Can someone explain an SSTO flight profile?

[Jestersage]

So far I am adjusting my next craft (a SSTO), and I noticed that while it is successful, it is quite different from a rocket's profile.

With rocket, AP, PE, and Surface/orbit speed just keep increasing. Any of these not increasing means the craft failed for the most part.

With SSTO, I found that it will climb to an AP of 35k, then it will decrease... but if I kept it oriented +15~+30, then eventually the AP will increase again followed by PE, allowing circulization. (I did not check whether the speed increase or decrease between the two AP).

So may I ask how does an SSTO fly in KSP? Is this "2x AP" normal?

[bewing]

There are two philosophies of SSTO spaceplanes in KSP. There are designs with plenty of wing, and designs with very little wing. Your ascent profile depends strongly on which philosophy you follow.

However, it sounds like you are climbing too fast. It's best if the Ap stays in front of you always. If you get too high while you are still too slow, then you will start to fall back down into denser air again -- usually that will really kill your rocket performance.

I go for the big wing philosophy myself. Which leads to an ascent like this: after launch, only use the F key to adjust pitch. Climb at a little over 10 m/s until 5km altitude, at low thrust and less than 5 degree pitch. Go to full thrust, break mach 1, climb 1km for each 100 m/s of speed, at 14km start bringing the nose down to allow a mostly horizontal speed run in airbreathing mode. As the jets approach max speed, allow the nose to rise. At just past peak speed, activate the rockets. Allow the nose to rise naturally and the jets to flame out. Then the key is to keep just enough pitch on the nose to climb while still increasing your speed. Then all you need to do is climb fast enough to avoid overheating and melting. Above 40km altitude, lock to prograde and finish the climb to orbit. This whole process can easily take over 15 minutes of flying time, but will get big payloads to orbit with very efficient fuel usage.

The small wing philosophy is totally different. It goes something like: raise your pitch to 45 degrees, go to full thrust, and coast then circularize when your Ap gets above 70km. This takes much more in the way of engines, and is therefore not so good for payload fraction.

 

 

[Jestersage]

EDIT: But I though F is "Invert SAS state?"

Edited November 14, 2019 by Jestersage

[Vanamonde]

Both rockets and spaceplanes reach a point in their ascent at which they're travelling close to to horizontally while building up to orbital speed. The difference is that rockets go mostly up to get there and then tilt over while planes gradually climb over a distance, then start to lose lift and thrust due to thinner air, and have to switch to rocket propulsion and try to achieve orbital speed starting from lower in the atmosphere. That's why planes are so much harder. They need enough wing to get lift early in the flight and thrust isn't terribly important, and then later in the flight wings just add drag and weight and you need to power through with thrust. Flight profiles for rockets are all pretty much the same, while those of planes will vary according to the plane's relative values of lift, mass, drag, and thrust. I don't think there is one right method for planes, but the rule is to get as high and as fast as you can on jets and wings, and when those start to become ineffective, switch to rockets before you lose speed. My design process was to build a rocket which could reach orbit starting from around 30,000m and then add enough wing to get it up there. 

[bewing]

6 hours ago, Jestersage said:

EDIT: But I though F is "Invert SAS state?"

When you launch a spaceplane, you always turn SAS on first -- unless you want to fly it with a joystick (which is usually annoying). So the function of the F key in that case is to momentarily turn off SAS. If your spaceplane is built to be stable then flashing the SAS to "off" for a moment should always cause the nose to drop. You can watch your "pitch" control indicator to make sure you always have positive pitch control.

 

[TheFlyingKerman]

Everyone prefer different profiles for their space planes. Usually I'd climb to certain altitude and accelerate to about mach 1. Then just switch to prograde lock and watch. I fine tune the wing incident so that the plane does a flat speed run, and start rocket engine when it reaches a favorable speed/altitude/pitch (3-5 degrees for Rapiers).

[DeadJohn]

I find that the flight profile for an SSTO spaceplane varies depending on its thrust to weight ratio (TWR) and drag:

• If it has a high TWR, enough to climb while accelerating through the sound barrier (roughly 340m/s) and continue accelerating all the way until the air-breathing engines run out of air, I'll climb at around 15 degrees. Heat buildup at the nose can be the critical factor; climb more steeply to reduce acceleration and heat.
• If it has a low TWR and high drag, with difficulty accelerating, I'll climb slowly and steeply to get to 10km where the thinner air decreases drag, level off to accelerate through Mach 1, then increase my climb to 5-10 degrees.

A lot depends on the specific engines you're using because different engines run out of air at different altitudes. With dual-mode CR-7 RAPIER engines, get to 20km however you can, then gradually climb to build up horizontal speed. Thrust will drop due to very thin air yet speed will continue to increasing with less drag. Watch your speed indicator; when it peaks switch the engines to rocket mode and nose up to maybe 20 degrees to increase your climb rate and continue accelerating.

When my AP reaches 75km I aim prograde, turn off engines, and coast higher. If drag drops my AP below 70km I'll do a boost burn to get back to 75km. Finally, when getting near AP, burn more to raise PE. This part is similar to normal staged rocket flight.

[Jestersage]

Thanks for the response, everyone.

Of course, in my way, I have to go with one of the most difficult SSTO: Liquid fuel Only SSTO that is hauling an Orange Tank.

I ended up using a variant of bewing's profile, except it goes full thrust. While a few of the early test gives me 1400 m/s before needing rocket assist, as of recently somehow he drops to 1350 m/s, which does eat into the overall 75k delta-V (530 m/s --> 200 m/s)

Right now I am fine tuning to see any parts cause drag, or have a more accurate flight profile.

[Laie]

1 hour ago, Jestersage said:

Of course, in my way, I have to go with one of the most difficult SSTO: Liquid fuel Only SSTO that is hauling an Orange Tank.

That usually calls for the "big wing" approach. Here's my recipe:

1.  climb "as fast as reasonable" from the get-go. There's no point in going hypersonic in the lower atmosphere (unless you have to, which is the case with many small-wing planes). However, striving for too much altitude early on is also lossy. My rule of thumb is that I should accelerate at least something like 1 - 1.5m/s², that is, it should approach the sound barrier when the mission clock shows 3-5 minutes. Any excess power beyond that can safely go into climbing.
2. break the sound barrier. This usually involves reducing climb, maybe even a shallow dive.
3. pull up, up, up, but don't overdo it. Ramjets and Rapiers gain tremendous power once supersonic, and at first you want to direct most of that into more altitude. Quickly you will climb at 200 m/s -- but beware, you want to level out again somewhere about 20km. If you're still climbing 300m/s at 15km, sheer momentum will carry you out of the breathable atmosphere.
   On my designs, I usually cross mach-2 (700m/s) at 10-12km while climbing 200m/s, from there I can essentially follow prograde and will end up in something like level flight.
4. Now, see to it that you still accelerate at a steady clip -- that's the point where things really start to differ between designs, and where sheer experience becomes important. You'll develop a gut feeling for how fast you need to be accelerating, and how quickly you may climb without eventually starving your engines.

While ramjets & rapiers  lose thrust with altitude, but for a good long while the thrust doesn't go down as quickly as air pressure. You will have much more thrust/drag at 15 km than at 5km -- that's why I recommend a speedy climb. However, when too slow you will have too little lift and need to angle up more, hence suffering from more drag. It's important to keep things in a kind of balance. Big wing planes can usually afford to climb first and level out for acceleration later, while small-winged ones sometimes need to go supersonic before they can climb at all.

For a good long while, the jets will also provide more power the faster they go. This is especially pronounced with the rapier. When small-wing planes finally go supersonic, they find themselves with lots of excess power and can both climb and accelerate at the same time. For those, it's often only a question of finding the right pitch angle to hold and maintain all the way through.

LF-only planes typically need more wing because the nukes have little thrust. They're dependent on wings keeping them in the air while they slowly accelerate to orbital speed.

[Jestersage]

15 minutes ago, Laie said:

That usually calls for the "big wing" approach. Here's my recipe:

1.  climb "as fast as reasonable" from the get-go. There's no point in going hypersonic in the lower atmosphere (unless you have to, which is the case with many small-wing planes). However, striving for too much altitude early on is also lossy. My rule of thumb is that I should accelerate at least something like 1 - 1.5m/s², that is, it should approach the sound barrier when the mission clock shows 3-5 minutes. Any excess power beyond that can safely go into climbing.
2. break the sound barrier. This usually involves reducing climb, maybe even a shallow dive.
3. pull up, up, up, but don't overdo it. Ramjets and Rapiers gain tremendous power once supersonic, and at first you want to direct most of that into more altitude. Quickly you will climb at 200 m/s -- but beware, you want to level out again somewhere about 20km. If you're still climbing 300m/s at 15km, sheer momentum will carry you out of the breathable atmosphere.
   On my designs, I usually cross mach-2 (700m/s) at 10-12km while climbing 200m/s, from there I can essentially follow prograde and will end up in something like level flight.
4. Now, see to it that you still accelerate at a steady clip -- that's the point where things really start to differ between designs, and where sheer experience becomes important. You'll develop a gut feeling for how fast you need to be accelerating, and how quickly you may climb without eventually starving your engines.

While ramjets & rapiers  lose thrust with altitude, but for a good long while the thrust doesn't go down as quickly as air pressure. You will have much more thrust/drag at 15 km than at 5km -- that's why I recommend a speedy climb. However, when too slow you will have too little lift and need to angle up more, hence suffering from more drag. It's important to keep things in a kind of balance. Big wing planes can usually afford to climb first and level out for acceleration later, while small-winged ones sometimes need to go supersonic before they can climb at all.

For a good long while, the jets will also provide more power the faster they go. This is especially pronounced with the rapier. When small-wing planes finally go supersonic, they find themselves with lots of excess power and can both climb and accelerate at the same time. For those, it's often only a question of finding the right pitch angle to hold and maintain all the way through.

LF-only planes typically need more wing because the nukes have little thrust. They're dependent on wings keeping them in the air while they slowly accelerate to orbital speed.

Thanks for the pointer.

It's already running in big wing mode (dark screenshot): https://i.imgur.com/oSOsgqt.png. May have to test out a bit more before I can push it: During one run I have 400 m/s left in 80k orbit, but not good enough for a rendezvous with a spacestation at 125k with an orange tank as cargo. So right now, I am trying to have a consistence profile that other people can use too.

[Laie]

9 hours ago, Jestersage said:

So right now, I am trying to have a consistence profile that other people can use too.

First find one that works for you

For most of stage-4 and beyond in my listing, I'm again looking at acceleration. But it's not possible to put a number to it as I did on stage-1, too much depends on circumstances.

Perhaps you should do a few flights where you don't try to go to space, but instead just find out how fast your plane can go. Take your time, perhaps turn off heat so that you can go forever without blowing up. Maybe also infinite propellant so that your pane no longer loses mass as it burns fuel. Just maintain 20km and wait until airspeed settles down, then climb very very slowly and see how things change with altitude.

The "atmosphere autopilot" mod is great for experiments like these.