"Best" TWR Values>

[Spricigo]

26 minutes ago, Starchaser said:

I'm sorry, but I had to take exception to this. I know you said 'most' real world rockets, but with 135 total launches of the 6 orbital shuttles, I think that's a fairly large slice of the total rocketry pie.

For this discussion not really relevant,  the STS throttled down to avoid being thorn to pieces, not because fuel efficiency concerns.

[regex]

1 hour ago, Starchaser said:

I'm sorry, but I had to take exception to this.

Why? It's technically accurate. Most real world rockets do not throttle. There are notable exceptions such as Angara-5 and Atlas V, but you'll note in all those cases that they throttle up after launch, which is within the context of my earlier statement about going "full blast". Most throttling in KSP, from what I've seen around here, involves throttling engines down.

E: Also, the Shuttle isn't really a "rocket", it's a spaceplane.

Edited October 29, 2017 by regex

[wumpus]

On 10/28/2017 at 11:36 PM, Spricigo said:

For this discussion not really relevant,  the STS throttled down to avoid being thorn to pieces, not because fuel efficiency concerns.

There's also the issue of SRBs vs. Hydrolox.  While the shuttle SRB Isp was wildly better than what you might expect from playing KSP, it still wasn't anything like hydrolox.  So even if they were throttling to reduce maxQ, they were also saving the high Isp fuels for after staging the SRBs.  Falcon Heavy doesn't have this issue at all, and presumably is also avoiding maxQ issues by (largely) saving the center stage until staging the side boosters (I have no idea what order Delta-Heavy lights the boosters).  Without the atmosphere, it would make no sense to save the center booster at all.

[OhioBob]

8 hours ago, wumpus said:

(I have no idea what order Delta-Heavy lights the boosters)

The Delta IV-H burns all three boosters at 100% thrust at liftoff, throttle back to 60% at 56 seconds, throttle up to 100% at about 252-257 s, cut-off at 327 s, and separation at 332 s.
 

[Reusables]

I'm on the opposite side of @Helmetman.

It's important for recoverable TSTO to reduce fuel, both for mass and cost. As having high TWR reduces the req dv by hundreds of m/s (around 300, depends on engine type), it's usually better to go for high TWR (Under 2).

I usually go for around 1.8~2 because in the range, engine mass is relatively low compared to the saved fuel mass and engine cost is effectively 0.

[OhioBob]

2 hours ago, Reusables said:

I'm on the opposite side of @Helmetman.

It's important for recoverable TSTO to reduce fuel, both for mass and cost. As having high TWR reduces the req dv by hundreds of m/s (around 300, depends on engine type), it's usually better to go for high TWR (Under 2).

I usually go for around 1.8~2 because in the range, engine mass is relatively low compared to the saved fuel mass and engine cost is effectively 0.

But fuel is cheap, engines are expensive.  If we can use a smaller engine, we've usually saved more than enough money to buy the fuel needed to pick up the extra ~300 m/s.  When using liquid fueled stages, it's typically cheaper to use low TWR.

[Reusables]

Just now, OhioBob said:

But fuel is cheap, engines are expensive.  If we can use a smaller engine, we've usually saved more than enough money to buy the fuel needed to pick up the extra ~300 m/s.  When using liquid fueled stages, it's typically cheaper to use low TWR.

Thus 'recoverable TSTO', as engines effectively cost zero with it. (As it's going to be recovered)

With disposable lifters I go for lower TWR - like 1.5, and use SRBs for cheap and high-thrust engines. Especially those kickbacks.

[Jim DiGriz]

The stock kerbal answer is that you want as much TWR as possible and you want to pitch over as much as possible.

The "perfect" launch profile neglecting the atmosphere and the surface of Kerbin would be roughly horizontal and would be instantaneous.  Since we can't do instantaneous burns a finite burn would result in falling through the surface of Kerbin.  So we have to pitch up.  Now the problem is that if you are at too shallow of an angle the aerodynamic forces drag you in a shallow gravity turn and the result is that you never escape from the surface (mostly you blow up from heating because you start trying to do orbital speeds at 20km alt or so and the atmosphere won't let you pitch up).  So you need to pitch up to the point where you actually make orbit, and then apply as much TWR as you can.

I added an alternative ascent profile to MechJeb to help people play around with this (the GravityTurn™ style of ascent).  To use it, build a rocket with a fairly stupid-high TWR (2.5 or so), some SAS, control, and some decent sized fins on a 2.5m body.  Then in MechJeb make sure to turn off AoA limiting so you get a pretty violent initial pitch over and then pitch over initially with the highest angle that allows you to still make orbit.  Something like 45 degrees.

You should be able to reach orbit in 28xx-ish m/s of dV.

Why are real rockets not this silly?

- For the payload you deliver the engine is huge and the fuel is immense.  The delta-V budget does not necessarily translate to $$$ budget.  What you want to do is get that payload to orbit with the least amount of fuel and cheapest engines, not least delta-V expended.  If you have to build a rocket 10 times as large as me to shave 50 m/s off your dV to orbit, then I'm the one who actually has the more economically viable rocket, not you.

- Real rockets are considerably less sturdy than Kerbal rockets.  Even in Realism Overhaul if Kerbal rockets were as flimsy and brittle as as much of a "wet noodle" as real rockets, we'd have a lot more failed launches.  You'd have to worry a lot more about sideslip and AoA and stress loading along the rocket at Max Q.

- Real rockets carry humans.  If you have a 2.5 TWR rocket, you're going to have max g-loads that would kill humans.  The shuttle throttled down and flew with a constant thrust acceleration (instead of constant thrust force) profile to not make the pilots go unconscious.

So all of that limits the TWR, and limits real rockets to flying closer to a zero-lift/AoA/sideslip gravity turn.

And note that drag forces are pretty minimal.  I've got a rocket I'm testing right now (Realism Overhaul tho) that the stupid autostaging blows the fairing off on the surface and it just costs about 40m/s out of 150 m/s of overall drag loss (in RSS where its 8900-9600 to orbit) by pushing the flat side of a brick, essentially, into orbit.

It used to be the case that you wanted to not exceed terminal velocity but that was back in the old pre-1.0 atmospheric soup.  The change from 4550 dV to LKO to 3300 dV to LKO going to 1.0 was from drag losses mostly going away.

Real rockets exceed terminal velocity (and Mach 2.0 even) fairly low in the atmosphere.  This creates a problem (accurately) in Realism Overhaul where if you have a short burning lower booster and you stage fairly low in the atmosphere to a sustainer the drag force will actually decellerate your rocket so that propellant in the tanks of your upper stage will slosh forwards and your upper stage will not light because it will not be ullaged.  The negative Gs may also be too large to be overcome by ullaging with RCS jets.  You can either hot stage or use ullaging motors with a sufficiently high enough TWR to overcome it.

 

[GoSlash27]

My experience has been right in line with Ohio Bob's suggestions.

The advantage of choosing a lower T/W is in mass and cost savings, not pure efficient attainment of orbit with the least possible expenditure of DV.
 Some other reasons to throttle back during the launch:
-Controllability issues around Mach 1. All the drag inherent in going too fast too low can manifest itself in a rocket that doesn't want to turn or doesn't want to fly straight. Keeping your t/w under 2 during the steeper portion of the flight can make a rocket easier to fly.
-Heating issues at supersonic speeds. If you keep the hammer down for too long while in the atmosphere, you *may* overheat to the point where parts start blowing up.
-Precision/ repeatability of launch trajectory. It's a whole lot easier to place your payload into a specific orbit when you slow the process down a bit. You find that you can follow a narrower launch corridor and expend the exact same DV every time. This helps with planning purposes, both during mission planning and vehicle design.

So I generally follow these guidelines when designing stages:
SRB booster: 1.2 initial t/w, 1,800 m/sec DV
LF&O booster: 1.4 initial t/w, 1,700 m/sec DV
Second (transstage): 0.7 initial t/w, 1,600-1,800 m/sec DV (larger diameter lifters have lower drag per mass)
Orbital and interplanetary ops: 0.5 initial t/w.

 During launch, I climb vertically to around 80 m/sec V, then initiate a 5° pitchover and follow prograde. LF&O engines are throttled back to maintain acceleration IAW the formula 2sin(pitch).
Pitch/acceleration
90/ 2
75/ 1.9
60/ 1.7
45/ 1.4
30/ 1.0

This is generally where my first stage conks out, around 800 m/sec, 30 km altitude, and pitch= 30°. Conveniently, this is where aerodynamic control no longer matters, so any control surfaces or streamlining stuff can be ditched here.

Second stage then kicks in, and I can firewall it at 0.7g. I pitch and throttle as necessary to keep my Ap 45 seconds ahead, and place it at the orbital altitude. Once it starts running away from me, I shut down, ditch the second stage if there's no further use for it, coast to about 5 seconds short of the Ap, then circularize.

 SRB first stages are cheaper, but also a bit more cantankerous, since they can't be throttled. A booster that starts out at 1.2g on the pad can be cooking over 3.5g when it finally burns out, so you may have to force the turn early and ride it through the heating.
 The acceleration can be artificially "throttled" by clustering SRBs and staggering their throttles, so they burn out at different times.

Best,
-Slashy

 

[GoSlash27]

On 10/21/2017 at 9:48 AM, wumpus said:

I was under the impression that they were pretty much stuck to ~1.2TWR, but one of Slashy's better attempts appears to be at 1.68 for KSP version 1.0.5.

Wumpus,
 It's been a little while since I designed that one, and I'm a bit hazy on the details... But IIRC the initial t/w was the result of synchronizing the fuel expenditure of the core LF&O engine with the burnout of the SRB boosters. That way, the quasi- asparagus tanks would run empty right at the moment the SRBs burned out.
 The 1.7 t/w at liftoff was a happy accident, not anything I actually aimed for.
 Best,
-Slashy

*EDIT* I forgot to mention up-stream: If the LF&O engine I have selected for cost- effectiveness happens to be more powerful than these figures, then I won't turn them down to match. They are considered minimums. If my engine wants to pull 2g off the pad, I'll let it.

Edited November 12, 2017 by GoSlash27