How do I SSTO?

[Capt. Hunt]

My Space programs are usually epic, I can send stuff almost anywhere with expendable launchers and shuttles, but the one nut I've been unable to crack has been Single Stage To Orbit.  I've seen tons of great designs on the Spacecraft Exchange, but I can't seem to build one myself that works.  Usually to get any space-planes into orbit, they end up as huge bloated monstrosities with no reserve Delta-V and a very small payload fraction, and usually I end up cheating with boosters or drop-tanks (thus not making a true SSTO).  What is the trick?  Some of the designs I've seen in the SE are tiny little things with huge payload capacities for their size, or enough Delta-V for multiple LKO launches without refueling.  Cupcakes designs certainly spring to mind.

[AeroGav]

The main problem i see time and time again is excess drag, requiring large numbers of heavy airbreathing engines to get through mach 1, which impacts dry mass fraction so badly it barely has the delta V to reach orbit, let alone anywhere beyond.

In an aerodynamically clean design you should be able to accelerate past mach 1 in level flight with just one RAPIER engine per 30 tons of weight, though you'll probably have to climb up above 6 or 7km before levelling off for the sonic run.

It is non intuitive because the drag model has very little to do with real life physics, and more to do with the craft tree structure.  Avoid mk2 parts as much as possible.   Avoid radially attached clutter, use service bays and cargo bays. Every stack needs a pointy object on the front, and a pointy one on the back.   You can even apply cones to the back of engines that have rear attach nodes, to avoid the drag penalty from unused nodes - just offset the cone inside the engine so it doesnt block exhaust.   Finally, when joining parts together, the attach nodes of the parts being joined should be the same size.     Eg.  1.25m engine attached to a 2.5m attach node on a 2.5m part creates excess drag.   Use a 2.5m to 1.25m adapter in between the two, or a 2.5m triple engine mount, to reduce drag.

 

[Reusables]

SSTO is single-stage-to-orbit. You can make one with conventional rockets, Just attach an orange tank to Twin-Boar and streamline it.

If you want spaceplanes: There are several counter-intuitive points in KSP aerodynamics. With excellent explanation of Aerogav, I'd like to add this one: The central node of mk3 mount is 2.5m, and must be filled for minimal drag.

[bewing]

Yes, it's mostly about minimizing drag and weight. You have to have studied carefully the weights and drags of all the parts.

After that, it's about understanding Isp, lift, knowing the KSP wing area vs. mass ratios, and the fundamental concept behind the gravity turn.

Then, it's about understanding heating during launch, and reentry.

Beyond that, you have to know everything there is to know about fuel storage.

Oh, and trivial little details, such as how to build a plane that can change in mass by a factor of 10 and still remain stable.

So, OK -- there's a lot to it. But you have to decide which of these subjects you want to tackle first, or you have to give us a plane to critique.

 

Edited April 3, 2017 by bewing

[Wanderfound]

Reinforcing...less is more.

Less engine, less fuselage, less intake, less wing [1], minimal ancillary gadgetry.

However...it's as much about the flying as the building. The best SSTO will fail if you fly it wrong, while even a bad one can often be coaxed into orbit if flown just right. The key is that you have to crank as much speed and altitude as you can from the jets before beginning the rocketry; you're aiming to get as close to 1,500m/s and 30,000m as possible before you light the fireworks.

Start small; the bigger they are, the harder they are to get right. But if you build something that's basically just a cockpit, some wings and fins, one RAPIER, an intake, an FL/T-800 LFO tank plus around 400 units of LF (for the open-cycle jet phase), it should go to space without too much trouble.

For the "how much wing" question, see:

For basic tricks (you may already know these, but just in case):

1) Minimise distance between CoM and dCoM (AKA CoM with empty fuel tanks). You don't want the plane shifting its balance point mid-flight.

2) CoL close behind CoM. How close is a judgement call; the closer it is, the more agile the plane; the further it is, the more stable the plane.

3) You want enough wing to keep your takeoff speed below 100m/s, and enough engine to hit that speed before the end of the runway. Anything more is a luxury.

4) In a tricycle setup, the rear landing gear should be just behind CoM. If it's a lightweight Mk1, you can get away with mounting gear on the wings. Anything heavier and you want to stick to fuselage mounting to reduce flex.

5) Drag is king. Don't mount anything on the surface of your ship if you can stick it in a cargo/service bay instead. If it's a Mk2 or Mk3, build in enough wing incidence so that you can keep the nose on prograde during the speed run; a few degrees is usually sufficient.

6) You need sufficient control authority to move the plane in three axes: roll, pitch and yaw. In each case, the further the control surface is from the CoM, the more power it will have. It's all about leverage.

7) One shock cone or intercooler will feed three RAPIERs. One ram air intake will feed two. Etc.

8) Make it look like a plausibly realistic plane. You can make freaky SF ships once you get good, but to begin with, you've got more chance of success if you copy the basic airframe of a real plane. They're shaped the way they are for a reason.

 

[1] This is a matter of preference, see the linked post above for detail.

Edited April 3, 2017 by Wanderfound

[bewing]

29 minutes ago, Wanderfound said:

Don't mount anything on the surface of your ship if you can stick it in a cargo/service bay instead.

..... except that you may need to test some of these things for yourself. I've tested the drag difference between having things surface mounted vs. service bays -- and the results were bad. Service bays have a lot more drag than you might think, so be careful.

 

[Spricigo]

8 hours ago, Wanderfound said:

However...it's as much about the flying as the building. The best SSTO will fail if you fly it wrong, while even a bad one can often be coaxed into orbit if flown just right.

QFT  However, worth noticing that KSP's control scheme sometimes get in the way of following a flight plan.

 

9 hours ago, Wanderfound said:

Reinforcing...less is more.

... less wing [1]...

[1] This is a matter of preference, see the linked post above for detail.

 

Let me say thank you. This very post you linked was a eye opener for me. After that I build a SSTO spaceplane, not my first but the first one I liked(actually pretty similar to your design above). Thank you for convince me that less wing is more. I’m still experimenting with some ideas based in this design philosophy but its already a game changer.

 

14 hours ago, Capt. Hunt said:

... and usually I end up cheating with boosters or drop-tanks (thus not making a true SSTO). ..

Of course your reason may be the challenge or the rule of cool. But, after designing a SSTO you like, notice that drop-tanks by itself aren't a bad thing. Going back to business: if results in lower cost* for mission go for it.

*Cost its not always funds. How much you value your time or how much you like doing something in a particular way often its more relevant.

[Urses]

On 3.4.2017 at 9:32 AM, bewing said:

..... except that you may need to test some of these things for yourself. I've tested the drag difference between having things surface mounted vs. service bays -- and the results were bad. Service bays have a lot more drag than you might think, so be careful.

 

This is Why i bagan to install unstaged fairings inline as fuselage for MK1 builds to have placement room. For Science and more. If the tool can't be included  (stowed) it works Perfect and is interactable for Kerbals but Greatly reduce drag for the craft. On other hand if you dont have anithing breackable inside of service bay you can missuse them as Aerobrakes.

Funny Kabooms 

Urses 

[Hodo]

First thing is not all SSTOs are planes.  So if you are good at traditional rockets, try building a rocket SSTO.   One that takes off like a rocket, and lands on its tail or lands like a plane.  

I have one that I built it is a fascinating test craft, that I may expand on later.  

SSTO space plane takes off like a rocket (from its tail on a launch gantry).  But lands like a plane.

And this was a SSTO "Rocket" and tail lander.

 

[Kerbital]

Q: How do I SSTO?

A: Tweak your engine cfg files