KSP Launches vs IRL Launches

[Olvirm]

How close is the typical launch profile in KSP that I have seen (grav turn at 10km, keep pitching over, burn until desired apoapsis, coast till apoapsis, circularization burn) to real life ones? From youtube videos it seems that the gravity turn starts very early, and they seem to reach their desired orbit without any coasting (unless the coasting and circularization is just cut out). I was just wondering if anyone could describe to me the typical launch profile of a real life rocket.

[soranno]

I`ll post before the people who know what they`re talking about get here, and see how I measure up

ksp`s atmospheric and aerodynamic properties call for the launch profile similar to that you have described, as it is to our benefit to get up out of the thickest atmosphere as fast as possible, and then begin accelerating to orbital speeds.

Real rockets launched make their turns earlier, and pitch down as they are aerodyamic in shape, and accelerate best when pitched over, gaining lift in a way not possible in stock ksp without attaching wings or other lifting surfaces.

To do so early in ksp just makes your job of orbiting harder as you don`t gain any benefit from an aerodynamic shape or lift and instead only have to plough through more thicker atmosphere

[doggie015]

Because earth is larger than Kerbin the high density part of the atmosphere is not as thick as there is more surface for it to spread over. Because the orbital velocity for Earth is much higher than Kerbins they need to start putting on horizontal velocity as soon as possible in order to save fuel (Kerbin orbital velocity is about 2.3 km/s and Earth orbital velocity is about 7.8 km/s!)

[Nibb31]

It makes sense to start the gravity turn as soon as you launch. Any energy spent on achieving vertical speed is wasted when the goal is to achieve horizontal speed.

The whole "go straight up and start your gravity turn at 15000m" nonsense is only because it's hard to gradually control the pitch rotation manually and it's easier to just switch on the ASAS to go straight up. If you control you flight with a proper flight computer as in RL (or MechJeb), then it's better to start rotating as soon as you're off the pad in order to get the most curved ascent profile you can.

In addition, real-life launch profiles get to orbit in one (staged) burn. They don't usually coast to apoapsis and circularize during the ascent like we do. This is because they can afford to calculate thrust and weight in order to do so, and also because most real rocket engines can't be stopped and restarted and are very limited in throttle. They also don't care about having perfectly circular orbits like we do, as long as the orbit doesn't decay and the apoapsis ends up where and when they want it. If they need to circularize, they will do so after reaching orbit.

Edited July 2, 2013 by Nibb31

[Canopus]

You can do the whole thing in one Burn with little thrust.

[Ralathon]

A gravity turn isn't "Fly up straight and flip 45 at X kilometers". It means you give a tiny nudge at the very beginning and the rocket slowly topples over throughout the launch. You can optimize that push to make sure you never have to coast and you just fall into orbit. It saves a lot of fuel to do this since you need less course corrections. You can do this in KSP as well to save some dV, it's just a lot more difficult.

Also, as others have stated the earth is a lot bigger than Kerbin while the atmospheric height is pretty much the same. Relatively speaking RL rockets need a lot less vertical and a lot more horizontal speed. Our 100km orbits are equivalent to 1Mm orbits in real life.

[The Viking On Mars]

Would it not make sense for them to turn earlier irl to prevent separated stages from hitting things like people and buildings, which we need not worry about in ksp?

[sgt_flyer]

i'll let you discover the similarities between this IRL launch and KSP ones

(it was a proton M which was launched today from baïkonour to put into orbit 3 glonass satellites )

[doggie015]

That was just MechJeb fighting with the ASAS over which way to go!

[razark]

Real rockets also pitch over at the start of flight to get them away from people/land in case they decide to "go Kerbal".

[KerbMav]

Only yesterday - while thinking about space planes - I thought of attaching some wings along two sides (facing north and south) of a rocket and begin an early turn - thereby creating lift upwards an starting to built up horizontal velocity.

Now, if this even makes sense, when exactly would I want to start tilting? Early means more thick atmosphere/lower terminal velocity - later means less lift due to thinner atmosphere.

And how to attach the wings to not starting rotating or tilting right away in the other direction? Maybe detachable wings on the other two (east and west) sides to be jettisoned on the planned tilt?

[Iren]

i'll let you discover the similarities between this IRL launch and KSP ones

(it was a proton M which was launched today from baïkonour to put into orbit 3 glonass satellites )

So Jeb was russian after all :/

[Eric S]

Part of the difference between KSP and RL launches comes from a difference in aerodynamics. Some rockets tend to be overly stable, wanting to stay on a prograde heading, so turning them 45 degrees in one shot just isn't possible, they lack the command authority to do it. Other rockets tend to be less stable, wanting to flip around if they get more than a few degrees off of prograde, at which point trying to do a 45 degree turn in one shot is just bad planning.

RL gravity turns are, as pointed out above, a small tilt at the start of the launch, and then just following prograde as the gravity turn gradually shifts the prograde from "up" to "sideways" which avoids both of these issues. If you've launched rockets using the FAR mod, you've probably seen both of these issues in action.

As for why they don't have a coasting phase, this may have to do with the fact that they need three times the velocity to achieve orbit, or it could just be because they're better at planning their gravity turns. I've generally found that if I find the right aggressiveness in my gravity turn, then I don't pick up as much vertical speed during my launch, so my horizontal speed is already approaching orbital velocity by the time I reach my desired orbital height.

[espm400]

That was just MechJeb fighting with the ASAS over which way to go!

Naw, they just started using FAR and over-cooked the turn a bit.

[Markotiks]

That Proton M launch was so Kerbal! Who's imitating who?!?

[Bunsen]

Another reason for the KSP's unusually vertical launch profiles is that Kerbal rockets are aerodynamic crap compared to Earth rockets (which sorta makes sense when you look at the sizes), so they suffer more severe drag losses low in the atmosphere. That makes it both hard and wasteful to pick up much horizontal velocity early in the flight.

[Olvirm]

So if I was to use FAR and aerodynamic rockets I should follow a more real life launch profile?

[longhornchris]

There are several key differences that change some of the details of the ascent profile:

1. Earth a lot bigger with more surface gravity
   
2. Earth has a much denser & taller atmosphere
   
3. Earth engines have much, much higher thrust to weight ratios - hence no asparagus staging
   
4. 
   

This is a major reason why Earth rockets go straight up, or nearly straight up then pitch over. For US launches there is usually a roll (for vehicles that have a 'wide' dimension) followed by a pitch to get out over the water quickly. The full pitch over doesn't occur till much higher to minimize atmospheric drag and the associated thermal and aerodynamic loads on the fairing.

Now, I don't expect KSP to implement all of this - but with FAR and deadly re-entry the impacts on the fairing are modeled, albeit not perfectly.

[rpayne88]

Pardon my French, but where the hell was the RSO? Or do the Russians just not care due to Boikonour's remotness

[rdfox]

For the record, all US launches since at least the start of the Saturn series have included a roll program, regardless of whether the vehicle is bilaterally symmetrical or not; it allows you to simplify the control logic for the gravity turn when launching to any chosen azimuth, and even for vehicles that are expected to only launch due east, it's advantageous because it allows you to keep the simple pitch-only gravity turn while orienting the launch pad however is most convenient for ground operations instead of forcing you to orient it so the rocket has the "this side down" markings facing east. (Atlas and Titan, as used in Mercury and Gemini, didn't use a roll program--I'm not certain they even had roll control--and thus the launch sites had to be "aimed" in the direction they were to be launched--there was a small crossrange capability, but not much, maybe a couple of degrees. This was because they were done to quickly get reliable ballistic missiles into service; later ICBMs had the same roll control as purpose-designed space launchers, because it meant that the orientation of the launch site wouldn't restrict targeting or tell the enemy what the missile's target was.)

The reason for direct-insertion orbital launches is, as said, most rockets are not equipped with restart capability, to save on complexity. However, if your upper stage is hypergolic, then you'll almost certainly *not* use the direct-insertion trajectory, as it's less efficient than a boost-coast-circularize trajectory, and with hypergols, you just need to be able to keep the tanks pressurized, apply ullage thrust (usually with tiny solids or RCS jets), and reopen the propellant valves to restart the engine. Indeed, the Space Shuttle used exactly that technique to reach orbit. (Originally, it actually launched on a half-rev suborbital trajectory, then did an injection burn of the OMS engines to raise apogee to the target altitude, then did a circularization burn, to make certain that the External Tank would come down where it was targeted. However, later missions, after they had a better idea of the ET's entry characteristics, launched directly to the target apogee, then did a single, slightly-longer circularization burn of the OMS engines, to get a bit more payload capacity, since the SSMEs had a much better specific impulse than the OMS engines.)

As for the RSO, apparently the Russians don't put a destruct system on their boosters; their range safety package is merely an engine shutdown command so that it will fall in their designated impact zone--but even that is inhibited until T+42 seconds, to try and make sure that the booster will not fall back onto the pad and thus require massive expense in rebuilding the launch site infrastructure. (As a side note, the shutdown command is incorporated into US range safety packages, too; arming the range safety package sends a shutdown command, and if that's enough to ensure that the vehicle will impact in the safe impact zone, the RSO may choose not to send the destruct command and instead let the vehicle fall intact, should it have something on board that would be more hazardous to have dispersed than allowed to impact intact.)

[Motokid600]

After you clear the tower in KSP wouldn't it make sense to pitch east like 5 degrees? If that.. Simply so Kerbin doesn't rotate under you. I mean if you go straight up to 10km wouldn't the assisted dV from Kerbins rotation degrade?

[Kimberly]

After you clear the tower in KSP wouldn't it make sense to pitch east like 5 degrees? If that.. Simply so Kerbin doesn't rotate under you. I mean if you go straight up to 10km wouldn't the assisted dV from Kerbins rotation degrade?

While this is true, you also want to get out of the lower atmosphere quickly, due to drag. I'm sure you could make a fancy calculation as to what the break-even point is, where it becomes better to make use of rotation than to reduce drag.

[Motokid600]

While this is true, you also want to get out of the lower atmosphere quickly, due to drag. I'm sure you could make a fancy calculation as to what the break-even point is, where it becomes better to make use of rotation than to reduce drag.

I imagine 3 - 5 degrees of pitch would be enoug to keep up with Kerbins rotation. With such a shallow pitch you'd have to travel through maybe... A few extra hundred meters of thick atmosphere? I'm gonna try experimenting with this.

[BFGfreak]

Or you can download FAR and curse at how you have to learn to fly all over again.

[Bunsen]

After you clear the tower in KSP wouldn't it make sense to pitch east like 5 degrees? If that.. Simply so Kerbin doesn't rotate under you. I mean if you go straight up to 10km wouldn't the assisted dV from Kerbins rotation degrade?

If your velocity measurement is in "surface" mode and your prograde velocity indicator is dead at the top of the navball, then you're co-rotating with the planet. Pointing straight up during launch keeps you pretty close to that. The atmosphere is also rotating with the planet, so you don't lose any of that free delta-v by minimizing drag. And on a vertical launch, I think the aerodynamics keep you nearly co-rotating with the atmosphere, but the Coriolis-induced lag at 10 km altitude would only be something like 3 m/s so it's almost impossible to notice.

The problem with immediately pitching over is that you spend a lot of the early flight just fighting gravity and drag, so you can accumulate horizontal velocity pretty quickly while your vertical velocity doesn't change much. Sometimes that ends with struggling to keep the rocket's nose pointed high enough against the horizontal relative wind. That's less of an issue if you maintain a good TWR and don't overdo the aerodynamic stability.

A perfect launch trajectory would include some immediate pitch, but the ideal angle is so tiny it's very hard to get it right by hand. Going straight up a few kilometers until the correct pitch angle is big enough to see costs almost nothing in delta-v, so most of us do it the slightly dumber and much easier way (though personally I find 10 km to be a bit much, 5 or 6 km is usually workable).

(Atlas and Titan, as used in Mercury and Gemini, didn't use a roll program--I'm not certain they even had roll control--and thus the launch sites had to be "aimed" in the direction they were to be launched...

Almost all guided rockets have roll control; even if there are no roll maneuvers it's necessary to prevent small aerodynamic or thrust asymmetries from inducing spin (just look at what happens in KSP with side mounted boosters and not enough struts). Atlas had a pair of little (heh, "little" means only a thousand pounds of thrust each) vernier engines with wide-angle gimbals, plus gimbaling of the outboard engine nozzles while they were still attached, and Titan had 2-axis gimbal control of each of its nozzles.