SSTO essentials?

[airelibre]

I'm thinking of building SSTO plane, but I have no experience in this whatsoever. I don't want to copy anyone's design, I'd rather come up with my own idea, but it would help to know what kind of things are essential. How much fuel, how many engines, etc.

Edited September 26, 2014 by airelibre

[Claw]

Well, here are some basic rules of thumb that I found are a good place to start. Plus they provide some flexibility when designing. You sort of iterate your way througn them, adding and adjusting as you build.

- 1 TurboJet for every 12 tons. Max of 1 per 15 tons.

- 150 units of fuel per TurboJet. Add more or less as your skills allow.

- 3 ram intakes per TurboJet. Minimum of 1 per TurboJet, Maximum of whatever you want. But as you add more, you have to double the amount to see much difference.

- 0.5 lift rating per ton. Minimum will depend on your TWR, maximum of 1.0 lift per ton. (More than that adds a lof of drag and weight for not much help in lift.)

The way these rules of thumb work out is that if you increase something in one area, you can decrease something in another area.

- For eample, if you are using a turbojet and your craft is only 7 tons, you need less fuel and/or lift rating. Or maybe use just 1 or 2 intakes.

I also find it's important to have some idea of what you want to do. At least some rough idea of cargo payloads, range, orbital altitudes, and all that. Of course, you can always just build a plane for the sake of building a plane.

Edited June 15, 2014 by Claw

[Hodo]

Your first SSTO space plane should be minimalist. Without going all airhog you can build a nice SSTO space plane using a single fuel tank I think its the LFT-400, with a RAPIER engine and single ram air intake. But it also depends on what mods you are using how simple you can go.

But the simplest SSTO space plane I ever built was this.

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[LethalDose]

Five bits of advice for SSTO design:

1. Patience
   
2. Keep your CoM in front of your CoL. But not too much
   
3. Moar patience
   
4. Start simple
   
5. A lot more patience
   

To build an SSTO from scratch, it's gonna take a lot of time, and a lot of tweaks. And, in my experience, a lot of frustration. Don't get married to any single design concept. What worked best for me was to figure out what could actually get to orbit with trial and error, then adjusting fuel and slimming down the design, and then adding the parts to make the SSTO do what I wanted it to do.

It's also worthwhile looking at other people's designs. You don't have to straight up steal their designs, but they may have done things you'd never have thought to do. I think the trick to making my SSTOs work was putting the wings over the fuselage. I saw that on some other designs and I never would have come up with that on my own.

Good luck and clear skies.

EDIT: Also be careful with multiple air-breathing engines. Sometimes, they won't get equal amounts of air, which means unequal thrust, which leads to a spin and a bad day. Actions groups are your friends and can be used to shut down turbojets manually, and switch modes on the RAPIERs.

Edited June 15, 2014 by LethalDose

[Claw]

Here we go. It took me a few minutes to find these.

This article by Keptin is really good at explaining the basics of aero. If you don't know this info, it'll be really important to avoinding hours of frustration.

http://forum.kerbalspaceprogram.com/threads/52080-Basic-Aircraft-Design-Explained-Simply-With-Pictures

I also wrote an in-game companion to show most of the ideas from Keptin's article.

http://forum.kerbalspaceprogram.com/threads/65638-Basic-Airplane-Space-Plane-Aero-Tutorial

As you get some practice, you can dig around in "The Drawing Board" (Specialists thread) to find links to more good SSTO space plane design articles.

http://forum.kerbalspaceprogram.com/threads/28352-The-Drawing-Board-A-library-of-tutorials-and-other-useful-information

[G'th]

I'm presuming you're using FAR, because otherwise planes are dumb. I'm also presuming you're using either RAPIER-type engines or a jet engine/rocket engine combo.

Anyway, for the jet stage, don't worry too much about how much fuel/deltaV you pack into it. Even with non-overpowered jet engines, the fuel you'll be consuming compared to your rocket stage is miniscule. What really matters with your jet stage is TWR. A high TWR (but not too high, as FAR will tear your plane apart) will get you up to orbital velocity (OV) much faster and generally more efficiently, and it will save you some fuel for getting into orbit, as you'll be going faster and higher when the time comes to switch to rockets. Generally you should carry enough to get you up to your rocket stage, and then ensure you'll have enough reserve in case you need to use powered flight when you come back down for landing.

For the rocket stage, fuel is extremely important. You need to ensure that by the time you switch to rockets, you have the dV to not only get up to your final desired orbit, but to circularize and then carry out whatever your mission is. You'll also need to consider deorbit. In my planes, a deltaV of ~2700 in my rocket stage (TWR, 1.2-1.4) usually leaves me with anywhere from 900-1200 dV leftover to do whatever I want once in orbit.

Another thing you want to look at with FAR installed is drag. You want your plane to be as streamlined as you can make it. Unlike normal spacecraft, its not that good of an idea to have too many bits and bobs stuck on the sides of your plane.

Now, as far as your ascent goes, the rule of thumb is after take off is to get up to ~ 10km altitude. Then, level off at a very low ascent angle (5-10 degrees above the horizon) and just gun it towards your desired inclination. At approximately 20km up, you should be going hypersonic and your engines should start losing thrust. This is when you want to switch to rockets. You can experiment here where exactly to switch to rockets, but if you let one of your jet engines flame out (and they're not set up to compensate for it) you'll have a bad time. Once you switch to rockets, you need to pull up to ~45 degrees and wait until your apoapsis hits your desired orbit, where you cut your engines and coast, just like a normal rocket.

As I mentioned, if you let one of your jet engines flame out (it isn't an issue if you only have one jet engine), you could run into problems if they're not set up to compensate. This is because one engine will flameout and throw off your center of thrust causing you to spin out. How to compensate for this is to have an odd number of engines and place them from the farthest points moving inwards to the center, with your last engine right in the center where you need your COT to be. Then, when your engines flameout, they'll flame out from the outside in, maintaining your COT. So, say you want three jet engines. You'd place two of them in 2x symmetry, and then place the third engine right in between them where your COT needs to be.

[Claw]

I'm presuming you're using FAR' date=' because otherwise planes are dumb. I'm also presuming you're using either RAPIER-type engines or a jet engine/rocket engine combo. [/quote']

I think your presumptions may be a bit of a jump for someone who admittedly has zero experience. Also, your opinion that planes are dumb without FAR does not mean they can't be done, or are in some way unenjoyable. Perhaps providing reasons why you prefer FAR would be more valuable for someone who is asking "Where do I start?"

[xtremespino]

Here's a few tips.

1. Use ram air intakes. Your rocket engine choice will change the number of intakes you need. Aero spikes with quite a lot of fuel don't require flying with jets at 35 km. However, if you use nuclear engines, the jets will have to do most of the work. The number of intakes can also be experimented with until the craft works.

2. Use turbo jets or RAIPER's. You will need at least some of these engines to make a good SSTO. I have made serval with basic jets to help in the low atmosphere, but they always had a turbo jet.

3. Be smart about weight. Use light materials and engines (however it's not always best to choose a light, but lower performance option). If there's a lighter counter part to a part, use that instead. But, remember that this is not always right. Heavy engines such as the nuclear motor are very efficient, and are worth the weight on some craft. I use them all the time but there not a good idea for you who is new to SSTO's.

4. Small is easier! Don't try make a huge SSTO to start with, limit yourself to 1.25 metre parts.

I generally don't worry about the amount of fuel in my SSTO craft, as it will usually add some VTOL engines and try to land in minmus without refuelling (doesn't always go well)

This is just for design, flight is a whole different thing.

Don't give up, that's the biggest tip of all.

Hope these tips help

[xtremespino]

I think your presumptions may be a bit of a jump for someone who admittedly has zero experience. Also, your opinion that planes are dumb without FAR does not mean they can't be done, or are in some way unenjoyable. Perhaps providing reasons why you prefer FAR would be more valuable for someone who is asking "Where do I start?"

I was thinking the exactly same thing

[Hodo]

I think your presumptions may be a bit of a jump for someone who admittedly has zero experience. Also, your opinion that planes are dumb without FAR does not mean they can't be done, or are in some way unenjoyable. Perhaps providing reasons why you prefer FAR would be more valuable for someone who is asking "Where do I start?"

As a FAR user and one who doesn't understand how you can not use FAR. I will say my reasons for suggesting it are simple. Aerodynamics mean something in FAR, planes work like uh.... planes. And it fixes many of the exploits like the "Krakken Drive".

[SpenSpaceCorp]

but it would help to know what kind of things are essential.

Well, I find that having some sort of propulsion certainly helps, because when Jeb makes Bill get out and push the thing into orbit. Wings also help, because Bob's arms start getting tired of flapping around 15km.

All jokes aside though, I find the RAPIER engines excellent over the turbojet and rocket method because you do not have to haul around dead weight during separate burns. i also find that you want more lift than usual, (so as to say), just because of the way the atmosphere is modeled. This is especially important at high altitudes, when you start struggling to keep altitude (around 20-25 km i find).

Oh, one more thing to consider: If at first you don't succeed, try try again! I just recently designed a small SSTO that is capable of delivering 7 kerbals into orbit from Laythe's surface, and it took a lot of tweaking, redesigning, and completing one portion, moving on to the next. Just keep at it, and you are sure to get the hang of it soon. Oh and one more piece of advice: when starting out with SSTOs, START SMALL, meaning small mass, therefore greater thrust, and greater lift ratio (with more wings).

Hope that helps!

Edited June 15, 2014 by SpenSpaceCorp

[Claw]

FAR is definitely someting to consider, but not required if you prefer stock.

Edited June 15, 2014 by Claw

[Pecan]

...Now' date=' as far as your ascent goes, the rule of thumb is after take off is to get up to ~ 10km altitude. Then, level off at a very low ascent angle (5-10 degrees above the horizon) and just gun it towards your desired inclination. At approximately 20km up, you should be going hypersonic and your engines should start losing thrust. This is when you want to switch to rockets. You can experiment here where exactly to switch to rockets, but if you let one of your jet engines flame out (and they're not set up to compensate for it) you'll have a bad time. Once you switch to rockets, you need to pull up to ~45 degrees and wait until your apoapsis hits your desired orbit, where you cut your engines and coast, just like a normal rocket....[/quote']

Perhaps that is true of FAR but for stock KSP it is way too low.

The great efficiency of spaceplanes lies in their use of air-breathing turbojets. Not only do these take their oxygen from the atmosphere, via air-intakes, so you don't need to carry so much 'oxidiser' but they are also very, very efficient on liquid-fuel, so you don't need so much of that either. To take maximum advantage of that you want to get as fast - horizontally - as possible with the jets before switching to rockets. You can go faster if you go higher because the air is thinner (less drag) but, and this is the balancing act you have to manage for flying spaceplanes, the higher you go the faster you have to go to get enough of that thin air to feed the engines.

As such you should climb quickly from takeoff to get above the thick, draggy, lower atmosphere then start to level-out above 10km and especially 15km. Aim to be going above 1km/s horizontally around 20km and then do the majority of your speed-building around 25km while hardly climbing at all (depending on your design). Without massive amounts of intakes per engine you'll probably need to reduce throttle somewhere between 25 - 30km; this reduces the amount of air the jet needs and keeps it burning and thrusing when it would otherwise 'flameout' at full throttle. Keep climbing slowly (10-20m/s) and reducing throttle as necessary as long as your horizontal speed is still building. Once you have to throttle-back so much that you're not accelerating any more it's time to light the rockets, turn off the jet and close the intakes (to reduce drag, although there won't be much that high anyway).

[tutrakan4e]

Never forget solar panels

[Claw]

Never forget solar panels

Haha, so very true and easy to forget on a plane.

[Jouni]

As a FAR user and one who doesn't understand how you can not use FAR. I will say my reasons for suggesting it are simple. Aerodynamics mean something in FAR, planes work like uh.... planes. And it fixes many of the exploits like the "Krakken Drive".

FAR may be good for planes (I haven't tried, because the ridiculously fuel-efficient jet engines are too silly), but it's bad for rockets. When reaching orbit takes just 3500 m/s, you can build SSTO rockets with a good payload fraction using basically any kind of engines.

This may be a big reason for why we still have the old placeholder aerodynamics in the stock game. Better aerodynamics wouldn't be just a spaceplane update, but it would also require rebalancing everything, and possibly resizing the planets.

[Hodo]

FAR may be good for planes (I haven't tried, because the ridiculously fuel-efficient jet engines are too silly), but it's bad for rockets. When reaching orbit takes just 3500 m/s, you can build SSTO rockets with a good payload fraction using basically any kind of engines.

This may be a big reason for why we still have the old placeholder aerodynamics in the stock game. Better aerodynamics wouldn't be just a spaceplane update, but it would also require rebalancing everything, and possibly resizing the planets.

It isn't bad for rockets, if anything it is good for them.

[Jouni]

It isn't bad for rockets, if anything it is good for them.

FAR makes rockets too easy. Given the way the rocket parts are currently balanced, something like 5000 m/s would be a reasonable requirement for reaching orbit.

[Hodo]

FAR makes rockets too easy. Given the way the rocket parts are currently balanced, something like 5000 m/s would be a reasonable requirement for reaching orbit.

You don't even need 3km/s to get to orbit in stock KSP.

[Claw]

Perhaps there's another place to have the "stock vs. FAR" debate other than a "please help" thread.

[Pecan]

You don't even need 3km/s to get to orbit in stock KSP.

Please demonstrate this with a rocket, which Jouni has specifically mentioned a couple of times.

ETA for Claw's comment - I think this has become a "stock vs hyperedit" debate ^^ - oh, and I'm not saying it can't be done, because I've never used FAR but I would like to see it done.

Edited June 15, 2014 by Pecan

[Hodo]

Please demonstrate this with a rocket, which Jouni has specifically mentioned a couple of times.

ETA for Claw's comment - I think this has become a "stock vs hyperedit" debate ^^ - oh, and I'm not saying it can't be done, because I've never used FAR but I would like to see it done.

I would show you but I no longer have a stock sized Kerbin.

[Heagar]

Not to take in account a functinal design is stupid, it doesn't mean you copy it.

You have your own goals, therefore your solution will be different in some kind. So getting ideas for a spaceplane from BSC or K-Prize e.g. is ok in my opinion. Take the ideas and create something new!

Only one tought is important: The first spaceplane should be small. Improve it and get it bigger in your style.

[Taki117]

Back on Topic:

Make sure that you have enough fuel to get to orbit and back. Also, if you plan on docking it with anything (i.e you are building a crew ferry) ensure you have RCS, and a fair bit of it. SSTOs are all about balance. Balancing CoL vs CoM, Balancing fuel flowing to engines, all that jazz. Don't be afraid to experiment with TurboJets and 24-77 or 48-7S engines. Also remember bigger isn't always better.

[FleshJeb]

I recommend TAC Fuel Balancer for all aircraft. It will help keep your center-of-mass (CoM) relatively stable:

http://forum.kerbalspaceprogram.com/threads/25823-0-23-0-23-5-TAC-Fuel-Balancer-v2-3-22Dec

Integrating both the aircraft and rocket fuel tanks into your design can be challenging. You can always use rocket tanks exclusively and take out some of the oxidizer while you're in the Spaceplane Hangar. (Oxidizer is used in a ratio of 11:9 with liquid fuel).

Good practice is to take a look at CoM vs CoL placement with completely empty tanks, since that's how you'll be trying to land the thing.

Also, remember that the plane landing gear is actually massless while flying. If they're on the plane in the editor, your CoM will be a lie.