Keep your time to AP at 1minute?

[guitarxe]

Why are there usually tips to keep your time to AP at 1minute? Does this apply to rockets or spaceplanes or both?

And more specifically, when you fly a spaceplane, since you are accelerating in mostly middle-atmosphere that is thick enough to drag your speed down significantly, what is more fuel efficient? Burn slowly all throughout the atmosphere while keeping the time to AP at 1min until you reach space, or burn full to 70km AP, regardless what the time to AP is, and then do additional small bursts when the drag slows you down?

[Eric S]

That's generally been rocket advice, to the best of my knowledge. Spaceplane ascents might use that rule after switching from air breathing to rocket propulsion in 1.0, but I haven't played around with enough spaceplanes in 1.0 to know if that has become the case. Prior to 1.0, my rocket engine phase was usually short enough that it didn't really didn't matter as my apoapsis was far more than a minute ahead of me.

[Coga19000]

It just shows that your orbit is rising faster than you are -because if you catch up with your Apoapsis (which is by definition the highest point in your current trajectory), well, your not going to space today.

One minute is a bit too much of a Margin of error, though. 35 to 45 seconds is good enough.

[cybersol]

That is likely old advice, in 1.0.2 its much more about targeting certain pitch angles up at certain altitudes.

For spaceplanes:

- Lift off

- 10-20* pitch up until speed above 270 m/s

- Raise pitch to maintain speed below 290 m/s (high drag above this speed)

- At 10-14km, pitch down to 0-5* and accelerate until 500 m/s

closer to 10km for only turbojets and to 14km for only rapiers

- Slowly raise pitch to 15* while building speed

5* pitch up per 50 m/s speed increase above 500 m/s seems to work well

- Grab the last bit of airbreathing speed by pitching back down to 5-10* upslope

Do this around 17km for turbos and 20km for rapiers

- As soon as your speed indicator stops going up, punch the rockets and/or switch modes on the rapiers

- Get out of dodge by aiming at 20* upslope until 32km

- After 32km follow the top of prograde marker until your at 5-6* pitch

- Cruise until your desired apoapsis

- Cruise out of atmospshere to apoapsis and circularize

You don't have to do all that exactly, but its what I do when efficiency is more important than lackadaisical piloting.

Edited June 6, 2015 by cybersol

[DarkGravity]

For rockets, if you have a very powerful rocket, you can get closer to your AP, or even pass it a small amount near the end of your burn to establish an orbit. With a weaker rocket, you want to keep AP ahead of you by a larger amount.

If you pass your AP in a weak rocket with say, another 2 minutes of burn needed to establish orbit, you will be sinking into the atmosphere as you burn, so you will need to shift your burn slightly toward radial-out which means even less of your fuel is going toward establishing orbit, and since you're past AP, you're not raising PA as efficiently as possible, and you'll end up with an elliptical (possibly highly elliptical) orbit even if you do somehow manage to establish orbit. Also, you will have wasted a lot of fuel just establishing that funky orbit.

[cybersol]

A modern rocket profile starts the gravity turn almost immediately, but the details depend on your preferred starting TWR. I prefer running full throttle at 1.7-1.9 starting TWR, but wimpsmore cautious people prefer 1.4-1.6 starting TWR in stock aero. For the lower TWR you will want to get more altitude faster and turn slower with gravity pulling you around more. Also, with more drag (i.e. ginormous fairing, etc.) you will want to gain altitude faster to reduce drag.

My rocket ascent profile:

- Starting TWR of 1.7-1.9 and pretty aerodynamic design

- Most important thing is to be SMOOTH, never do anything too fast

- At a speed of 50 m/s, SAS off and pitch over to 75*

- Pull your gravity turn a little faster by being near the lower edge of progade until 280 m/s

- Straighten up to almost exactly prograde from 290-400 m/s

- Then pull near prograde edge again to try to reach 45* picth at around 13km

- Aim for 22.5* pitch by around 26km

- Level off to 5* by 40km

- Continue until your reach target apoapsis

- Main engine cutoff, coast to apoapsis and circularize

EDIT: Okay, now that I wrote both of my ascent profiles out, its time for 1.0.3 to land and change them

Edited June 6, 2015 by cybersol

[match]

Keeping the your apoapsis ahead of you is the first priority while gaining orbit speed in atmosphere. The time is arbitrarily set far enough out to allow for losses during ascent. It is short enough to be close to your apoapsis where your thrust through prograde is contributing more to horizontal speed rather than vertical speed.

It all depends what your goal is and the tools you have at hand, if you have a draggy payload, staying in the atmosphere any longer than necessary is bound to cause more issue than the efficiency granted by a picture perfect ascent profile. Is a case of theory vs application.

[MarvinKitFox]

It just shows that your orbit is rising faster than you are -because if you catch up with your Apoapsis (which is by definition the highest point in your current trajectory), well, your not going to space today.

One minute is a bit too much of a Margin of error, though. 35 to 45 seconds is good enough.

For rockets, Coga speaks Great Truth. For me the sweet spot is 35 seconds.

For spaceplanes, I know less, but I think Cybersol's comments are spot-on.

[Coga19000]

Oh right, forgot about the spaceplanes.

I don't have a lot of experience with those, but generally you want to milk the atmo for all its worth. When either the Intakes fail you or the engines fail to get you any useful boost, open the closed -cycle and wear your 50-degree pitch on until you see the Apoapse you like. If you go like this, however, make sure you have some extra Liquid Fuel packed among with the LOx for the rockets. It is even easier with Nervs, but in this case you can turn them on a bit before you shut down your airbreathers.

[guitarxe]

Would someone mind giving my craft a try? I'm no longer sure if my design is bad or my flying is bad.

http://kerbalx.com/guitarxe/Heavy-MK18.craft

[cybersol]

Checking it out now and will add more later. But first of all, waaaaaaaaaaaaaay to much wing. I'm guessing you are using KJR or Claws mod or similar because it explodes on physics load every single time for me. Its actually a pretty cool explosion though.

Edited June 7, 2015 by cybersol

[guitarxe]

Yeah, I am using KJR, I didn't realize it didn't work without it.

[rkman]

It applies to rockets. If AP is more than about a minute ahead there is a big chance it will 'run away' from you so you end up with a significantly steeper ascent than you planned, costing significantly more gravity loss than you planned for.

If AP is to close there is a big chance you will overtake it, then descent and never make it to orbit.

[guitarxe]

Ok, so I went back to the drawing board, and I don't understand how I can have any less wings? I don't have enough lift past 11km with any less wings:

http://kerbalx.com/guitarxe/Back-to-the-Drawing-Board-MK3.craft

[rudi1291]

Ok, so I went back to the drawing board, and I don't understand how I can have any less wings? I don't have enough lift past 11km with any less wings:

http://kerbalx.com/guitarxe/Back-to-the-Drawing-Board-MK3.craft

Had a quick look at your craft. Good news: It wasn´t exploding every time i tried to launch it.

But something was still exploding: the tailplane. Having those wings right next to the engine exhaust might not be the best idea

The wingsize is ok, but you need to do something about the engines. If you have them like this, they will cause to much drag

[Laie]

That is likely old advice, in 1.0.2 its much more about targeting certain pitch angles up at certain altitudes.

Nope, it's as good advice as it ever was. In effect it prescribes an ascent profile... that is different for every rocket, depending on TWR and drag, but works out well more often than not. Much like Mechjeb's 50% ascent profile, it is not perfect or in any way best, but reasonably efficient for almost any design.

Checking your time-to-apoapsis (TTA for short) allows you to find a suitable pitch on the fly, good when you're controlling an unfamiliar or specialty rocket.

Try to fly your rocket so that once your TTA reaches one minute, it stays there. Control TTA by pitch at first, and if necessary by throttle. Doing so will teach you a lot about how ascents work. Your climb rate, horizontal velocity, TWR... they all play into TTA. It's one of those things that are much easier to learn by doing, rather than reading. Within due time, you will begin to form your own ideas about how to do it better and start to abandon the one-minute rule. As I said, it's not perfect... but a good starting point.

For planes, TTA can be a very important guideline, but it's not as simple as "keep it pegged at 1m and everything will work out".

[cybersol]

Nope, it's as good advice as it ever was.

I disagree completely here. This was the key advice for the old style of lingering to milk your air-breathers, which really doesn't work any more. In the new aero with any actual payload, you won't get near 1 min to AP until after you go transonic and are at the very edge of the air. By then its not bad advice for the brief time you can use it; but you are never going to space if that is the only advise you use so that you fail to get through the transonic region effectively.

[ajburges]

It's still good advice if you know what it's for.

An important aspect of rocket gravity turns is the Oberth effect. You get more kenetic energy thrusting prograde and prograde thrust is also the most efficient way to raise velocity. Keeping Ap a fixed duration ahead of you while increasing orbital period means you are thrusting closer to Ap WRT orbital period the longer you burn. Being closer to Ap means prograde is closer to a tangent of the surface. Burning strictly (near) prograde and building horizontal velocity as you climb to Ap is what gravity turns are all about!

You have less than 1% atmosphere after 40 km anyway so drag isn't a huge item there.

To clarify the 1 minute to Ap rule is not a good rule if you need non-prograde forces still. Space planes transitioning to anaerobic mode need to get out of there or use lift to allow low TWR engines to build speed. However radial thrust trends to hurt them more than help anyway. They are better staying near the prograde circle even to pull up.

You shouldn't see much issues with Ap running away from 1 minute under 30 km unless you over-designed your craft, are late with your gravity turn, or not aggressive enough with your gravity turn.

As a hint if you never touch 100% throttle using this rule, you can give the stage less relative engine mass for a dV/cost savings.

Edited June 12, 2015 by ajburges

[guitarxe]

I find that if I take a very shallow ascent profile, and reach a target of 75km AP, I am at 40km alt with about 5 mins to AP. If I just coast from this point on, I will lose a lot of speed by the time I reach AP, which will actually dip below 70km unless I keep pumping it up.

What's better to do in this case? Lower thrust such that I burn for much longer, but at lower thrust, while keeping the time to AP lower, or should I take a steeper ascent profile and pay more dV at the circularization burn? What will end up less in total?

[Laie]

@guitarxe: in that situation, crawling to AP at low throttle is better than kick-and-coast. If we're talking about a rocket, it would be better still if you you kept the TTA much lower to begin with.

As I said a few posts earlier: just for the hell of it, try to (mostly) ignore all other data and steer by to time-to-apoapsis. Start your gravity turn as usual until your TTA reaches one minute, then use the throttle to maintain TTA, and see what happens.