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Minimum vertical speed for ascent?


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As in the title. For years I'm trying to improve my gravity turn curve, and I still haven't figured out the sweet spot. I know what to do until I am about 20km up. First part is good, 1.3TWR on liftoff, first turn at 1km/100m/s, 45° at 10km, and so, but then I am never sure. What vertical speed do I need to keep till I reach desired apoapsis and turn off the engines?

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I wouldn't bother with the exact numbers. It's not about the speed number or even the attitude at whatever altitude (although that's typically a decent indicator). It's about TWR. Rockets with more or less TWR will want a more or less aggressive gravity turn.

Another decent indicator is time to apoapsis. If it's above 1 minute and rising, tilt more until it stabilizes around one minute. Keep that going 'till orbit or until you reach the desired apoapsis and then throttle down until you reach apoapsis and then circularize.

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I tend to find that as long as I'm over about 41km, then just keeping the vertical speed a positive number is all that's necessary. Below that, there tends to be too much heating and drag for a nice ascent to orbit.

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Hard to figure and very craft dependent.

Since orbiting is going sideways really fast, we want to commit as much as possible of our thrust to increase horizontal velocity.  So,  the most correct and most useless answer is: ignoring drag*, the ideal vertical velocity is 0.

A slight less correct and slight less useless answer is that you need to follow the trajectory that minimizes the drag/gravity/steering losses** (in other words: maximises the rate of energy gain) , whatever that trajectory is. 

And,  finally,  the technicaly incorrect,  speculative,  out of my cheeks but practical answer is that atmospheric drag is higher in lower atmosphere, at higher speeds,  at higher AoA; gravity drag will be higher at lower altitude and speeds; higher AoA will make steering drag higher. Thus,  initially climb fast enough to build up a good upward momentum that is just enough to maintain you outside the atmosphere until you reach orbital velocity.  If you are going fast and high enough (~1,5km/s, 40km)  just keep a positive climb rate. 

*collision with terrain is just a particular,  violently high, kind of drag. 

**For the sake of argument different kinds of drag,  something that reduces your energy 'dragging'  you back. 

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For me, it mostly depends on the TWR of my orbital-insertion stage.  I often use some pretty small engines (LV-909, poodle on significantly larger.  I'll even use LV-Ns (expect a stage in between) if I'm using them to leave Kerbin).  Doing this often means a higher TWR than 1.3 (if it takes more than a minute to circularize, you probably need TWR>1.3) or more stages.

I won't argue that this is actually more efficient than a 1.3 launch, the following link pretty much blows me away (and they launch with low TWR).  One reason I stick with low-TWR insertion stages is I want to keep using the same stage/engine to leave Kerbin (not an issue in the challenge).

If you haven't already downloaded Kerbal Engineer/Flight Engineer, go do so.  The critical things you need to watch in flight are apoapsis and time to apoapsis (they can be seen in map mode after minimal upgrading, but this often requires keeping the mouse on them.  I'd rather slap KE on all my ships.  The two easy ways to do orbital insertion are:

1. Create  a maneuver node at AP.  Burn like any other burn (you might delay the burn with experience).
2. Use KE (or the map, but at that point I'd just create the node and be done with it).  Burn at 1:00 before "time to AP" and burn until "time to AP" starts increasing.  Repeat at :45, :30, :15, and then finish around zero.

The point of my post is that a minimal speed for ascent in the first stage requires a maximal TWR in the second stage.  The way the rocket equation works, you can typically afford to make Isp of the first stage much lower than you can the second (and good Isp typically means poor thrust).  I really don't recommend going for a minimal ascent speed.

 

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27 minutes ago, wumpus said:

For me, it mostly depends on the TWR of my orbital-insertion stage.  I often use some pretty small engines (LV-909, poodle on significantly larger.  I'll even use LV-Ns (expect a stage in between) if I'm using them to leave Kerbin).  Doing this often means a higher TWR than 1.3 (if it takes more than a minute to circularize, you probably need TWR>1.3) or more stages.

I won't argue that this is actually more efficient than a 1.3 launch, the following link pretty much blows me away (and they launch with low TWR).  One reason I stick with low-TWR insertion stages is I want to keep using the same stage/engine to leave Kerbin (not an issue in the challenge).

If you haven't already downloaded Kerbal Engineer/Flight Engineer, go do so.  The critical things you need to watch in flight are apoapsis and time to apoapsis (they can be seen in map mode after minimal upgrading, but this often requires keeping the mouse on them.  I'd rather slap KE on all my ships.  The two easy ways to do orbital insertion are:

1. Create  a maneuver node at AP.  Burn like any other burn (you might delay the burn with experience).
2. Use KE (or the map, but at that point I'd just create the node and be done with it).  Burn at 1:00 before "time to AP" and burn until "time to AP" starts increasing.  Repeat at :45, :30, :15, and then finish around zero.

The point of my post is that a minimal speed for ascent in the first stage requires a maximal TWR in the second stage.  The way the rocket equation works, you can typically afford to make Isp of the first stage much lower than you can the second (and good Isp typically means poor thrust).  I really don't recommend going for a minimal ascent speed.

 

Good advice. Quick note, you don't need the maneuver node at AP if you have KER. You can have it display that information on the fly with perfect accuracy at all times. 

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My tests gave me a TWR between 1.8 and 2.2, faster meant more fuel-efficient by having less time to fight gravity, but also the sound barrier in thick atmosphere below 30km increased the fuel use :) Once circularized outside atmosphere, TWR only affects the burn time.

I made a basic rocket made of one engine and fuel tanks, and affected TWR by limiting the engine thrust. But my test did not include the fact, that TWR can be "limited" with additional fuel instead of thrust limit...
I have not tested the hypothesis that while lower TWR means burning more fuel per time unit (fighting gravity), the rest dV is offset by having ... more fuel onboard.

My rule of the thumb is that insertion engines should have ASL thrust as priority, where space engines should have ISP instead (while having at least 0.3 TWR, I am not a fan of these long burns). :)

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9 hours ago, wumpus said:

Burn at 1:00 before "time to AP" and burn until "time to AP" starts increasing.  Repeat at :45, :30, :15, and then finish around zero.

My method is to start the circularization burn before apoapsis and reduce the throttle just enough to keep reducing time to AP. How much before the Ap Io start to burn depends on craft, the idea it to reach orbital velocity just a moment before apoapsis. 

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If you really look hard into these things, you will find, that the fact is a general answer probably does not exist.  You can optimize your ascent profile for a particular rocket/configuration, but that same profile will be non-optimal for another rocket/configuration.  Personally, I use a throttle lookup table versus apoapsis altitude along with a smoothing function (spline?).  One of my favorite solutions, though, is to lock throttle to 1-((current speed)/(orbital velocity@current altitude)) .  This gives surprisingly good results.

Edited by James McConnel
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Rule of thumb: you always want the nose as close as possible to prograde, and you want prograde as close as possible to the horizon [1]. However, limiting factor: you don't want prograde to drop below the horizon.

So, I'll generally do the 45 degrees at 10,000m thing to get out of the low-atmosphere soup, but from then on I keep the nose on prograde and watch the time to apoapsis. If it's getting too close to zero, pitch up a few degrees; if it's racing away from you, drop back to prograde. A bit of practice and you can get your circularisation burn down to almost nothing.

 

[1] Within reason. I'm talking about the latter part of the gravity turn, not going straight sideways at launch.

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