Designing SSTO Rockets

[Destroyer713]

After being told several times that they are much easier to design and fly than a spaceplane SSTO, I decided to give a try at making a rocket SSTO! Problem is, I'm not exactly familiar with designing these, so I'm not entirely certain how to make one that can lift the payloads I need, but minimize the fuel costs.

For one thing, I'm not sure what engines are ideal for the task, particularly since it's not as simple as slapping on Rapiers and calling it good. Is it more efficient to use a bunch of lifter engines (Mainsails, Mammoths, etc.), or should those be used along with more efficient engines (Poodles, Rhinos, etc.)? Having the more efficient engines would be great once you got out of the atmosphere since they have high ISP, but then your lifter engines become dead weight.

When it comes to parachutes, how many do you tend to need? Same goes with drouge chutes and airbrakes, which I assume you'll need in order to slow the craft enough for the main chutes to safely deploy. Are radial chutes more ideal, or is it better to stick some on top of the side tanks?

Oh, and fairings. Are they even worth using in most cases? Particularly if payloads are often fuel tanks with docking ports on the sides and maybe a Cupola module on the top. Then there's the issue with wobbly payloads inside the fairing, since the only connection between my payload and the rocket is often a docking port (A decoupler would just add velocity to the payload and screw up a rendezvous, I would assume). There any way to deal with that at all?

I appreciate any advice you guys are able to give me!

Edited September 15, 2015 by Destroyer713

[Highguard]

As with designing anything, start small and work your way up as you develop experience.

Start with single-engine designs and just see how much each engine can carry to orbit, if anything.

The amount of dV used in/near vacuum isn't usually enough to make it worth carrying specific engines for that.

Once you've found an engine that can SSTO 5t for example, progress to 10t just by bolting two of them together! (Except where you find a better single-engine design)

Except for very heavy payloads (>100t), I've never found a need for more than 2-4 parachutes on the launch vehicle, although it depends on how much fuel you reserve for a powered landing. Generally I stick them as 'nose cones' on side-tanks if the slots are available, otherwise as radial-mount inside a service bay where I also put the batteries, probe core, SAS unit and what-have-you. Airbrakes are great for keeping orientation during re-entry as well as just the braking effect but quite often you don't need them at all anyway.

Fairings very much depend on the payload, although for fuel tanks and command pods they shouldn't be needed. Docking-ports are good for bringing a payload back down but otherwise decouplers are just as good. The payload isn't going to be affected much by the separation force anyway, but it can be tweaked in the VAB if you have a problem with it.

[moogoob]

Oh, and fairings. Are they even worth using in most cases? Particularly if payloads are often fuel tanks with docking ports on the sides and maybe a Cupola module on the top. Then there's the issue with wobbly payloads inside the fairing, since the only connection between my payload and the rocket is often a docking port (A decoupler would just add velocity to the payload and screw up a rendezvous, I would assume). There any way to deal with that at all?

About fairings - sometimes they're useful, sometimes not worth the weight and hassle. I tend to plop a nose cone with reversed decoupler on the tip of said fuel payloads and just eject it before circularization - that's more than enough for what you have planned.

As to wobbly payloads, that's what struts are for. A rigid payload is worth any drag or mass added by struts.

I use a decoupler on one side and docking port on the other - the extra push doesn't screw up your orbit that much.

[GoSlash27]

Destroyer713,

Finding the best SSTO rocket for a given payload is just a matter of math and using a spreadsheet so you can apply the same math to all the engines at the same time.

You would apply this series to all the rockets:

1) How much can one of these engines lift at 1.4G at surface thrust?

Tmax* Ispatm/1.4*9.81*Ispvac= pad mass

2) What percentage of this mass needs to be fuel in order to place this vehicle into orbit?

Wet-to-dry ratio = e^(3,600/9.81Isp)

(Wet-to-dry ratio-1)/ wet-to-dry ratio =fuel percentage

3) Pad mass* fuel percentage= fuel mass

4) Fuel mass/8= tank mass

5) Pad mass-fuel mass-tank mass-engine mass= single engine payload mass

So now that you know what payload a single engine can lift to orbit, it's just a matter of scaling the launch vehicle to accomodate your desired payload.

If you do this with all engines, you will quickly see which engine will yield the lightest and cheapest vehicle for your payload. You can also figure exactly how much fuel you need.

Best,

-Slashy

Edited September 15, 2015 by GoSlash27

[Admac]

I love SSTO Rockets!

You want to use Atmo engines for it for the most part, since it's simpler, if less efficient. In the same vein, choose the biggest ones you can.

Knowing TWR is super important. You'll be starting at 1.2 or so, and finishing closer to 5-6. This can be made more reasonable and a lot smaller if you are okay with some first stage expendable SRBs (I'd recommend them) then optimize for a 1.0 TWR on SRB separation, allowing you to end closer to a 3-4 TWR, saving a lot of engine weight.

Landing is tricky, so assume a powered landing.

Airbrakes are essential since they are very light and allow you to be more accurate on landing (Turning them off and on as you over/undershoot) Then enough parachutes to bleed off speed, then a powered landing for the final seconds is often enough. To get enough parachutes to land safely is a LOT more than what you need to get to 20m/s fall speed.

To stick landings on the fields near KSP you want the lowest part of your rocket to be the radially attached tanks (3,4,6x symmetry have worked for me). landing legs are better, but landing on engine bells is cool too if you have a steady hand. Often as little as 40 m/s of DV for the powered landing is all you need.

[Norcalplanner]

About fairings - sometimes they're useful, sometimes not worth the weight and hassle. I tend to plop a nose cone with reversed decoupler on the tip of said fuel payloads and just eject it before circularization - that's more than enough for what you have planned.

As to wobbly payloads, that's what struts are for. A rigid payload is worth any drag or mass added by struts.

I use a decoupler on one side and docking port on the other - the extra push doesn't screw up your orbit that much.

The utility of fairings depends on several factors, including how many radial parts are on the payload. My general rule is that if there are two or more medium sized radial docking ports, then I'll use a fairing. Otherwise, I'll do just like moogoob suggests and slap a nose cone on the end - the only change I would make is to omit the decoupler. The day I learned I could attach docking ports to end nodes was an eye opener.

[RocketBlam]

Here's what I would do.

1. Design a vertical rocket that can get to orbit in one stage (technically, it's already an SSTO at this point, but you want a spaceplane.) This isn't too tricky, just take a Swivel and keep putting fuel on it until it can barely take off, and if you haven't added too much weight elsewhere, you're all set.

2. Turn it over on its side and put wings on it. Then put gear on it.

Sounds trivial, but the first challenge is getting the D/V and TWR to get to orbit. If you can conquer that, the wings just add controlability.

[Alshain]

Remember it is Single Stage To Orbit not Single stage to everywhere in the solar system. A lot of people forget that. For SSTO rockets, I like to build my launcher capable of landing again after delivering the payload (usually via probe), but the payload may stage of unused parts and leave them behind if necessary.