Gravity turn and horizontal slip

[Dillweed]

I've been playing for a while but here is a problem that seems to keep creeping up. I will ascend to 10km, start my gravity turn to 45 degrees and my vertical acceleration will just die. My horizontal speed will shoot up very quickly and I'll end up just moving horizontally, usually perpendicular to the ground. Sometimes my vertical speed actually decreases. I thought this was only a problem with my larger rockets but I've started to have problems with my more modest sized ones as well. I'm running Flight Engineer, so I can tell the V vs. H speed. I also have a pretty cromulent TWR (1.5+). I've found that this problem is not so bad if I wait until I'm at 20k to do the turn, but I know that crazy inefficent. I was wondering if anybody has a more scientific explanation about what is happening/what I can do about it.

An example of a rocket I'm having trouble with, MK1 Tug

1 Jets to 8.5k/ ~ 250 m/s

2 Skipper to appo

3 T45 to circularize

Edited October 25, 2013 by Dillweed

[sjwt]

Different ships, different flight plans, their is no one rule.

[phunk]

At an angle of 45 degrees, you need a TWR of at least 1.414 for the vertical component of your thrust to exactly equal gravity and not accelerate at all. You need more than that once you take air resistance into account and more because you're going to want to keep accelerating, not just match gravity. It probably works better at 20k because both the air density and your fuel weight have reduced significantly by then.

[DMagic]

The issue is probably that you are turning too far. You don't have to go down to 45^o, in many cases it's better to very slowly turn over. Go 5 or 10^o at a time and it should be easier to control. For that particular design the heavy and unstable looking payload on top might be causing issues, too.

[Stochasty]

Make your turn gradually, and try to stay close to your prograde vector throughout the turn.

The best profile varies from ship to ship, but you want to aim for 45 degrees around 25km or so, and near horizontal around 40-50 km.

[ScottyDoesKnow]

I usually go 22.5 at 10k, 45 at 25k

[Taki117]

Most of my turns start at about 7-8k and go straight to 45 degrees. None of my designs are outlandish though.

[Kasuha]

Gravity turn means kicking the ship off the vertical at one point, then burning prograde and following the gravity-induced fall of the prograde marker. If you do it right, you end up in orbit right when the marker gets horizontal. If you don't do it right (which is much easier), you need either to burn radial out to raise your AP, radial in to raise your PA without raising your AP, or to coast a while which are all less efficient ways of raising your orbit.

My experience tells me that 45° at 10 km is usually too much to perform gravity turn. You just force the ship into orbit because it is strong enough to handle it. Weaker ships need different approach.

[jfx]

Dont use jets on a rocket. They are fat, heavy, expensive and clunky to use. Also, start turning gradually (5-10° at a time) at ~5k. I usually start the turn after the long stock boosters are dropped.

[Dillweed]

Thanks guys. Yeah, my TWR was actually right around 1.4 on that rocket. Finally, any way to change the status to "answered"?

Edited October 25, 2013 by Dillweed

[Snillum101]

Thanks guys. Yeah, my TWR was actually right around 1.4 on that rocket. Finally, any way to change the status to "answered"?

Edit your original post

[WafflesToo]

Conceptually, your design is similar to my 'Val' SSTO.

I know this is going to sound insane, but try pulsing the rocket as you fly from 10k up to 17k or so (basically until the air runs out). I can't explain why it works but I can save several tons of fuel getting into orbit that way.

Edited October 26, 2013 by WafflesToo
my phone hates video links

[Dillweed]

I find the usefulness of jets decreases pretty quickly with the bigger ships.

[Mr Shifty]

Many existing rockets -- the Space Shuttle included -- have a flight profile that tips them to horizontal early and lets their vertical speed drop to less than zero until the horizontal speed is fast enough to overcome gravity. Horizontal speed is what you need for orbit. The only reason you have vertical speed is to keep out of the soup. On airless worlds, for instance, it's most efficient to get just enough altitude to miss the mountains, then burn horizontally as much as you can without losing altitude.

[numerobis]

I know this is going to sound insane, but try pulsing the rocket as you fly from 10k up to 17k or so (basically until the air runs out). I can't explain why it works but I can save several tons of fuel getting into orbit that way.

It's because you flame out when your throttle is too high for the air that you have. Reduce throttle, reduce the air requirement, your jets can keep going.

Just because this is completely unphysical is no reason for it not to work.

[WafflesToo]

It's because you flame out when your throttle is too high for the air that you have. Reduce throttle, reduce the air requirement, your jets can keep going.

Just because this is completely unphysical is no reason for it not to work.

Can't, TWR is already too low at that point in the flight as it is (using basic jets instead of turbojets, they work better in this application believe it or not). I'm basically just using the rocket to keep my vertical velocity up while the jets build horizontal velocity (which helps increase airflow). I've noticed doing it that way lets me run the jets longer and to higher altitude which I suspect is where I'm seeing the fuel savings.

[numerobis]

I misunderstood what you meant by pulsing.

You can combine the ideas: leave the jets on, throttle back, but also have the rocket on, to gain another couple km.

[Kulebron]

I've been playing for a while but here is a problem that seems to keep creeping up. I will ascend to 10km, start my gravity turn to 45 degrees and my vertical acceleration will just die. My horizontal speed will shoot up very quickly and I'll end up just moving horizontally, usually perpendicular to the ground.

...

I also have a pretty cromulent TWR (1.5+)

I recommend increasing TWR to 2.0, because 1.5 is really very low.

Then look at this flight path.

[Diazo]

I recommend increasing TWR to 2.0, because 1.5 is really very low.

Err, not really?

Especially once you get into the heavy lifters, a TWR of 1.5 is high.

I think my last heavy lifter had a TWR of 1.1 on the pad before ignition.

Having said that, it was very touchy and required a very uncompromising ascent path to reach orbit.

So perhaps what I'm saying is that any TWR can reach orbit, but the higher your TWR the easier it is.

D.

[phunk]

1.1 on the pad is not the same as 1.1 at your gravity turn.

[Psycix]

I always build towards a TWR of 2 (>2 for a majority of the stage burn), and then throttle back for terminal velocity.

Asparagus is great for maintaining a constant TWR.

Depending on the design, my upper stage can be even more powerful, just to get that thing into orbit as fast as possible after clearing the atmosphere.

Although I might lose out on engine weight, I go for minimising gravity and atmospheric losses.

[WafflesToo]

I misunderstood what you meant by pulsing.

You can combine the ideas: leave the jets on, throttle back, but also have the rocket on, to gain another couple km.

Those were the flights that this vehicle performed at its worst; either failing to reach orbit or only barely. I don't have the mathematical muscle to explain why it's working out that way, it goes against all conventional wisdom, my Kerbal instincts keep screaming at me that it's wrong... but I can't deny that it seems to be the best approach. It just works.

[WafflesToo]

Err, not really?

Especially once you get into the heavy lifters, a TWR of 1.5 is high.

I think my last heavy lifter had a TWR of 1.1 on the pad before ignition.

Having said that, it was very touchy and required a very uncompromising ascent path to reach orbit.

So perhaps what I'm saying is that any TWR can reach orbit, but the higher your TWR the easier it is.

D.

I'm with you. Most of my lifters are 1.2 or less; often with less than 1 at the terminal stage and I routinely reach orbit for around 4.6km/s

OP is using jet engines for their first stage and they are a lot finickier to handle than rockets which I think is at the heart of his problem.

Edited October 26, 2013 by WafflesToo
I can't brain today

[Kulebron]

Err, not really?

Especially once you get into the heavy lifters, a TWR of 1.5 is high.

I think my last heavy lifter had a TWR of 1.1 on the pad before ignition.

Having said that, it was very touchy and required a very uncompromising ascent path to reach orbit.

So perhaps what I'm saying is that any TWR can reach orbit, but the higher your TWR the easier it is.

D.

I tried low TWR, and even though my rocket had 4600 m/s of delta-v, it did not reach the orbit. So with low TWR you use too much fuel for hovering. Basically, substract 1.0 from your TWR, which is cancelled by gravity and see how much is left, and how fast you can get to 10 km. 0.1g, 0.5g and 1.0g make big difference.

If you have low TWR, sure you get into orbit, but before that the rocket spends a lot of fuel and lowers its weight to start accelerating.

The upper stages, of course, may have low TWR, because you don't fight gravity there.

[goldenpeach]

At an angle of 45 degrees, you need a TWR of at least 1.414 for the vertical component of your thrust to exactly equal gravity and not accelerate at all.

[...]

How can you calculate that?