How does throttle work out in real life?

[Pizza on Pineapple]

I've found that if I need to be really efficient (IE gain height), I don't actually full-throttle it at liftoff so I can get some height and then I increase the throttle when I've esacped some of the atmosphere.

So in real life, how do they work around the fact they are in the thick of it? Do they limit the thrust?

[dnrob7]

It's all pre-calculated a 1000 times over. They know exactly what speed and angle they want to be going at any given point. The boosters are designed around the weight of the payload, the pilot does nothing.

[tourist]

from the Falcon9Heavy:

Just like the Falcon 9, FH provides engine-out capability for a large portion of its first stage flight. All 27 engines are ignited on the ground, about three seconds before launch. All must reach operational conditions and liftoff thrust for the launch release command to be issued.

The engines are monitored constantly in flight and computers can shut down any engine at any time to prevent RUD (rapid unplanned disassembly). Following the unplanned shutdown of an engine, the flight computer would re-plan the ascent trajectory to reach the cutoff target with the remaining engines by extending their burn and potentially cutting a booster return, forgoing re-usability to ensure success of the primary mission.

The two boosters would burn for roughly 195 seconds before separating from the core to begin their journey back to the launch site. Continuing powered ascent, the core would throttle up its engines and burn for just over a minute to continue boosting the velocity of the stack, achieving a much higher speed than any previous Falcon cores which makes its recovery more difficult given its much greater energy at separation.
http://spaceflight101.com/spacerockets/falcon-heavy/

Flightcomputer has routines and guidance, it sort of works the same way. The player is given much more room to error. There is MechJeb and at least one mod for chances of parts failing.

[ZeratulR]

One of the main principles of the efficient ascent is to keep your speed as close to the terminal velocity as possible (in KSP; in the real world it's way more complicated, at least I think so) or, in other words, keep your atmosphere efficiency as close to 100% as possible. At least it's true for the first 15-20 km of Kerbin's atmosphere. The main reason for this is the fact that the longer you stay close to the surface, the more speed you lose because gravity is stronger when you are closer to the planet's centre of mass. There are situations when you want to decrease throttle because you are already over (or close to it) your craft's terminal velocity but in practice, a situation like that means that your craft has too many engines (or too many powerful engines).

As a rule of thumb when I get to the rockets with good aerodynamics characteristics (*cough* fairings *cough*) I try to have TWR of at least 2.0 (2.2-2.3 or even slightly more is better) in the first stages of my rockets. With them, I usually go full-throttle for the above-mentioned first 15-20 km of ascent and I think it's pretty efficient way to launch.

As for the real life, I wouldn't be surprised if the things like that were guarded commercial (and sometimes military) secrets.

[tourist]

NASA is public and the commericial ventures are both contracted by the military for the vast majority of hardware, if not all, and also the commercial ventures are required to be insured. The insurance companies want risk assessment and a demonstrated reliability.

The secret payloads, secret payloads of child slaves for Mars are what they don't want you knowing about.

[CptAWatts]

Solid boosters are the same as in KSP. They just blast at max until they are done. As far as engines with controllable thrust they have computers that calculate the most efficient burn as well as the angle. Just like we sometimes do in KSP they also run it through a simulated enviroment so they get a rough idea of how the ship will perform.

[Langkard]

In an ideal setting (insert spherical cow joke here) with an infinitely tall atmosphere of uniform density, a TWR of 2.0 would put a rocket at terminal velocity when going straight up. Kerbin (and the real world) are not that ideal situation, however, so your rocket needs to catch up to terminal velocity, which is always increasing as the atmosphere thins. For this reason, the most fuel-efficient rockets thrust as much as possible initially (or use SRBs) then throttle back some as the atmosphere thins to approximate approaching terminal velocity. Almost impossible to achieve staying at terminal velocity from launch to orbit, though. The TWR needed to approach terminal velocity decreases also through a gravity turn as the flight path lengthens the time in-atmosphere.

There was a great post on the KSP forums explaining it some years ago...

Ah, here:
http://forum.kerbalspaceprogram.com/index.php?/topic/73033-twr-and-terminal-velocity/

[Knarrwinkel]

Originally posted by Langkard:

Ah, here:
http://forum.kerbalspaceprogram.com/index.php?/topic/73033-twr-and-terminal-velocity/

That's from 2014, when the atmosphere was a placeholder made out of pea soup and when drag was based on mass and was directly applied to the CoM of the craft. You can safely disregard anything that was said in there.

[Mightylink]

Its pretty much auto pilot unless something goes wrong.

[Langkard]

Originally posted by Knarrwinkel:

Originally posted by Langkard:

Ah, here:
http://forum.kerbalspaceprogram.com/index.php?/topic/73033-twr-and-terminal-velocity/

That's from 2014, when the atmosphere was a placeholder made out of pea soup and when drag was based on mass and was directly applied to the CoM of the craft. You can safely disregard anything that was said in there.

The post applies to KSP AND the Real World. KSP has since made the atmosphere more like the RW. The post applies even more now.

We're talking about fuel efficiency here. You can get a rocket into orbit in KSP with a TWR of 1.5 or above 3.0. The most efficient way to do it is with a proper gravity turn and the most fuel efficient velocity. It is impractical to attempt staying on the terminal velocity curve, but it can be approximated.

Originally posted by RoofCatA:

...
Ignore "terminal velocity" bs in KSP, as there is no terminal velocity for rocket launch. Read wiki for terminal velocity in case you are curious about it. Short - it is about free falling only and in a strict vertical direction.

This is incorrect. You are confusing the terminal velocity for a falling object with rocket terminal velocity. They result from the same physical conditions, but ARE NOT THE SAME.

Please see the KSP wiki. http://wiki.kerbalspaceprogram.com/wiki/Atmosphere#Terminal_velocity

Terminal velocity is important because:

1. It describes the amount of velocity which a spacecraft must burn away when it is close to the ground.

2. It represents the speed at which a ship should be traveling upward during a fuel-optimal ascent.

In KSP, rocket terminal velocity varies with altitude and with the planet. There is even a chart on the wiki showing the terminal velocities for various altitudes on some of KSP planets.

[Langkard]

I'll try once more, since you insist on not understanding. Just because the term "terminal velocity" is usually applied to falling object does not mean that it ONLY applies so.

What is the equation for terminal velocity?

TV = sqrt(2*mass*acceleration / (area * air density * drag coefficient))

In the special case of a falling object not under thrust, the acceleration in the formula is simply the acceleration due to gravity. This is the most common definition and the one you and Wikipedia are using; but it is a special case, not the only case. The equation also applies to objects moving in other directions.

In the case of a rocket thrusting straight up, the acceleration in the formula is the difference between the vectors of the rocket's engine acceleration up and gravity acceleration down. Moving upwards at an angle is more complicated because not all of the acceleration (or in the case of upward travel, deceleration) of gravity is applied directly against the thrust of the rocket. For a rocket moving at a continually changing angle (such as a gravity turn) the calculations become even more complicated, because the acceleration of the rocket isn't applied entirely to the direction of travel.

Terminal velocity, as used in KSP, is simply the term used for the speed of an ascending rocket at which it will no longer accelerate because of the effects of atmospheric drag and gravity. It is called that because the terminal velocity equation is used. It confuses people (like you) who were taught that it only applies to falling objects. It might be more properly called terminal acceleration perhaps to avoid confusing some people.

Now, go read an aerodynamics book and a book on physics and not just Wikipedia and maybe we can discuss further.

[Toastie Buns]

Oh hey it's the old Terminal Velocity joke.

TUNE IN FOR THE THRILLING CONCLUSION OF PART...Uh...Twenty?

[Langkard]

Toastie Buns! We're trying to have a childish discussion here. This discussion comes up about once every 6 months or so. Think of it as homecoming.

[Manwith Noname]

Is Charlie Sheen invited?

[Pizza on Pineapple]

Originally posted by Langkard:

<Snip>

So why do the rockets throttle back mid-flight? As I see it if they are "chasing" an equilbrium point there is still scope to maximise speed, surely?

Or is it a case of "the most efficient" form is to stay a "little bit" below (whatever that means) the equilbrium point as any more use of fuel has succeedingly poorer returns?

[Manwith Noname]

You can have losses due to drag but obviously, the higher you go, the less of a problem that becomes. Also, as you get closer to orbit, the craft will be, lighter, so TWR increase along with less atmospheric drag.

I've yet to manually launch a rocket in KSP whereby I can burn full throttle all the way and finish in a circular 80-100km orbit.