could work.

[Alshain]

I think Slashy's point was that SRBs make cheaper disposable boosters than LFO engines do.

Nobody questioned that. I think you are misunderstanding. I've said it MANY times on the forums, if your lifter has fuel lines and your payload is less than 120 tonnes, you are throwing away money. Leave the Onions and Asparagus in the garden where they belong, they are no longer a good thing for most rockets.

The conversation is about whether you push the limits of your center stack engine, or add more/bigger SRB's as DeltaV.

I say, push the limits of your center stack and use cheaper or fewer SRB's since the SRB's are more expensive than a fuel tank in cases where you want to squeak out a little delta V. All you have to do is get that center stack engine up to the altitude where it's TWR is sufficient, kickbacks and even thumpers are not always the solution to that. If my TWR is in the 1.1 range, I slap on a pair of Hammers and by the time they burn out, my center stack TWR will be 1.25. Better still, they aren't on the rocket long enough to bother with the expense of a nose cone. If I'm using 2 instead of 4 SRB's, thats a massive savings right there, for the cost of a fuel tank.

It does largely depend on which size you are working with though and how much fuel you actually need. But if all I need is an X200 fuel tank, I'd much rather use Hammers with that than Kickbacks at 2700 apiece.

Edited November 6, 2015 by Alshain

[severedsolo]

Not sure how people are launching with less than a TWR of 1.5? If I go lower my craft generally just sits on the pad thrusting and not moving. Usually I go for 1.6 just to make sure I'm actually going to lift as even 1.5 sometimes just sits there.

It's always been that way for me despite reinstalling the game numerous times on different PCs.

You may perhaps be running into the "sticky launchpad" bug. It usually only affects the Lv2 pad I think (but really, that's all you need 99% of the time). I find that it releases if you can kick it to 1.5/1.6 TWR when it sticks.

Generally I go for a 1.2-1.4 TWR (it depends on the craft). What's more important to me is how well my craft flies the gravity turn. If for example, I have a massivly underpowered main engine, I will set my boosters in the 1.4-1.5 range, to offset the losses on my 1.0-1.1 main engine.

[GoSlash27]

The conversation is about whether you push the limits of your center stack engine, or add more/bigger SRB's as DeltaV.

Ahh... that's the source of the confusion; we're talking about 2 different things. You're talking about using SRBs to add DV to a parallel stack. *I'm* talking about using SRBs as the first stage in a series stack.

Best,

-Slashy

[Alshain]

Ahh... that's the source of the confusion; we're talking about 2 different things. You're talking about using SRBs to add DV to a parallel stack. *I'm* talking about using SRBs as the first stage in a series stack.

Best,

-Slashy

Oh geez, I would never ever do that no matter how much cost it saved. That's just painful to fly. No gimbal = no go for me, I'd never get an efficient gravity turn on those things and my cost savings would be swallowed.

Edited November 6, 2015 by Alshain

[paul23]

Uh alshain: why do you keep calling it a gravity turn?

A gravity turn is a turn where you do not use power (gimbal) to turn around. And only use gravity to let your rocket "fall over" slowly. Sure a stage can start as a powered turn and then transform in a gravity turn but you keep misusing the term.

[Red Iron Crown]

Uh alshain: why do you keep calling it a gravity turn?

A gravity turn is a turn where you do not use power (gimbal) to turn around. And only use gravity to let your rocket "fall over" slowly. Sure a stage can start as a powered turn and then transform in a gravity turn but you keep misusing the term.

Rockets are inherently unstable aerodynamically, they all require some form of control to keep pointing in the right direction, be it gimbals or fins.

[Alshain]

Uh alshain: why do you keep calling it a gravity turn?

A gravity turn is a turn where you do not use power (gimbal) to turn around. And only use gravity to let your rocket "fall over" slowly. Sure a stage can start as a powered turn and then transform in a gravity turn but you keep misusing the term.

There is a small problem in your logic.... this is a game. I could spend forever running test runs and tweaking my rocket moving fins around to just the perfect place to make this happen, but because the slightest change in payload mass changes this effect and KSP is not a perfect simulator, I would have to do it for every single rocket I launched. That just isn't worth it. So, when I say 'gravity turn', I think you knew what I meant.

[paul23]

There is a small problem in your logic.... this is a game. I could spend forever running test runs and tweaking my rocket moving fins around to just the perfect place to make this happen, but because the slightest change in payload mass changes this effect and KSP is not a perfect simulator, I would have to do it for every single rocket I launched. That just isn't worth it. So, when I say 'gravity turn', I think you knew what I meant.

Actually no, I do not. Do you mean a powered turn, or just turning in general, or turning due to the difference between aerodynamic center and center of mass.

I really thought you were talking about a gimbal-free turn (which is more efficient, as the propulsion vector is always in line with the direction of flight). I really thought it was weird that you talked about importance for gimbal when talking about gravity turning. - First thing that came up was to fix the inherent instability - but than active active control surfaces are better I think?

Edited November 6, 2015 by paul23

[cantab]

In my experience many rockets are mildly unstable and need to be actively held on the prograde vector during the gravity turn. That said if you have a sufficiently stable rocket, which is possible with fixed fins, and have enough control to do the initial pitchover (or put the rocket on the pad at an angle) then you can fly the whole ascent hands-off. Even then, though, a good way to make the initial pitchover is with a gimballing engine.

[Alshain]

Actually no, I do not. Do you mean a powered turn, or just turning in general, or turning due to the difference between aerodynamic center and center of mass.

I really thought you were talking about a gimbal-free turn (which is more efficient, as the propulsion vector is always in line with the direction of flight). I really thought it was weird that you talked about importance for gimbal when talking about gravity turning. - First thing that came up was to fix the inherent instability - but than active active control surfaces are better I think?

I mean a gravity turn.