terminal velocity ascent and when to do gravity turn?

[Nerd^3(USA)]

based on a rocket that can exceed terminal velocity under full thrust. if that rocket is holding terminal velocity straight up, would it be better to turn sooner and reduce throttle to hold terminal velocity or keep going straight up until said rocket could no longer hold terminal velocity? just occured to me that once you do start the gravity turn, the prograde marker will start to fall toward the horizon. at what point would be the most efficient elevation at which to start the gravity turn, based upon the TWR of different ships? if you start too soon, a low TWR ship won't rise fast enough to make orbit, while a high TWR ship would end up needing to exceed terminal velocity in order to raise the AP to the point of atm escape.

the next question is, how fast do you want to do the gravity turn? too slow and you end up with very little horizontal speed and a very high vertical speed (need more vertical speed to make orbit more efficiently). too fast and you end up low in the atm and still at lower terminal velocity speed which means you waste fuel because you have to exceed terminal velocity to continue to ascend.

[Zombo]

OTG Idemus eredeti hozzászólása:

there is a very in depth explaination on YouTube by Scott Manley about grav turns. He explains the common pitfalls and shows various ascent profiles in KSP and the resulting Delta-V usage. However a lot of people have hit the nail on the head here...ANY course correction from the prograde marker will cost you Delta-V in the long run. The art here is reducing that to as few deviations from that prograde marker as possible, while still getting into a circularized orbit at the right altitude.

Following the prograde marker only works when its switched off the "Surface" to "Orbit", and you have already started to turn some. The marker will stay pointing straight up aslong as you do. So following it isnt really the case, its actually following you. The prograde marker is an indicator of your ships actual trajectory.

What you want is a smooth arc from vertical to horizontal, starting the turn around 30,000m and finishing above 70,000m+... As you approach the end of that arc and reach space, you do your circularization burn and finish off your orbital insertion.

Dont just turn a full 45% degrees at whatever height you decide to start you turn. You want an nice arc for the most efficient launch. And theres less risk of you side slipping and bleeding off thrust.

I lean over 10-15 degrees before this starting at 10,000m, just to make sure I dont start falling over the wrong way later on. Sometimes the rocket will start to turn on its own as the tanks empty, so you want to make sure it goes the right direction if it does. It also allows for a bit more speed to pick up. What you dont want is the prograde marker to slip below where you are aiming. If you start to slip sideways due to low TWR, it will bleed off your momentum severely. I usually restart the launch when that happens. The remedy, you have to stay going straight up until 30,000m when you start your turn. This prevents you from side-slipping, and you will be going slower. But by the time you start your turn at 30,000m the side slipping problem is gone. Its only a problem at lower altitudes.

[Nerd^3(USA)]

using mechjeb, i do agree with you. i even went and flew it up manually a few times, just to see if i could get close to what mechjeb was doing. the gradual turn is NEEDED. i tried the turn 45 degrees until 30km and ran out of fuel before my PA hit 100km. from there, i went back and started my turn at 10km and slowly started aiming closer and closer for the prograde marker. by the time i hit 30km, i was at the back side of the prograde marker. i then went to the front side of the prograde marker till 40km and then on to 90 degrees (level with horizon) until my PA hit 100km and then hit small thrusts to keep PA at 100k until i hit the edge of the atmosphere. with that, i was only 2m/s off what mechjeb got with 10km start turn and 45 degree ascent profile. the big part is to ascend under terminal velocity until you hit about 25km when the terminal velocity goes up VERY fast. that's when you can really get some serious horizontal speed and get very close to horizontal with the horizon.
if you want to try the ship i was using, it's an mk1-2 command pod, 3 fl-t400 tanks, 3 fl-t400 tanks radially around the lowest tank, 2 fuel lines (one on each side of the 3 tanks) connecting the 3 radial tanks to the lowest main tank with an aerospike engine. mechjeb got 620m/s remaining fuel and i got 618m/s fuel remaining. oh yea, i was in surface mode for the navball until it automatically switched.
forgot about the av-r8 winglet. lol
here is a pic. http://steamcommunity.com/sharedfiles/filedetails/?id=309918036

[[OTG] Idemus]

Zombo eredeti hozzászólása:

OTG Idemus eredeti hozzászólása:

there is a very in depth explaination on YouTube by Scott Manley about grav turns. He explains the common pitfalls and shows various ascent profiles in KSP and the resulting Delta-V usage. However a lot of people have hit the nail on the head here...ANY course correction from the prograde marker will cost you Delta-V in the long run. The art here is reducing that to as few deviations from that prograde marker as possible, while still getting into a circularized orbit at the right altitude.

Following the prograde marker only works when its switched off the "Surface" to "Orbit", and you have already started to turn some. The marker will stay pointing straight up aslong as you do. So following it isnt really the case, its actually following you. The prograde marker is an indicator of your ships actual trajectory.

What you want is a smooth arc from vertical to horizontal, starting the turn around 30,000m and finishing above 70,000m+... As you approach the end of that arc and reach space, you do your circularization burn and finish off your orbital insertion.

Dont just turn a full 45% degrees at whatever height you decide to start you turn. You want an nice arc for the most efficient launch. And theres less risk of you side slipping and bleeding off thrust.

I lean over 10-15 degrees before this starting at 10,000m, just to make sure I dont start falling over the wrong way later on. Sometimes the rocket will start to turn on its own as the tanks empty, so you want to make sure it goes the right direction if it does. It also allows for a bit more speed to pick up. What you dont want is the prograde marker to slip below where you are aiming. If you start to slip sideways due to low TWR, it will bleed off your momentum severely. I usually restart the launch when that happens. The remedy, you have to stay going straight up until 30,000m when you start your turn. This prevents you from side-slipping, and you will be going slower. But by the time you start your turn at 30,000m the side slipping problem is gone. Its only a problem at lower altitudes.

I didn't say you should just keep your nose pointed directly at your prograde marker the whole time, but as close to it as possible. The science goes like this...you are using thrust to propel a pointy object through atmosphere and away from center of gravity of another object. The goal is to reduce effort for circularization by using the arc created by the pull of gravity. If you add minimal, constant force at a decreasing amount to a ballistic object, you can extend that arc into orbit, where at apoapsis, you are just short of the lateral speed you need to be going to have gravity perpetually pull you around the object (orbit obviously). Meaning, if you follow the perfect ascent profile for your rocket, you shouldn't have to deviate very far from your velocity vector. The most optimal ascent would have a very subtlely changing trajectory, not taken in stages. In the perfect ascent, your velocity vector and your heading WOULD stay very close together and you would burn from launch to orbit (if only with a small percentage of full thrust towards the end). However we aren't computers and there are a lot of variables, close is close enough.

[Toastie Buns]

Am I the only one who doesn't kill the engines?
I do full orbits from the launchpad, none of this stop the engines until apoapsis silly business.
I theorise that in killing the engines, I'm losing speed, which is never a good thing. I just aim to get myself above 2000m/s so I'm at orbital speed and this never seems to fail me.
Grav turn starts around 350m/s-400m/s, regardless of craft mass. This means I completely ignore the whole 10,000km business that also, seems silly. I think it's more coincidence that 10,000km = >300m/s for most craft.

[[OTG] Idemus]

You are right, in that speed matters more than altitude. However, speed is often based on altitude in this case, considering atmosphere is incredibly dense until just prior to 10km. I wouldn't say killing the engines is necessarily a bad thing. Firstly, while running your engines or not, gravity is still shaving off essentially the same amount of vertical momentum at a given altitude. Realizing that the faster you ascend, the shorter the time those forces are effecting you, the difference between shutting down your engines for even 30 seconds, and not, is very minimal at higher altitudes. Where the altitude and gravity is dense is the area where that makes a huge difference. All that being said if you do everything properly on ascent in a well-balanced vessel, you should be able to run the engines pretty much the whole way without losing substantial amounts of Delta-V to course correction or drag. However, in situations where the final stage engines have substatially lower ISP in atmo, it may be more efficient to build up velocity with boosters to go ballistic until you get out of atmo before firing up the final drive engines. In these cases, you may not care that you are wasting Delta-V on a sub-optimal ascent path, because your 909 driven craft now has enough altitude and velocity to not become an unintentional flaming reentry vehicle without a parachute.

[sigma female]

I just do it at 10,000 and go flat at 45,000.
I think a Scott Manley video taught me, not sure because I had game for long time.

[Nerd^3(USA)]

OTG Idemus eredeti hozzászólása:

You are right, in that speed matters more than altitude. However, speed is often based on altitude in this case, considering atmosphere is incredibly dense until just prior to 10km. I wouldn't say killing the engines is necessarily a bad thing. Firstly, while running your engines or not, gravity is still shaving off essentially the same amount of vertical momentum at a given altitude. Realizing that the faster you ascend, the shorter the time those forces are effecting you, the difference between shutting down your engines for even 30 seconds, and not, is very minimal at higher altitudes. Where the altitude and gravity is dense is the area where that makes a huge difference. All that being said if you do everything properly on ascent in a well-balanced vessel, you should be able to run the engines pretty much the whole way without losing substantial amounts of Delta-V to course correction or drag. However, in situations where the final stage engines have substatially lower ISP in atmo, it may be more efficient to build up velocity with boosters to go ballistic until you get out of atmo before firing up the final drive engines. In these cases, you may not care that you are wasting Delta-V on a sub-optimal ascent path, because your 909 driven craft now has enough altitude and velocity to not become an unintentional flaming reentry vehicle without a parachute.

i do agree with using boosters. those things put out quite a bit of thrust for very little investment, if you're playing career. though, i wouldn't just pack on a bunch of boosters without being concerned with the amount of thrust, and therefore the amount of acceleration. by balancing it out, you won't have to pack on quite as many boosters, if you can ascend a bit below terminal velocity. even if you DO exceed TV a bit, you're still going to be getting the needed altitude for the higher dV for your main engines.

[Toastie Buns]

Nerd^3 eredeti hozzászólása:

I wouldn't just pack on a bunch of boosters without being concerned.

You've changed. I thought we raised you better.

[Nerd^3(USA)]

Toastie Buns eredeti hozzászólása:

Nerd^3 eredeti hozzászólása:

I wouldn't just pack on a bunch of boosters without being concerned.

You've changed. I thought we raised you better.

i posted that? i LOVE packing on boosters. the more the merrier. once i finish mowing the lawn, i'm gonna make one with little more than boosters, just for sh**s and giggles. lol

oh, that quote was taken out of context. you gotta take into account the amount of thrust with the boosters you are putting on. if you have too much thrust, you're going to be exceeding TV very early and therefore wasting the efficiency of those boosters.

[J-Curwen]

Dutchgamer1982 eredeti hozzászólása:

I found that the most efficient way to rise up is as follows :

step 1 get speed as fast as possible to 90-110 m/s (you don't want to fight gravity to long)
step 2 keep your speed under 110 m/s untill 14000m (the trick layer is a little more than 10km. ou want to be clear ahead of it)

Thats mostly right. But you are going to slow. The terminal velocity will rise while getting higher. 110 m/s is good till 1km but after that you are losing fuel for fighting the gravity for to long.

Altitude (m) Velocity (m/s)
75 100.9
1000 110.5
2000 121.9
3000 134.5
4000 148.4
5000 163.7
6000 180.6
7000 199.3
8000 219.9
9000 242.6
10000 267.7
12500 342.4
15000 437.8
20000 716
32000 2332

source: http://wiki.kerbalspaceprogram.com/wiki/Kerbin

[[OTG] Idemus]

Sandor eredeti hozzászólása:

Dutchgamer1982 eredeti hozzászólása:

I found that the most efficient way to rise up is as follows :

step 1 get speed as fast as possible to 90-110 m/s (you don't want to fight gravity to long)
step 2 keep your speed under 110 m/s untill 14000m (the trick layer is a little more than 10km. ou want to be clear ahead of it)

Thats mostly right. But you are going to slow. The terminal velocity will rise while getting higher. 110 m/s is good till 1km but after that you are losing fuel for fighting the gravity for to long.

^ this is very important. It's great to minimize drag on your way up...it's also great to minimize gravity's effect on your ascent as well. Too slow and you are burning Delta-V on extra unneeded lift. Too fast and you burn Delta-V right out the back of your ship cause you can't go any faster. It's an illusion that you are saving fuel because the meter creeps down real slowly, making the fuel 'last longer'. It is however, only an illusion as you would be climbing much faster were the fuel going down faster. ISP is going to be the same for an engine whether it's at half or full, so use full throttle when you can.

[Nerd^3(USA)]

dV rises as you get higher in the atmosphere. the issue is to get be as efficient with your fuel as possible. you want a combination of high climbing speed while going as much under TV as possible. the problem is, finding that "most efficient" climb rate while staying as much under TV as you can. too slow and you are burning extra dV because you are ascending too slow. too fast and you are exceeding TV which causes your engines to burn excessive dV because it's fighting against atmospheric resistance. though, i'm not sure if staying UNDER TV is as good as it is to stay near or AT TV.

edit: i MAY be confused about dV rising as you get higher in the atmosphere. i thought i saw it rising during some testing. maybe it was going down? lol

[Toastie Buns]

Efficiency is for casuals and scrublords. The only real way to Kerbal is to attach 20 of the largest SRBs you have and aim for the sky.

Nerd^3 eredeti hozzászólása:

edit: i MAY be confused about dV rising as you get higher in the atmosphere. i thought i saw it rising during some testing. maybe it was going down? lol

dV is a measure of velocity per unit of fuel, which obviously means that if you are going SONIC SPEEDS the dV of your next stage will climb rapidly. It still has 1000 dV in its stage, but because you've added, lets say an extra 300m/s through PURE SPEED, the total dV of stage 2 would be 1300 dV. This is why the number increases. If you stage, then wait before you strike up your boosters you'll see this number roll back down to 1000dV

[Nerd^3(USA)]

ohhhh, thanks for the clarification, toastie. i didn't notice it drop back down.

[Nerd^3(USA)]

Toastie Buns eredeti hozzászólása:

Efficiency is for casuals and scrublords. The only real way to Kerbal is to attach 20 of the largest SRBs you have and aim for the sky.

Nerd^3 eredeti hozzászólása:

edit: i MAY be confused about dV rising as you get higher in the atmosphere. i thought i saw it rising during some testing. maybe it was going down? lol

just 20 SRB's? i've put up to 32 SRB's on a couple of my launch stages. i just love seeing all the smoke and flames! LOL