Do simulator games use a realistic trajectory to get into orbit?

In videogames (such as KSP or Spaceflight simulator) people put rockets in orbit with the following procedure:

Turn on the engines and move vertically for a bit.

Start a roll program to turn the vertical trajectory into a ballistic one.

Keep going until the Apoapsis of the trajectory reaches the desired height.

Turn off the engine and wait until the rocket (almost) reaches Apoapsis.

Burn prograde to increase periapsis until orbit is achieved.

I understand the reasons behind each of this manouveurs, however I’m wondering if this is how real rockets get into orbit.

Cutting off the engine and letting the rocket loose vertical speed looks counter-intuitive to me (you basically spend a lot of fuel to accelerate and then you let the rocket slow down).

So my question is: how do real rockets go in orbit? Is there somewhere an explanation of the manouveurs and trajectory the they follow to get in orbit?

6 Answers 6

I understand the reasons behind each of this manouveurs, however I’m wondering if this is how real rockets get into orbit.

Cutting off the engine and letting the rocket loose vertical speed looks counter-intuitive to me (you basically spend a lot of fuel to accelerate and then you let the rocket slow down).

In most real launches to low Earth orbit, the burn continues from liftoff until orbital insertion, without a coasting phase. Some (like Antares) do coast between the first stage and second stage burn; the exact design of the launcher determines which approach is more efficient.

In flights to higher orbits, for example geosynchronous orbit, a coast phase is always used; this approximates a Hohmann transfer orbit.

In KSP, neither the trajectory nor the rocket design, typically, is optimal, and the planet is much smaller than Earth, so the tradeoffs are different.

Physics

In regards to the physics, KSP is fairly realistic, other than it not modeling n-body physics (which isn’t really relevant in scope of orbiting Earth/Kerbin).

In regards to the engineering, KSP makes its parts much stronger than the real life components. The tradeoff here is that when something finally breaks in KSP, it explodes and disappears. Engines are also more powerful, and fuel is (slighly) less heavy. This is all done to “round the numbers” in favor of the player to make sure that the limited part set can get most reasonable jobs done.

In regards to the controls, KSP’s control system is not as fine-grained as a real world spacecraft. Given a keyboard and mouse and moslty eyeballing the thrust gauge, there is only so much precision you can reasonably expect from a player.

In regards to the ascent profile, due to the stronger craft and less fine-grained controls, KSP spacecraft are able to ascend using sharp(er) turns than is realistically possible in real life. In KSP, a simple ascent profile is to go straight up for about 10km, and then make a 45° turn and start building horizontal speed.

In real life, a rocket won’t be as easy to turn (e.g. a 45° turn) due to the stress that puts on the vehicle; most importantly it’s the incoming wind from still moving straight upwards (due to inertia) that is going to start beating down on the side hull (which you expose by orienting your craft away from its prograde vector).
To avoid that, the best approach is to orient your spacecraft as close to its prograde vector as you can, because that minimizes exposure of the less aerodynamic side hull compared to the aerodynamic nose.

This means that in reality, spacecraft tend to make their 90° (from vertical to horizontal) turn slow and steady (i.e. pretty much immediately after launch). And this is actually not that hard to do: even a minor (but consistent) deviation over a long period can do most of the work for you and doesn’t require active steering (other than necessary corrections due to imperfections in the initial setup).

Gameplay

In KSP’s defense, there is a boundary of realism that you should avoid when it negatively impacts gameplay. The level of precision required (for both thrust and attitude adjustment) to achieve a “real” ascent profile is quite high, requires a lot of calculation, and any imperfection in the simulation would render any calculation impossible to get accurate anyway. So rather than risking the player’s calculated approach being useless after all that effort, KSP opted to simplify things slightly to keep things much more intuitive.