"Return-to-KSP" SSTO design(s) & questions

[Solarapple]

So after a summer-long hiatus I've decided to return to KSP to conquer Kerbin's moons, Duna/Ike, and Laythe. So I'm gonna have an SSTO carry base parts which are then taken and landed by a tug and a sky crane. Here's the issue: solar cant into space. I'm currently trying to make an MK2 spaceplane design for carrying 2t to orbit and returning. When burnout happens, I'll run out of rocket fuel at 25 kilometers above Kerbin. I've tried these ascent profiles:

• 45 degrees on takeoff, 25 degrees/10 km, 10 degrees/15 km
• 40 degrees on takeoff, 35d/600 m/s or 5 km, 30d/1000 m/s or 10km, 20d/? m/s or 15km, leveling at burnout
• Same as previous but no leveling (20 degrees/20km)
• 30 degrees entire flight

SSTO designs:

• 6x Rapiers, 8x air intakes, very fast
• 4x Rapiers and 2x 45s, 4x air intakes, very slow (not pictured)
• 4x Whiplashes and 2x 45s, 6x air intakes, in-between speed 

Images below:

Spoiler

If it's not working: http://imgur.com/a/mJ3dc

 

So I need help perfecting this design because I also want to make an Mk3 part 40t ssto to go along with it.

What are good parts for SSTOs depending on design?

• Engines (rocket and jet)?
• Air intakes?
• Wings and control surfaces?
• Stability (SAS)? My SSTOs are pretty agile because of the 1.25m SAS.

What's a good spaceplane design for SSTOs?

• CoM and CoL distances?
• Position of wings?
• Fuel weight? Fuel oxidizer amount?

And lastly, What is the best ascent profile depending on design?

Please respond if you have answers to ANY of these questions. I could really use the help.

Edited December 15, 2015 by Solarapple

[SpaceplaneAddict]

Best ascent profile? 

 

 

You're going WAY too steep. My way to rise up is to pitch according to thrust. Below 15km, right click an engine and see if it is gaining thrust. If not, pitch down.If it is, continue to pitch up until the accelaration is nice and steady. Pitch down to 5 degrees at "speed run" altitude. Pitch to 10 degrees when rockets ignite.

[Rune]

Yup, these days shallow ascents win the day, and the best ascents usually have very little pilot input. My go-to flight manual in 1.05 is to leave the runway at 5ºAoA, let it build height, then at 10,000m levelling to 5ºAoA again by briefly switching to prograde autopilot, then doing nothing until the RAPIERs give out. That is ideal for low TWR SSTOs (TWR 0.5), and low TWR SSTOs are the ones that get the awesomest mass ratios. It may seem slow, but in the end the climb to orbit is still about 15 minutes, because when you go fast, you climb fast even if you go shallow. If you find that you are blowing up following this tip, then you have way too much TWR, but if you still want to go that route, just increase the second angle slightly, or don't level off at 10,000 at all.

As to engines, look nowhere but RAPIERs. They may seem underpowered when you take off, but anything more than 0.45 TWR, and you can get to a ~1,300m/s airbreathing cutoff speed. And that is the key, they have the highest cutoff speed available, meaning they leave the smallest job to the rocket.

Intakes are easy nowadays. Basically anything that keeps the engine lit will do. The best results are apparently with a single shock cone per RAPIER, tough, low drag and enough air.

Wings, whatever floats your boat, but he BigS line offers handy fuel tankage "for free". Check their heat tolerances while you are at it.

And general design principles... well it depends on the mission. But some universal tips are to check the CoL/CoM with empty tanks, like it'll be when you land (you'd be surprised how often that gets overlooked), and to limit yourself to a bit under 20mT per RAPIER for best payload fraction results. The fuel/payload ratio depends entirely on the design dV, but if we only count "ground to LKO", payload fractions of around 30% are perfectly doable. The LF/LOX level is something best done by trial and error, noting fuel levels on orbit and adjusting accordingly... and that's pretty much it. The rest is experimentation, so have at it!

 

Rune. Did I hear someone asking for SSTO advice? To the rescue!

[Wanderfound]

One shock cone per RAPIER is less than ideal these days; one intake per pair of engines is closer to the mark. But they have to be the right intakes.

At speed, a single shock cone can supply three RAPIERs; stationary, it can just barely supply one. So, if you're running nothing but shock cones, using enough intake to keep the engines lit on the runway will result in having way more intake than you need at speed. Shock cones work best when mixed with some other intakes that can compensate for their runway weakness.

You need to look at the stationary speed of the intake, the air supplied by the intake at speed, and the air required by your engines. The high-speed engines don't actually need that much air; the real air-guzzlers are the high-bypass turbofans (Goliath, Wheesley). Most intakes are specialised for high speed (e.g. shock cones) or low speed (e.g. circular intakes).

Good low-speed intakes have a higher stationary speed; usually, these intakes aren't great at high speed, but there are a couple of crossovers. The most obvious of these is the intercooler; it has good high speed performance, but also sucks enough air to keep a pair of jets lit on the runway. As a rule of thumb, a shock cone is good for three jets when fast, one when slow. An intercooler will do two jets, fast or slow.

This one is running a pair of Whiplashes on nothing but a single intercooler, and it'll keep the engines whirring happily up to the conventional jet ceiling:

 

That's a FAR ship, so the flight profile shown may not work for stock (or for anything with sensible TWR in FAR; that 'un is rather overpowered), but the information re: intakes applies the same in stock or FAR.

Edited December 15, 2015 by Wanderfound

[Solarapple]

Thanks for all of the advice so far! Though there is one more question about delta v. Whenever I switch to rockets I'll always have only around 1000 delta v to work with. How do I make sure I have plenty when I switch to rocket mode?

[Val]

I wrote some ascent profile tips in the SSTO runs out of fuel thread.

There are also some examples craft, but if you're looking for pure RAPIER builds this one is probably more relevant.

And there are more crafts links, including ascent profiles, in my signature.

Hope this can be of help.

[McFarnsworth]

I'll try and give you some specific examples and base my comments on the plane in your first screenshot.

- ditch the rearmost air intakes, all of them. They cause massive drag and you don't need them.

- replace the front intakes with the shock cone ones, and keep the precoolers. That should give you a very low drag solution to power 6 rapiers.

- put the cargo bay where the CoM is, so it doesn't screw up your plane's balance on reentry with an empty bay.

- replace (at least) one center lfo + ox tank to a pure liquid fuel one, you're carrying way more oxidizer than you need if you do the ascent right.

- remove the fuel lines, rapiers don't need them, and they cause drag.

Doing all of the above should get your plane comfortably into orbit. You can probably get away with using only 4 rapier engines even.

As for the ascent, depends a bit on the plane but there's actually very little control input needed. Choose a starting angle where the plane will reach a little over 300m/s at an altitude of around 7000m (doesn't nearly have to be accurate, these are just ballpark figures). Once there, pitch down to around 5° - 10°. You'll pick up speed while the planet rolls away from you. (if you start overheating your plane is too powerful or your angle is too shallow) Switch to rocket mode when the speed starts to drop.

[The Optimist]

On December 15, 2015 at 5:30:51 AM, Wanderfound said:

One shock cone per RAPIER is less than ideal these days; one intake per pair of engines is closer to the mark. But they have to be the right intakes.

At speed, a single shock cone can supply three RAPIERs; stationary, it can just barely supply one. So, if you're running nothing but shock cones, using enough intake to keep the engines lit on the runway will result in having way more intake than you need at speed. Shock cones work best when mixed with some other intakes that can compensate for their runway weakness.

You need to look at the stationary speed of the intake, the air supplied by the intake at speed, and the air required by your engines. The high-speed engines don't actually need that much air; the real air-guzzlers are the high-bypass turbofans (Goliath, Wheesley). Most intakes are specialised for high speed (e.g. shock cones) or low speed (e.g. circular intakes).

Good low-speed intakes have a higher stationary speed; usually, these intakes aren't great at high speed, but there are a couple of crossovers. The most obvious of these is the intercooler; it has good high speed performance, but also sucks enough air to keep a pair of jets lit on the runway. As a rule of thumb, a shock cone is good for three jets when fast, one when slow. An intercooler will do two jets, fast or slow.

This one is running a pair of Whiplashes on nothing but a single intercooler, and it'll keep the engines whirring happily up to the conventional jet ceiling:

 

That's a FAR ship, so the flight profile shown may not work for stock (or for anything with sensible TWR in FAR; that 'un is rather overpowered), but the information re: intakes applies the same in stock or FAR.

I see that your ship has multiple stages.

 

Therefore, as a definition kraken, it is not a SINGLE STAGE to Orbit craft, but more of a jet-assisted spaceplane.

Please don't take offence :P.

[PhylumCnidaria]

I'm pretty sure that that's because he wants to trigger engines at different times without using action groups for some reason...

[Val]

6 hours ago, The Optimist said:

I see that your ship has multiple stages.

 

Therefore, as a definition kraken, it is not a SINGLE STAGE to Orbit craft, but more of a jet-assisted spaceplane.

Please don't take offence :P.

That is not how SSTO is defined.

A multi stage vehicle can be an SSTO as long as it only uses a single stage to orbit.

And for SSTO, staging refers to whether or not stages are dropped using decouplers or similar. Activating the engines in stages does not preclude being an SSTO.

[Wanderfound]

8 hours ago, PhylumCnidaria said:

I'm pretty sure that that's because he wants to trigger engines at different times without using action groups for some reason...

Actually, it's because I always toggle the engines with action groups, so I frequently forget to look at the staging at all.

That one looks like I did set it up for career players, though; staged activation works for spaceplanes when you don't have action groups to light the fireworks with.

[Rune]

On 15/12/2015 22:04:27, Solarapple said:

Thanks for all of the advice so far! Though there is one more question about delta v. Whenever I switch to rockets I'll always have only around 1000 delta v to work with. How do I make sure I have plenty when I switch to rocket mode?

Ok, how do you pack enough dV? Well, I use handy rules of thumb. The sweet spot on the rocket equation is at a mass ratio of ~2.73 ("e"), so I play it safe and go for 3, but ignore tankage. That means about 3 times more fuel tank weight than payload, or 66% of the weight being fuel. That would get you more than ten times the Isp in m/s, very roughly. VERY roughly, I often play fast and loose with this "rule". But it means than anything below half the takeoff weight in fuel is a no-go, because then I'm way under 2km/s, unless I have a very good reason for that. Then you figure out how much liquid fuel you need to get that to rocket takeover conditions... I do that mostly by trial and error, though I have some other rules of thumb... but they are not even this "precise", so I encourage you to look for your own.

 

Rune. A good experiment is worth a thousand theoretical explanations.