I was thinking about my next rocket design at school then...

[Jake SIlverman]

During my last period I was bored because had nothing to do so I watched the Falcon Heavy test launch on youtube. When I was done I was curious so I looked up its launch profile but could not find any mention of  a prograde burn which made me look at all the other rockets such as the Saturn V, Gemini Launch, Ariane 5 and still could not find the slightest hint at a circularization or prograde burn. Any ideas on why?  This is driving me insane!

Edited November 10, 2018 by Jake SIlverman

[sumghai]

[MOD - Moved to Science & Spaceflight subforums, as the thread is mainly about real-life rocketry]

[Gaarst]

What do you mean by prograde burn?

Real rockets do a single burn to orbit, there is no circularisation burn.

[cubinator]

Real life rockets go to orbit in one continuous burn, barring staging events. The most efficient trajectories are calculated beforehand, so the rocket knows exactly how to turn itself so that it reaches the desired orbit without having to coast beforehand. The main reason they don't have to do a circularization burn like we do in Kerbal Space Program is because Earth is much bigger than Kerbin. If you want to do a single burn to reach orbit around Kerbin, you have to turn very fast to get horizontal before reaching orbital speeds. Kerbin has an atmospheric height similar to that of Earth's, but it's orbital velocity is only about 2300 m/s whereas Earth's orbital velocity is over 6000 m/s. Additionally, Kerbin's radius is 600 km and Earth's radius is about 6000 km. That means when you're getting into orbit around Earth, you spend a lot more time burning horizontally when already in space to get enough velocity to clear the curve of the planet, which is very shallow. On Kerbin, the time spent in the atmosphere is a much bigger part of getting into orbit, and you end up much higher relative to Kerbin's curve before you've turned over completely. This usually puts you on a trajectory where you need to wait a while before you reach a place where you can circularize your orbit. When launching into Earth's orbit, you can spend a lot of time keeping horizontal during your initial burn and keeping your apoapsis close - allowing you to circularize your orbit without having to wait for a second burn.

You can better understand this effect if you try launching a rocket in RSS or a 10x scale KSP system. You will notice that you spend much more time circularizing, and that you can do that during your initial burn much easier than on a small planet like Kerbin.

Edited November 10, 2018 by cubinator

[Mad Rocket Scientist]

In real life, orbital velocity is much higher than in KSP, so the circularization time is much higher, and it sort of spills over into the ascent.

@cubinator, while I'm pretty sure some rockets in real life do use precomputed trajectories, many use Powered Explicit Guidance Ascent System (PEGAS) or a variant thereof. PEGAS can accept any target orbit and compute an optimal trajectory to reach it, and can correct for errors in-flight. Someone made a version of it in KSP here if you're interested in more details:

EDIT:

See also: The space shuttle PEGAS document:

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19740004402.pdf

Edited November 10, 2018 by Mad Rocket Scientist

[wumpus]

10 hours ago, Gaarst said:

What do you mean by prograde burn?

Real rockets do a single burn to orbit, there is no circularisation burn.

Note this is only true for LEO (which was most of the launches OP asked about). geosynchronous orbits definitely require a "coast" then re-burn, and a lot of the appeal (to customers) of the falcon heavy was the ability to re-light the engines for  geosynchronous circularization.  Geostationary orbits are even more complicated, as they typically do an inclination change beyond GTI and then  require at least one more burn to get into position (ESA can launch from the Equator and ignore this).

[magnemoe]

17 minutes ago, wumpus said:

Note this is only true for LEO (which was most of the launches OP asked about). geosynchronous orbits definitely require a "coast" then re-burn, and a lot of the appeal (to customers) of the falcon heavy was the ability to re-light the engines for  geosynchronous circularization.  Geostationary orbits are even more complicated, as they typically do an inclination change beyond GTI and then  require at least one more burn to get into position (ESA can launch from the Equator and ignore this).

True, however for most GEO and similar satellites the satellite do the circulation with their own engine and inboard fuel and the rocket only does an GTO or geosynchronous transfer orbit trajectory. 
Again its exceptions the falcon 9 Iridium missions do an circulation burn and then release the satellites. 

Also as other pointed out you need an far longer burn to get into orbit in real world, real world upper stages also tend to have pretty low TWR. I have done burns who don't require circulation burn in KSP with low TWR bases or interplanetary ships, here the base or ship is the upper stage. The tricks here is to not burn directly pro-grade but aim a bit above the horizon and walk Ap ahead of you, in the start you have to trust say 20 degree up to not fall down into the atmosphere but as you go faster you start aiming more and more prograde. 

Some launches even put the initial Ap from boosters and first stage higher than the orbit to get away with lower TWR on upper stage. 
On Starliner they have to use two engines for centaur so they can avoid this as the steep trajectory would give too high g force on reentry on abort. 
https://www.youtube.com/watch?v=OislF7OG_BI
I assume manned falcon 9 missions will use badge landing even if not needed because of payload since they can fly an flatter trajectory.