TWR

[Spaced Out]

In real life, how is the optimal TWR for different rockets determined, and is there a typical range that their optimal TWR falls in? And also, what are the factors that determine the optimal TWR for rockets? And does the optimal TWR change throughout the flight?

[Superfluous J]

Very roughly speaking, "optimal TWR" is "as low as you can get away with" because, also generally speaking, the lower your TWR, the lighter your craft.

[qzgy]

44 minutes ago, 5thHorseman said:

Very roughly speaking, "optimal TWR" is "as low as you can get away with" because, also generally speaking, the lower your TWR, the lighter your craft.

Isn't a lower twr meaning your craft is heavier? For X amount of thrust, a heavier craft would have a lower twr.

Also, I believe some early Japanese rockets had insane get, maybe 10:1 or something. 

[Superfluous J]

For a given engine, higher mass craft have a lower TWR but that's not looking at it the right way.

For a given payload mass, the lower the TWR of the ship you build to deliver it, the lower the mass of the delivery ship. In general, of course.

[Human Person]

8 hours ago, Spaced Out said:

In real life, how is the optimal TWR for different rockets determined, and is there a typical range that their optimal TWR falls in? And also, what are the factors that determine the optimal TWR for rockets? And does the optimal TWR change throughout the flight?

I'll start with the last one as it will explain the others too. 

1. Liftoff TWR: Probably the most critical number is your TWR at liftoff. High TWR at this point really pays off as it minimizes gravity losses wich are most severe early on (Basically all the delta v you burn vertically is lost delta v). The first stage should also build up some good time to apogee, as the next stage(s) are typically lower on TWR.

Limiting your TWR at Liftoff is your budget (engines are expensive) and the structural integrity of your rocket. High acceleration and/or high dynamic pressures cause a lot of stress on the structure, so lower TWR might enable you to build lighter. 

2. Wet Mass/ Dry Mass ratio: the higher it is, the higher is delta-v on a stage. Engines are heavy and add up to your dry mass. This is more relevant in upper stages than in lower stages since vacuum optimized engines are much heavier compared to their thrust than sea level engines. This is why upper stages are usually lower on TWR (oftenly even lower than 1)

3. Rated burn time: Since I started playing in RO, I found that this is the hardest factor you have to consider: Don't push your engines past their rated burn time or they will be very likely to fail. This sometimes causes me to build stages with higher TWR than strictly needed.

4. Time to orbit: This number ranges from around 6 minutes all the way to well over 20 minutes an it determines the trajectory you have to fly. The higher this is, the steeper is the required trajectory and the higher is the orbit you will end up in. There are some tricks to get into lower orbits, like burning past your apogee and pitching up (sometimes quite hard), but they will reduce efficiency due to steering losses (also called cosine losses) and such trajectories are quite hard to fly. You have to perfectly nail the timing on your circularization, cancelling out your vertical velocity right when you reach orbital velocity.

 

Typical wett mass TWR per stage (based on my craft, what I think is flyable. Not all rockets work as those I describe here. There are many counter examples):

1st stage:

1.15 to 1.4 (less is too inefficient, more is overkill* (for RO, you can go a bit higher in Stock)

 

2nd stage:

0.7 to 1.2: Depends how much work the first stage did. The further you get on the first stage, the lower can your TWR on the 2nd stage be. There are rockets that go even lower on 2nd stage TWR (e.g. Ariane 5), but they are optimized for GTO and bejond.

 

opt. 3rd stage:

depends on the purpose of this stage. 3 stage to LEO rockets are rare (example: The Proton Rocket). 3 stage rockets are mostly used for bejond LEO applications. Once you are in orbit, burn time (and thereby TWR) isn't an issue anymore. The TWR can be nearly arbitarily low (depending on how much of the stage is needed to circularize. Keep the Time To Orbit in mind).

 

*literally, as the rocket could be destroyed by aeroforces if the first stage had a higher TWR. An exception to this rule are boosted rockets, where the boosters give just a "short kick" to get the rocket going. 

Edited August 28, 2017 by Physics Student

[Nefrums]

The question here is how much fuel can you add before twr gets so low that you get no benefit of adding more fuel.

As dry mass ratios of fuel tanks is very different irl and in game and the cost of adding more fuel is very low. This tends to end up with a liftoff twr around 1,2 irl.

The main thing that is optimized irl is payload/cost. Amount of dV spent to get to orbit is irelevant.

[Racescort666]

5 hours ago, Physics Student said:

2nd stage:

0.7 to 1.2: Depends how much work the first stage did. The further you get on the first stage, the lower can your TWR on the 2nd stage be. There are rockets that go even lower on 2nd stage TWR (e.g. Ariane 5), but they are optimized for GTO and bejond.

I just ran the numbers for Atlas V and the Centaur has an initial TWR of 0.33:1 for a GTO payload. Data: http://spaceflight101.com/spacerockets/atlas-v-551/

[kerbiloid]

ICBM, SLBM 2.5..6+.

Launch escape system (LES) 8..21

Orbital maneuvering systems of space ships 0.05+

Ion engines 0.0001-..0.001+

In KSP, with MechJeb, you can set upper T/W limit in Ascent mode, and just launch several rockets to choose optimal T/W. For my ones it was ~2.

[Human Person]

4 hours ago, Racescort666 said:

I just ran the numbers for Atlas V and the Centaur has an initial TWR of 0.33:1 for a GTO payload. Data: http://spaceflight101.com/spacerockets/atlas-v-551/

Interesting. I didn't know it was that low. 

saying 0.33:1 instead of just 0.33 is a bit redundant, isn't it? 1:3 would make sense.

Edited August 28, 2017 by Physics Student

[Racescort666]

3 minutes ago, Physics Student said:

Interesting. I didn't know it was that low.

Yeah, I knew the RL10 was a pretty low thrust engine but I was surprised to find it to be that low relative to Centaur. I suspect that Atlas V is booster thrust limited though and they can compensate for the low thrust upper stage with a steeper ascent. This would be evidenced by ULA adding more boosters to increase payload. Although, Starliner is supposed to be flying on a 2 engine Centaur. I'm not sure if this improves payload or just allows them to fly a different ascent profile.

[Pand5461]

20 hours ago, Racescort666 said:

I just ran the numbers for Atlas V and the Centaur has an initial TWR of 0.33:1 for a GTO payload.

Centaur is the circularization / 3rd stage in the above classification because boosters work as 1st stage and core as 2nd. It just adds last 2 km/s to get to the orbit and the flight is almost horizontal already.

[wumpus]

On 8/28/2017 at 1:55 AM, 5thHorseman said:

For a given engine, higher mass craft have a lower TWR but that's not looking at it the right way.

For a given payload mass, the lower the TWR of the ship you build to deliver it, the lower the mass of the delivery ship. In general, of course.

You are basically assuming that a high-TWR takes a heavy engine.  Typically most high TWR craft have entirely solid first stages thrust is really dependent on the geometry of the cross section of your SRB, and doesn't require a fancy, heavy, engine.

With liquid engines, adding more fuel (barely) increases delta-v at the cost of TWR, so often the TWR is pretty low as they eke the last bits of delta-v out of the liquid engines.

With SRBs, adding more fuel also increases thrust, so TWR really is limited by geometry, aero forces, and g forces on your rocket (that thing isn't light to start with, and you expect it to support how much additional weight thanks to each g-force?).

Judging from ICBM design, optimal TWR can be pretty high for suborbital designs.  Since aero losses should be more significant in orbital trajectories, I'd expect optimal TWR to orbit to be as least as high as with ICBMs.

The *huge* catch is that often liquid designs fit the goal better than SRBs (especially when you don't want to risk your spacecraft exploding.  SRBs are famous for that).  Liquid rockets almost always have other issues that make 1<TWR<2, and typically under 1.5.  I think the space shuttle was around 1.5 (and limited to around 3 or so by geometric tricks), but that was probably to limit g-forces on the SSMEs (and astronauts, but I suspect even more the SSMEs).  Come to think of it, I'm really curious why the shuttle didn't use a constant acceleration under SRBs.

[sevenperforce]

6 hours ago, wumpus said:

The *huge* catch is that often liquid designs fit the goal better than SRBs (especially when you don't want to risk your spacecraft exploding.  SRBs are famous for that).  Liquid rockets almost always have other issues that make 1<TWR<2, and typically under 1.5.  I think the space shuttle was around 1.5 (and limited to around 3 or so by geometric tricks), but that was probably to limit g-forces on the SSMEs (and astronauts, but I suspect even more the SSMEs).  Come to think of it, I'm really curious why the shuttle didn't use a constant acceleration under SRBs.

The SRBs dropped in thrust as the SRBs dropped in weight so that the stack wouldn't get going too fast too soon.

[Iskierka]

6 hours ago, wumpus said:

You are basically assuming that a high-TWR takes a heavy engine.  Typically most high TWR craft have entirely solid first stages thrust is really dependent on the geometry of the cross section of your SRB, and doesn't require a fancy, heavy, engine.

With SRBs, adding more fuel also increases thrust, so TWR really is limited by geometry, aero forces, and g forces on your rocket (that thing isn't light to start with, and you expect it to support how much additional weight thanks to each g-force?).

Judging from ICBM design, optimal TWR can be pretty high for suborbital designs.  Since aero losses should be more significant in orbital trajectories, I'd expect optimal TWR to orbit to be as least as high as with ICBMs.

"Most high TWR craft" is very few orbital rockets, however. In fact, I can only think of a few to be all-solid first stage (two to have flown, at the moment. Might be a couple more), when I could give a huge list of all-liquid launch vehicles. It's also incorrect to say that solids don't have heavy engines - as the entire container is an engine, and the entire thing must contain the pressure of the burn, solid booster casings are very heavy, are very dependent on the thrust they produce, and likely work out worse than liquid engines. The advantage of solids is they're cheap so it doesn't matter they have low performance in Isp and mass ratio, you can pack more to meet a bigger mission quite easily.

ICBM design also isn't aiming for optimal at all. It's aiming for response time, and to minimise a potential risk of early-stage intercept (which is the most vulnerable part). They go way beyond what is helpful for TWR, and have considerably more aerodynamic loss because of it - much faster in low atmosphere, and must turn earlier to deflect their trajectory away from the vertical, whereas an orbital vehicle spends significant time thrusting up to gain apoapsis hang-time for upper stages to work.

6 hours ago, wumpus said:

With liquid engines, adding more fuel (barely) increases delta-v at the cost of TWR, so often the TWR is pretty low as they eke the last bits of delta-v out of the liquid engines.

Adding more fuel significantly increases delta-V, at very minimal cost to TWR. That's the point of them being higher Isp, big changes for little cost. They go low TWR as there's nothing to be gained from more, and whether it was solid or liquid, more TWR would be more mass, and therefore a little bit more fuel and welcome to the rocket equation. Even solid payload assist motors largely only look at about 1g, tops.

There's also never been note that rocket engines have struggled with g-force. Even in high-g malfunctions they have not had failures related to loads. Liquid engines are exceptionally high performance, and notably the SSMEs, which are hydrolox fuel and therefore incur a significant mass penalty for requiring two fuel pumps, were higher TWR than the STS SRBs, at 68.5:1 versus 64.4:1. Yes, this does not account for the fuel tank mass associated with hydrolox, but it also doesn't account for getting nearly double the specific impulse once a little altitude is gained.