How to orbit without turning off engines

[Nendra]

Hello Guys, just wanna ask. In real life we usually see rocket which did not turn off the engine to orbit. I mean, how to orbit the rocket without turning off the engines and wait until it reaches the apoapsis. Can we orbit the rocket by still turning on the engines? For example, the Apollo mission, it uses it third stage to orbit, and did not turn the stage off. The Soyuz, it orbits using the third stage without turning off the engines. So how to orbit a rocket without turning off the engine on the stage? Thank you.

[James Kerman]

Welcome to the forum @Nendra.

The stock KSP system is basically a toy solar system so it takes a lot less energy to get to orbit. One thing you could do is use weaker engines (or thrust limit them) or you could try some of the rescale mods to increase the requirements.

[Nendra]

Ok, so by using Thrust limiter on reaching orbit, we can orbit the spacecraft without turning off the engine right? 

[Nendra]

6 minutes ago, James Kerman said:

Welcome to the forum @Nendra.

The stock KSP system is basically a toy solar system so it takes a lot less energy to get to orbit. One thing you could do is use weaker engines (or thrust limit them) or you could try some of the rescale mods to increase the requirements.

Ok, thank you for the information, Buddy.

[James Kerman]

1 minute ago, Nendra said:

Ok, so by using Thrust limiter on reaching orbit, we can orbit the spacecraft without turning off the engine right? 

You can do this with the throttle during flight as well without having to tune thrust. In stock this is less efficient as you will be subject to more drag and gravity losses during ascent.

When playing with the rescaled system, you will probably need to burn all the way up anyway so tweaking probably won't be necessary.

[Nendra]

24 minutes ago, James Kerman said:

You can do this with the throttle during flight as well without having to tune thrust. In stock this is less efficient as you will be subject to more drag and gravity losses during ascent.

When playing with the rescaled system, you will probably need to burn all the way up anyway so tweaking probably won't be necessary.

Alright, Thank you for the information, Buddy 

[Goody1981]

2 hours ago, Nendra said:

Hello Guys, just wanna ask. In real life we usually see rocket which did not turn off the engine to orbit. I mean, how to orbit the rocket without turning off the engines and wait until it reaches the apoapsis. Can we orbit the rocket by still turning on the engines? For example, the Apollo mission, it uses it third stage to orbit, and did not turn the stage off. The Soyuz, it orbits using the third stage without turning off the engines. So how to orbit a rocket without turning off the engine on the stage? Thank you.

As the others have alluded to, it’s about getting the timing right. You want to reach your apoapsis just as your periapsis hits your desired altitude.

I’ve found two ways to do this:

First way... Let’s say you want a 75km x 75km orbit. Do your normal launch/gravity turn, and click the thing in the bottom left that displays your apoapsis and time to apoapsis - this is the important bit of information you will need.

Most guides tell you to get your Ap to whatever height you want, then cut throttle, coast to Ap, then circularise. But if you want your engine burning the whole time then you need to play with the throttle. Basically you are just trying to time it perfectly to reach an Ap of 75km at exactly the same time as your Pe reaches 75km. So you are just watching both numbers and trying to judge it as best you can. If it looks like you’re gonna hit an Ap of 75km before your Pe hits 75km, then throttle down. If it’s the reverse, throttle up. That’s if you want to hit a very specific orbit.

The second way... you care less about the specific altitudes and you just wanna be in orbit. This time you are changing where you’re pointing to try and reach Ap at the same time as your Pe hits 70km (space). Again, most guides say to always point directly prograde (I guess it is most efficient) but this can cause your Ap to race away from you. If you point below prograde while thrusting, you bring it back closer to you, while still raising your Pe. If you accidentally reach your Ap and go past it, you can point above prograde to get it back in front of you. By doing these little changes (only 5 or so degrees) in pitch you can keep your Ap constantly 5-10 seconds in front of you, watch your Pe, and when the Pe is close (say 60km) just point prograde and you will reach your Ap at the same time as your Pe hits 70km.

Depending on the thrust of your engines you’ll probably need to use some throttling in the second method as well, so perhaps the best way is a combo of both ways. Note since you’re pointing off prograde and/or throttling down, this probably isn’t the most efficient way to orbit, but like you said it is realistic  I always use a combo of both ways (I haven’t used the map screen to get into orbit for years!)

That’s a lot of words - but hope it helps!

Edit: I guess there is a third way - build and test your rocket and ascent profile dozens of times, refining it until you can full throttle launch off the launchpad, hit “point prograde” then sit back and get perfectly into orbit, running out of fuel on your final stage, just as your Ap and Pe reach 70.00km simultaneously. (Hard work)

Edited June 26, 2019 by Goody1981

[Nendra]

1 hour ago, Goody1981 said:

As the others have alluded to, it’s about getting the timing right. You want to reach your apoapsis just as your periapsis hits your desired altitude.

I’ve found two ways to do this:

First way... Let’s say you want a 75km x 75km orbit. Do your normal launch/gravity turn, and click the thing in the bottom left that displays your apoapsis and time to apoapsis - this is the important bit of information you will need.

Most guides tell you to get your Ap to whatever height you want, then cut throttle, coast to Ap, then circularise. But if you want your engine burning the whole time then you need to play with the throttle. Basically you are just trying to time it perfectly to reach an Ap of 75km at exactly the same time as your Pe reaches 75km. So you are just watching both numbers and trying to judge it as best you can. If it looks like you’re gonna hit an Ap of 75km before your Pe hits 75km, then throttle down. If it’s the reverse, throttle up. That’s if you want to hit a very specific orbit.

The second way... you care less about the specific altitudes and you just wanna be in orbit. This time you are changing where you’re pointing to try and reach Ap at the same time as your Pe hits 70km (space). Again, most guides say to always point directly prograde (I guess it is most efficient) but this can cause your Ap to race away from you. If you point below prograde while thrusting, you bring it back closer to you, while still raising your Pe. If you accidentally reach your Ap and go past it, you can point above prograde to get it back in front of you. By doing these little changes (only 5 or so degrees) in pitch you can keep your Ap constantly 5-10 seconds in front of you, watch your Pe, and when the Pe is close (say 60km) just point prograde and you will reach your Ap at the same time as your Pe hits 70km.

Depending on the thrust of your engines you’ll probably need to use some throttling in the second method as well, so perhaps the best way is a combo of both ways. Note since you’re pointing off prograde and/or throttling down, this probably isn’t the most efficient way to orbit, but like you said it is realistic  I always use a combo of both ways (I haven’t used the map screen to get into orbit for years!)

That’s a lot of words - but hope it helps!

Edit: I guess there is a third way - build and test your rocket and ascent profile dozens of times, refining it until you can full throttle launch off the launchpad, hit “point prograde” then sit back and get perfectly into orbit, running out of fuel on your final stage, just as your Ap and Pe reach 70.00km simultaneously. (Hard work)

Wow, thats a lot of information. Thank you very much, Buddy.

[Goody1981]

2 hours ago, Nendra said:

Wow, thats a lot of information. Thank you very much, Buddy.

No worries  pleasure!

[Superfluous J]

Also astronautics engineers are tasked with cutting the cost of something that costs literally millions of dollars. 1% savings can be the difference in them getting paid or fired. So unlike us (even in the slightly-less-of-a-toy-but-still-just-a-toy-compared-to-real-life "realistic" mods) who can just keep trying things until it works good enough, they are very motivated to get exactly as much utility out of the craft, and do design the craft to have exactly the utility it needs.

And coasting upwards is wasting some of the thrust you used, so they design their rockets to use less thrust lower in the atmosphere, so the engines weigh less and they can push them further.

Whereas we just toss a few more SRBs on it and call it a win.

[Nendra]

24 minutes ago, 5thHorseman said:

Also astronautics engineers are tasked with cutting the cost of something that costs literally millions of dollars. 1% savings can be the difference in them getting paid or fired. So unlike us (even in the slightly-less-of-a-toy-but-still-just-a-toy-compared-to-real-life "realistic" mods) who can just keep trying things until it works good enough, they are very motivated to get exactly as much utility out of the craft, and do design the craft to have exactly the utility it needs.

And coasting upwards is wasting some of the thrust you used, so they design their rockets to use less thrust lower in the atmosphere, so the engines weigh less and they can push them further.

Whereas we just toss a few more SRBs on it and call it a win.

Ok, Understood. Thanks...

[ajburges]

A few notes:

Due to limits of RL systems, staging involves noticeable coasting. Multi-mission craft (like the Falcon) will coast before circularization. Also I've heard RL missions will use pitch instead of throttle to tune Ap and TTA.

Many KSP players are too shallow on gravity turn for their TWR. The better stage design is the first stage is to get you an ap of 50 km with significant horizontal velocity and TTA. Then, an underpowered second stage that takes almost twice that TTA to burn the remaining dv to orbit! 50 km will get you clear of most atmo to control drag loses, the horizontal velocity will control gravity loses. You normally want vacuum engines suitable for a node smaller than the stage when you do this. Matched nodes vacuum engines topically have more thrust than you need.

Edited June 27, 2019 by ajburges
Phone keyboard shenanigans

[Nendra]

1 hour ago, ajburges said:

A few notes:

Due to limits of RL systems, staging involves noticeable coasting. Multi-missioncraft (like the Falcon) will coast before circularization. Also I've heard RL missions will use pitch instead of throttle to tune Ap and TTA.

Many KSP players are too shallow on gravity turn for their TWR. TThebetter stage design is the first stage is to get you an ap of 50 km with significant horizontal velocity and TTA. Then, an underpowered second stage that takes almost twice that TTA to burn the remaining dv to orbit! 50 km will get you clear of most atmo to control drag loses, the horizontal velocity will control gravity loses. You want vacuum engines suitable for a nose smaller when you do this.

Ok, Understood. Thanks

[The Aziz]

Yeah, in real life, rockets spend much more time going sideways when they build up the horizontal speed (due to the size of the Earth), so it's almost natural that they don't have to turn the engines off. In stock KSP, the planet is much more compact, as in, the planet diameter-to-atmosphere height ratio is significantly different (if you just scale Kerbin to the size of the Earth, the atmosphere would reach much much higher), ergo, the flight profile is also quite different.

[Mahnarch]

Turn good.

Burn good.

Don't asplode!