• 0
martinborgen

SSTO planes, profile

Question

Hello all. Ive recently been refining my spaceplane building and flying (particularly cargo lifters), and come to a question regarding ascent profile.

I normally keep a quite shallow ascent to keep the speed up for the rapiers to keep their 200+ Kn of thrust. When nearing the 10 Km mark, i try to gain as much speed as possible with a shallow climb. Now at what point do you switch the rapiers to close cycle mode? I've found that I can get up to 1520m/s depending on the heat tolerance of the parts used, but the last ~40m/s are gained very slowly. Am I better switching to close cycle at some point before this?

 

And the main question: when switching to close cycle, do you pitch up, if so, how much? I've found that often I loose a lot of speed in the pitch manouvre, and I feel like I might just keep going straight  (actually about 5-10 degrees of positive pitch)- the curvature of Kerbin and the speed you'redoing by now will guarantee you leave the atmosphere soon anyways. 

 

Share this post


Link to post
Share on other sites

12 answers to this question

  • 1

Posted (edited)

On 3/20/2017 at 6:47 AM, AeroGav said:

Flying straight into prograde means zero lift so you spend longer in the atmosphere, meaning more drag  overall despite the slight reduction at first.       

Just wanted to make a note here. In all my post-1.2 testing, as long as you're above 22km or so, gravity losses almost always outweigh drag, so the faster you're going horizontally, the better. Even if this means staying in the atmosphere longer. My most efficient runs, I actually get my PE above 0 while still below 40km in altitude. My circularization burns barely exist. This does, of course, depend on having a low-drag aircraft, but if you're following prograde, even mk2 parts are pretty low-drag.

Edited by Jarin

Share this post


Link to post
Share on other sites
  • 0

First off, as oon as you DO switch, stay prograde. You'll want to keep minimal drag to build up speed faster until you hit a good apoapsis.

As for when you switch, it varies from plane to plane. You'll want to download KER or MechJeb, or equivalent to actively find your TWR during flight. Then, fly a test. Once your TWR drops to about 1 or 0.8, that's when I'd switch. Do as many tests as needed to find the sweetspot :)

1 person likes this

Share this post


Link to post
Share on other sites
  • 0

This question about how to play the game has been moved to Gameplay questions. 

Share this post


Link to post
Share on other sites
  • 0

I usually just lock in to a 5-15 degree climb (depending on TWR/thermal) until my apoapsis is out past 100km, then coast. I do the cycle swap once my absolute speed starts to drop.

1 person likes this

Share this post


Link to post
Share on other sites
  • 0

I agree with foamyesque about doing the mode change when the speed starts to drop, because airbreathing is so much more efficient than closedcycle. I think you want to switch to prograde as early as possible (pitching down, not up, and generally using the F key to do it) after the mode change. But "as early as possible" depends on the heat tolerance of your parts. As soon as you go prograde, your speed will start climbing rapidly -- and that can melt your nose if you do it too soon.

Share this post


Link to post
Share on other sites
  • 0

Man, many of us could write on this topic for days.  First, if you can post some screenshots and ideally the ship itself to kerbalx, you'll get much more specific and useful feedback.

I build my spaceplanes climb at ~10 degrees, reduce a bit to 7-8 at 11-12km, switch over when the speed starts to drop, then follow prograde.  Implicit in that profile is having the right combination of lift, drag, and power - achieved with a lot of testing and redesigning.  1520m/s top speed on jets is good - (I'm finding that 1550-1650 is achievable without exotic construction by paying LOTS of attention to reducing drag - I hide EVERYTHING in cargo bays).   But 1520 is good work.  I use more engines than some (my usual heavy lift SSTO runs 10 rapiers to lift 76T with a 140T fueled airframe) and get to orbit quickly and without heat problems.  

If you have to pitch up to maintain your climb, I would say you need to add power, reduce drag, add lift, or all three.  Mk3 profiles (I'm assuming that's what you're talking about since you said cargo lifters) have AWFUL drag from the fuselage if you're pitched up - it really gets simple if you do whatever it takes so that you can do your burn for orbit on prograde.  Use static wing incidence and just the right amount of wing, not too much or too little.  Add enough power to push apo during that burn but not too much dead weight in engines.  Hide all the extra bits and bobs in cargo bays.  Delete or reduce everything that isn't absolutely critical.

Share this post


Link to post
Share on other sites
  • 0

If you have nukes on-board then their Isp/thrust gets useful somewhere above 10km. As your air-breathing RAPIERS start to fade towards 20km, you can fire the nukes to fill the gap until you switch the RAPIERS to closed cycle. This can overcome the drop in hard-earned speed you often see at this altitude. 

Share this post


Link to post
Share on other sites
  • 0

Untitled_zpspctazepz.png

This is the generic flight profile I use flying to orbit.    

Subsonic climb - Keep speed below 240 m/s as this marks the start of the high-drag transonic region.   Keep the nose less than 5 degrees above prograde.    At lower altitudes, you may in fact use nose angle just 2 or 3 degrees above prograde, if your craft has a good amount of lift.

Transonic Acceleration - When it becomes impossible to prevent airspeed exceeding 240 m/s without pulling the nose more than 5 degrees over prograde  , then it is time to go supersonic.   Reduce the amount of nose-up input so you are flying with the nose less than 3 degrees above prograde.  If you're over 5km, toggle the nukes on until over 440m/s.

Supersonic Climb - Once over 440m/s, turn the nukes off and pitch up again so the nose is 5 degrees above prograde.   Aim to level off at the acceleration altitude.

Speed Run - Accelerate in level flight to the highest practical top speed.     RAPIERs act like ramjets in many ways, producing more power as you get faster, but even so they do peak at mach 3.75 (1150 m/s) and after that thrust gets weaker the faster you go. The decline is quite slow out to mach 4.3 (1350 m/s) but becomes more rapid after that.   Which altitude for the speed run?  I prefer to fly higher, where drag and heat are less, but no more than 22km, as above that thrust falls faster than drag, so getting higher actually reduces top speed.   When is it time to quit and switch to rocket mode?     I tend to fly with the Aero Data GUI enabled (press alt-f12, physics tab, tick enable aero data gui).   When drag exceeds 75% or more of total thrust, it's time to switch.  

Climb to Orbit - Now that the rocket engines are active, just maintain a nose angle of 5 degrees above prograde.  This gives best lift:drag ratio in supersonic flight and will give lowest total drag.     Flying straight into prograde means zero lift so you spend longer in the atmosphere, meaning more drag  overall despite the slight reduction at first.       

2 people like this

Share this post


Link to post
Share on other sites
  • 0
1 hour ago, AeroGav said:

Flying straight into prograde means zero lift

Not if you have incidence on your wings.. :)

 

2 people like this

Share this post


Link to post
Share on other sites
  • 0

I usually skim the surface until I'm at around 300-400m/s (assuming I'm using R.A.P.I.E.R.s; I skip this step on my Whiplash and Panther-based SSTOs) and then pull up into a fairly steep climb (as close to 45 degrees as I can manage) until around 10km. Then I go almost completely level (with only a slight climb,) set my SAS to stability hold and get as much speed as I can on my jets. Due to the curvature of Kerbin my climb will naturally increase during this period, and if I set things up right I'll be getting close to my engines' maximum speed and maximum altitude at about the same time. When my speed starts to decrease I switch to closed-cycle (or activate my rockets if I'm not using R.A.P.I.E.R.s) but I don't pull up. I keep going under rocket power until my AP gets to about 65km, at which point I activate prograde hold, cut throttle and coast to about 60km. At 60km, I throttle up again, still pointing prograde (which will be nearly horizontal at this point) until my AP breaks 70km. Then I cut throttle again and coast up for a final circularizing burn (which won't be that long since I'll already have a lot of horizontal velocity by that point) at AP.

I'm not sure if this is absolutely the most efficient way of getting to orbit, but it works for me.

Share this post


Link to post
Share on other sites
  • 0
4 hours ago, Jarin said:

Just wanted to make a note here. In all my post-1.2 testing, as long as you're above 22km or so, gravity losses almost always outweigh drag, so the faster you're going horizontally, the better. Even if this means staying in the atmosphere longer. My most efficient runs, I actually get my PE above 0 while still below 40km in altitude. My circularization burns barely exist. This does, of course, depend on having a low-drag aircraft, but if you're following prograde, even mk2 parts are pretty low-drag.

True, an ascent that's as shallow/close to a straight line as possible is best.     But,   you statement there makes it sound like lift is a bad thing and that ideally you'd decouple the wings at the moment you switch modes.   If you have a lot of lift potential, I say use it, by pitching to a 5 degree AoA.

If you watch this video from the 4 minute mark,  I am flying level at 22km, which is the altitude for getting max speed at level flight on the RAPIER.     My AoA is only 1.5 degrees, to keep us from climbing.  By 1400 m/s. drag has risen to 75% of my thrust.    This means that even though my RAPIERs have 4x the ISP of the nukes,  I am better off starting them  when three  quarters of that thrust is being cancelled out by drag.

When I start the nukes,  I add nose up pitch trim  (hold down the ALT key and tap S) until the craft settles into a 5 degree positive AoA.   This is the AoA that gives best l/d ratio in supersonic flight.   Drag rises from about 60kn to about 105kn, but lift rises by a lot more, our l/d ratio goes from 2.5 to one to nearly 4 to 1.    The increase in drag is temporary  - in less than 30 seconds we gain 3km in altitude, which sends our drag below 60kn again.

Conclusion - lift is a good thing, you just want to make sure you're getting the best possible trade when you use them to exchange drag for lift.  Once you hit closed cycle mode and are no longer holding the plane artifically low to so the jets can breathe,  maintainng the best l/d is the only consideration.

Share this post


Link to post
Share on other sites
  • 0

Posted (edited)

It depends on the plane; different ships require different profiles.

If you're flying with RAPIERs, though, 30,000m is the altitude you need to be thinking about. You don't want to switch to rocket mode until you've extracted the last possible Newton from the jets, and about 30,000m is where they die.

You want to hit that altitude carrying as much horizontal speed as possible. However, in a draggy plane like a big cargo bus, you also want to hit it with a fair bit of vertical speed to get you out of the high-drag zone as quickly as possible.

If it's a plane with enough thrust/drag that it doesn't need to level off to get through transonic, what I like to do is this:

1) a 10° climb until 10,000m.

2) Drop the nose to the horizon (0°) and crank the speed up to 1,500m/s.

3) Lift the nose to 15°. Keep the jets airbreathing until switching mode at 29,000m.

4) Keep the nose at 15° until the apoapsis reaches 55,000m, then drop it to surface prograde and keep it there until you reach space.

(note, I build wing incidence into my planes; add 5° or so to those angles if you're in a ship that requires substantial AoA for level flight. You want the drop in step 2 to almost completely negate your climb rate, but not actually put you into a dive)

If it's a plane that does need to level off to break the sound barrier, then you just add an extra couple of steps at 5,000m or so where you level off, accelerate to 450m/s, then resume climbing.

You need to be careful of overheating in step 2 (just raise the altitude if it's too hot) and you'll lose a bit of speed during the change of direction in step 3, but the lower the altitude at which you do step 3 the more time your jets have to regain that lost speed.

Get it just right and you can go past 20,000m at 1,600m/s with the apoapsis already well over 30,000m.

Edited by Wanderfound
1 person likes this

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now