SSTO climb profile

[cpt_nosh]

Hi, as many others (I hope at least) I have my difficulties finding a good climb profile for my SSTO. Currently, I'm using Rapiers and normally end up at around 13k feet speedlocking at around 300m/s.

I read and seen some ideas about diving and then pushing through, and I know the best speed for that engine is over mach 3. Can anyone give me a hint?

[EtherDragon]

If you are speedlocking at 300m/s at 13k feet, that is an issue.

Try holding at 10km, and accumulating some speed. You should be able to get 500k/s, or more, at that altitude. And as you climb toward 20km, you should be seeing speeds in excess of 1km/s. About that time is when the engine starts wanting to cut out (this is where I pitch up and go to Rocket Mode, usually...).

EDIT:

One thing I found that helps during the transition from Air-Breathing to Rocket Mode is to close the air-intakes. Their Drag goes down when you do this.

Edited May 5, 2015 by EtherDragon

[Guest Darth Gerbil]

Climb as fast as possible to the 13k mark, the level out with your prograde vector at 5 degrees. (You should reach 20k with about 1.5 kilometres per second then slowly pull up from 20K so that when you transition into rocket mode, you are already pointing at a steeper angle. When you transition, raise your apoapsis as quickly as possible because rapiers are rather inefficient as a rocket engine.

[Old Foxboy]

I guess your plane is constructed a) too heavy with too much drag and c) too few engines.

With the rapier, you should be able to kick through Mach 1 below 5km and go continuously up, accelerating while climbing. Don't try to achieve the jet stage ceiling, but rather try to achieve vmax at an altitude where you still have enough air + power for the engines. Then pull (gently) up, zoom into a steep climb and kick in the rocket stage when air is running out. This is the most efficient way of operating the Rapiers:

1st: Get them to optimum performance speed at a reasonable height as soon as possible to avoid wasting fuel down low&slow.

2nd: Fly into the danger zone of speed + heat.

3rd: Pull gently into a zoom and kick in the rockets.

[cybersol]

Helpful tips are that if you have fewer than 1 rapier per 13 tons or you have to drop altitude to punch through mach 1, you may be able to save fuel by adding engines (despite the engines extra weight). Also I've found that at around 0.0166 intake air per engine means you basically never have to worry about flame out.

I initially climb as fast as possible, keeping my speed below 270 m/s to stay under the worst of the mach 1 drag. Then between 12-14k and I switch to 0-5 degree upslope to gain speed to ~470-500 m/s. Being careful to stay above 450 m/s so as not to re-enter the mach 1 drag, I climb at 20-30 degrees to 17km where I level off to around 5-10%. I build up speed until the rapiers lose thrust above 22km and my speed acceleration heads almost to zero. There I switch rapier modes and close intakes and try to get above 32km as fast as possible. After 32km I point near prograde until main engine cutoff as my apoapsis rises above 75 km.

[mikeb30165]

Totally agree with Garth Gerbal, but for one odd thing: I discovered (by accident) that by holding 5 degrees while firing in closed-cycle mode from 29K onwards - not pulling-up to reach apoapsis ASAP - I got about 100 more delta-v into orbit. (at 29K, Rapiers auto-switch modes when using the new-shaped ram-inlets). Total speed would be about 2150K, dribbling off to 2050 by the time 70km was reached, leaving a circization burn of just 350K. According to mechjeb, total Dv used was about 4200 which is good for a space plane.

Not sure why this happens, but noticed the same behaviour, many times with other planes & also when when flying rockets. My guess is (probably wrong!) that the atmosphere is thinner then it used to be from 30K onwards so drag-losses while coasting are lower. Add in other factors like that the Rapier's ISP just about max's out from 30K, circulisation is closer to the ground, steering losses are lower, your nose is deviated less rom the petrograde during the early part of the coasting etc and you seem get a little more left in the tanks after ascent. (anyone else had this experience?).

To help with the original question, give yourself plenty of power (TWR of 1.3/1+ at sea-level, 4/1+ at 30K) and only put on as few wings/control surfaces as you can getaway with on your plane; the emulated trans-sonic drag, is very, very heavy and can stop an underpowered plane dead in its tracks at mach 0.99.

Hope this helps..

(Quick update: just seen Cybersol's suggestions: this is basically what I do too. works a treat!)

Edited May 5, 2015 by mikeb30165

[Old Foxboy]

My guess is (probably wrong!) that the atmosphere is thinner then it used to be from 30K onwards so drag-losses while coasting are lower.

Your guess ist right. Kerbin simulates a near-Earth atmosphere while being 10 times smaller than Earth and having an atmosphere of half the height, but the first 30km of altitude feel like flying at earth, so after 30km the atmosphere thins out dramatically fast.

[eddiew]

I guess your plane is constructed a) too heavy with too much drag and c) too few engines.

c) is very common in 1.0.2... old bi-rapier designs simply can't make orbit anymore.

If you happen to have 3 rapiers, replace the middle one with a turbojet and watch the thing soar to orbit in a blaze of orange fury If not, try to find space for a turbojet!

[Old Foxboy]

Well, i roughly did it with 3 rapiers on a shorter MK 2 craft with a cabin. Maybe if i build one more tank, optimize the relation between oxidizer and liquid fuel and eliminate piloting errors, i can lift a real payload or add a docking port. But yeah, exchanging one rapier with a turbo will save a lot of fuel for the initial climb.

[FlipNascar]

c) is very common in 1.0.2... old bi-rapier designs simply can't make orbit anymore.

If you happen to have 3 rapiers, replace the middle one with a turbojet and watch the thing soar to orbit in a blaze of orange fury If not, try to find space for a turbojet!

You don't need to find the extra space for a turbojet. Just make the middle Rapier (or all of them...) Turborapiers! They still work.

Although I have to disagree with you... I've yet to find any point where adding a Turbojet has been more effective than either amending the ascent profile or adding another Rapier... But, a) early days each to their own designs! c) I shall now have to add more Turbojets and see if that makes a difference on my fat unorbital pigs.

[Radam]

Best way I found so far is have a rapier or turbojet for every 12t of the craft, minimise drag, and have about 1:4 lifting surfaces to mass. And then just race to 250ms at the end of the runway point up to 20 to 23 degrees or so. And just wait untill it switches, maybe even switch manually. Then point the nose down so that apoapsis stays below 70km for as long as possible.

Btw my craft has 6 rapiers, 4 turbojets, 2 nukes...

[FlipNascar]

Best way I found so far is have a rapier or turbojet for every 12t of the craft, minimise drag, and have about 1:4 lifting surfaces to mass. And then just race to 250ms at the end of the runway point up to 20 to 23 degrees or so. And just wait untill it switches, maybe even switch manually. Then point the nose down so that apoapsis stays below 70km for as long as possible.

Btw my craft has 6 rapiers, 4 turbojets, 2 nukes...

But with 12 engines how much payload are you lifting?

[Radam]

But with 12 engines how much payload are you lifting?

36t of cargo and 120t craft at runway. Well I havent yet designed proper reentry systems.

[FlipNascar]

^ Props.

[Radam]

Also, mechjeb helps alot. Using Smart A.S.S. surface mode, punching in 90 degree heading and 20 degree pitch (thereabouts) results in a somewhat unstable (mechjeb is less stable than stock SAS) but handsfree ascent. At least until rocket mode.

[KarateF22]

If you are at 13k altitude locked at 250 m/s, you are actually probably fine, just unable to get over the transonic drag hump. I have a bi-RAPIER + single Nuke SSTO with a similar problem and my solution was to climb to 12.5k then descend to 10k in order to break the sound barrier. Once you break 500 m/s you can begin climbing again no problem as drag falls after 400 m/s while RAPIER thrust skyrockets.

[cpt_nosh]

So, thanks to you guys, I kinda figured out the ascending profile with air breathing engines. Though ran into some more problems. At what Altitude and Speed should I normally switch to rocket engines? Right now I normally don't have enough liquid and oxidizer to achieve a low kerbin orbit? Is there some kind of weight to fuel ratio for this?

Thanks in advance. I'm on my journey to regain my pre 1.0 SSTO capabilities.

[xandalis]

Climb as fast as possible to the 13k mark, the level out with your prograde vector at 5 degrees. (You should reach 20k with about 1.5 kilometres per second then slowly pull up from 20K so that when you transition into rocket mode, you are already pointing at a steeper angle. When you transition, raise your apoapsis as quickly as possible because rapiers are rather inefficient as a rocket engine.

Just tried this guideline on my SSTO I've been testing; previously, I'd made it up once or twice after several attempts at various pre-1.0 profiles, usually with less than 100 dV left to an 80kmx80km orbit. This got me up on the first shot, with about 200 dV left. The plane in question has 6 Rapiers, and weights about 60t at start. Wasn't a perfect ascent though, leveled out a little under 10km by mistake, so I nearly cooked off some parts during the 10-20km accel. Anyway, thanks for that tip; I'll probably tighten up my ascent on future tests, and also probably tweak the design a bit. The current design has 8 shock-cones and 12 pre-coolers (I'd been trying to get a 20+km cruise altitude), so I can probably shave those numbers down a bit.