Why one should not fly to slow on ascent

[Jebs_SY]

Hello everyone,

I tried to make a video, showing that the gravity losses are much more important than the drag losses and that this is the cause, why one shouldn't fly to slow on ascent trying to minimize the drag losses.

If someone is interested: o/

 

BR

Jebs_SY

[invision]

whats an atmosphere? 

[Alshain]

That video isn't relevant to any KSP version after 1.0 because the "turn 45 degrees at 10km" method of ascent has been obsolete for that long.  Your gravity losses have nothing to do with high speed vs low speed, it has to do with vertical velocity vs. horizontal velocity.  Gravity eats vertical velocity, but not horizontal velocity.

In general you don't want to be too fast because it makes turning on time difficult to do.  1.5 launchpad TWR is ideal, it is possible to turn off SAS, take your hands off the keyboard, and let the rocket turn on it's own.  You can go between 1.2 to 1.8, but not much faster.

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Edited August 12, 2017 by Alshain

[Kerbal101]

@Jebs_SY I measured actual 2.2 TWR to be ideal, but anything less than 1.4 TWR is wasting fuel..
..but then I found out that trading fuel for speed - adding an extra can, an exchange between dV and TWR (>1.4) wins over high TWR.
So using less fuel, but having 10% left is worse than using more fuel, but having 20% left;- thanks to taking an extra fuel can. Which works superb for single-stage stuff.

..but on second thought if there is staging, usually first 1-N stages are specifically designed to leave Kerbin and allow higher stages (payload) do something useful.
So here the extra fuel saved by TWR/dV exchange of no use, yet higher TWR means taking more payload.
Which means investing in TWR, while having enough dV pays off more for multistaged stuff.

As of ascend profile, like @Alshain said - the ideal one is a hands-off/SAS-off gravity turn. Or .. in other words, the horizontal profile. But the rocket and its trail should not go "surf-boarding", in other words rocket points nose upwards, gases leave horizontally;- thats too little vertical speed.

Edit: watched your video to the end, its a great comparison. Interesting to know that mechjeb can show gravity loses, never used it - always looked at "dV stage" in KER like you did. Also have TWR display there, to see the fuel economy depending on distance to gravity sources.

Edited August 12, 2017 by Kerbal101

[Spricigo]

You missed the point.  You don't need to slow down with this rocket because it is not powerful (or draggy) enough to reach the critical velocity before getting out of the dense part of the atmosphere. Try switching that Reliant for a Vector and see how it goes. 

Apart from that,  I have to agree with @Alshain that the way you fly exacerbate the gravity losses. If you want to demonstrate how a "faster" ascent is better than a "slow" ascent, don't let the effect of a vertical ascent  be so pronounced. 

[wumpus]

On 8/11/2017 at 11:17 PM, Alshain said:

In general you don't want to be too fast because it makes turning on time difficult to do.  1.5 launchpad TWR is ideal, it is possible to turn off SAS, take your hands off the keyboard, and let the rocket turn on it's own.  You can go between 1.2 to 1.8, but not much faster.

For higher TWR launches, it often helps (even more than usual) to angle the rocket in the VAB.  Of course, control of this gets iffy so expect to either revert a lot or ignite your stages before releasing the launch clamps.

One thing to remember is that people aren't always clear on how long a rocket maintains its launch TWR.  A two stage rocket using a SRB first stage will gain TWR faster as it climbs than most rockets, while a three-stage (or more) rocket will typically gain TWR much more slowly than most two stage rockets.  Adding hammers might wildly increase TWR, but that TWR won't last long.  Typically it makes much more sense to use smaller engines (especially using less SRBs to wrap a first stage) than to throttle the engine or set SRBs to less thrust.

Don't forget that KSP is an extremely flexible game (note the train enthusiasts on this site), you should consider being flexible in your rocket design.  A tall, ungainly rocket that has great difficulty turning (read losses control the moment you turn in a thick atmosphere) might be well off taking an extended old-school trip straight up before turning.  Such a design (assuming your noodle can handle it) would  be more efficient with enough kickers wrapped around it for a TWR at least 2.0.  It won't be efficient, but it might be one of the few ways to get it in orbit.

[Jebs_SY]

Hi all,

good points here. I have to say, that I missed to point out a fact in the video... with "fast" ascent, I didn't only mean "don't fly slow", I also mean, "try to get to orbital velocity (PE>70km) as fast as possible" (meant time wise).
And at the same time flying as shallow as possible while also not flying to long in the thickest atmosphere, too.

The video should be dedicated to situations, which I (often?) see at twitch, where people slow down the throttle, cause they feat atmospheric drag. So extending the time where they fight gravity drag at the same time.

BR
Jebs_SY