Tips and tricks for SSTO construction

[wasmic]

So, I have recently managed to create my first SSTO, using a TurboJet and two LV-909's. The problem with it is, that once I got it into orbit, I only had enough fuel for a deorbit burn, and it was only capable of carrying one kerbal. I also made a RAPIER SSTO, I got it into a highly decaying 152*29 km orbit before it ran out of fuel, and it had a problem with always yawing to the right once it got up to speed, despite being build with symmetry on the whole time. The yaw stopped the moment that the RAPIERS went to rocket mode. So I have a few questions:

1: Since I get flameouts at 23 kilometers, should I put more intakes on?

2: What's the recommended ratio between LF and Oxidizer?

3: Why do the RAPIERs yaw to the right?

4: What other tricks should I know about?

[ComradeGoat]

Your yaw is happening because, as the engines approach flameout, they reduce power. If you have more than one, they won't reduce power symmetrically, inducing yaw. You can counter this by having a HUGE RUDDER, and/or by throttling back manually.

The RAPIER is a pretty poor engine, BTW. A turbojet and 2 LV 84-7Ses works much better. The trick is keeping them fed with air until you're at almost orbital velocity. Climb rapidly to 20,000, then gently, bringing your speed up to about 1,300 m/s before you hit 25,000 metres. Try and level off at 30,000 and fly horizontally until your periapsis appears. Keep flying horizontally, and your apoapsis should start to climb into space. When this happens, pitch up to about 45º and reduce power to avoid flameout. Keep this angle, reducing power to keep the engine burning, until you reach about 40,000m, then go horizontal again and keep reducing power for as long as the jets will run, to keep your apoapsis from falling. Once the jet dies at zero power, turn it off and switch to your LV 48-7Ses. You should only need about 100 m/s or less to circularise at apoapsis.

A plane with one of the small 360 unit fuel tanks, one turbojet, 2 LV 48-7S engines and 4 radial intakes should be able to reach orbit and have loads of delta V left, as long as you get the ascent profile right.

Also, try to keep your centre of lift just behind your centre of gravity. That will make your plane nice and stable.

[wasmic]

Thank you. I am quite experienced in making regular planes, but I've always had hard times with SSTOs.

[MrUberGr]

Yep I've also have had hard times with the rapier. It isn't the one in all solution that many people where complaining that would render their previous SSTOs useless. I haven't been able to get one in orbit.

As for the matter of running out of fuel, you could build a refueling station at some point, and go refuel there before you venture to new places!

[Tex_NL]

There is an easy trick to avoid 'flame out yaw'

With an even number of jet engines the left engine will always flame out before the right. Don't ask my why, this is just the way the game engine works. Instead use an odd number of jet engines. First place the outer ones and place the center engine last. The last placed engine will always be the first one to flame out. When the center engine flames out you know it's time to throttle back and you won't suffer catastrophic yaw.

[Hodo]

I do not trust the automatic switch over from air breathing to rocket mode. I set it to an action group, and switch over as soon as it starts to flame out, around 23-25km.

Here is my basic SSTO with a RAPIER, I built it in 15minutes when .23 came out.

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The trick with your first SSTO is start small and work your way up from there. ((even though I totally did not do that myself))

I start by climbing at best possible rate of climb to 15km, then I level off and slowly climb at about 10deg pitch up, till 20-25km. My goal at that point is speed, I want to be going at least 1750m/s before I have to switch over to rocket. When I have pushed as fast as I can before the engines flameout I switch over to rocket mode pitch up to 30deg and climb the rest of the way to an AP of 100km, this is my personal test altitude for all my SSTOs, if they don't get there they aren't useful to me.

I found running just pure LFO tanks work best, but if you wanted to I see nothing wrong with replacing one LFO tank with a straight liquid fuel tank. But you do not want to much excess liquid fuel as it is dead weight in space.

[John FX]

Here is something I knocked up in an hour to have some fun with on Duna. It uses jets to get an Ap of about 100Km and a NERVA to get to Duna where it drops the subject in a drop pod. The drop pod has the basics to keep a kerbal alive..

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I found I only needed 8 intakes per jet (16 total) so I adapted the nosecone to look a bit better. I have some small adjustment tanks to finetune the LF/Ox mix to maximise efficiency.

[tavert]

More intakes mean better fuel efficiency, so go nuts until you think it looks ugly.

Instead of switching to the rockets when your jet first flames out, instead you should nurse the throttle to keep using the air-breathing engine for as long as you can. Hit x immediately to get the engine back on, and throttle back up to just below wherever you previously had the throttle when you just flamed out. Keep this up and you should get more speed and higher altitude out of the jet.

Don't fly an SSTO like a rocket. Instead you want to level off your flight path at the highest altitude you can sustain enough throttle to keep accelerating - drag drops off pretty noticeably above 30 km, and it takes very little throttle to counteract it and continue building speed at high altitudes. The more intakes you have the higher this will be. When you're happy with the speed you've gotten off the jets, then switch to the rockets and pitch up to climb the rest of the way out of the atmosphere.

[WhiteWeasel]

Two turbojets and one rocket engine is a good combo for me. If you don't mind redesigning, swap out the the lv 909 for an aerospike. An aerospike has more than three times the thrust (50<175).

[Mic_n]

Unless you're using a particularly large SSTO, that's not particularly great advice, IMO. Once you're off the pad and up at decent altitude, the biggest limiting factor isn't total thrust, it's available thrust, and with a jet engine, that availability is limited by air. At the optimum altitudes to gain speed, you're generally nowhere near full throttle.. much closer to the other end of the scale, and two engines at 5% throttle compared to 1 at 10% is just extra weight. Add to that that to keep your thrust vector centred behind your centres of mass and drag (to prevent torque moments) you need to have at least one 'pair' of engines, and there are no lightweight jets available like there are rockets.

You also don't really need a whole lot of thrust from those rockets to get orbital. You can actually get that final 'nudge' to circularise from ion engines if you do it right. A pair of LV909s has more thrust than you need as is, and they're considerably more efficient.

If you're just building something to get a crewman into orbit, it really doesn't take much at all. 1 jet, 1 'standard' fuel tank, a pair of mini radial engines and probably 4 intakes will get a Jeb into space and back. Well, maybe not a Jeb, 'coz he'd probably use all his fuel blasting his Ap Munwards, but it'd probably get a Bob into space.

[Horn Brain]

Don't put all your intakes at the front of the vehicle. At high speeds this can move your center of pressure in front of the center of mass and cause you to become unstable.

Do landing tests before you do orbital flights. There's nothing more frustrating than getting your SSTO to orbit and then having it flutter to the ground like a lead leaf on reentry. Make sure it can fly nearly empty by tweaking most of the fuel out of the tanks. This will save you much agony.

Make sure fuel drains symmetrically so that the CoM stays in the same place. For smaller craft, just use one big tank and make sure the CoM is at the center of it. For larger craft, I have found that it's pretty easy to set up the fuel flow so that the engines on one side drain from the front and the engines from the other side drain from the rear, and then I make sure to use the same tanks on the front and rear. It doesn't have to be absolutely perfect, but it should be as near as you can tell.

[barrenwaste]

The turbojet and the atomic rocket are by far the two best engines for small sstos. You can clip the atomic rocket into a fuel tank so that it doesn't stick out horribly from the back of your space plane. Also, if you clip the ram air intakes inside the fuel tanks you can airhog like crazy without making your space plane look all whacked out. Also, don't worry about different fuel types, you can't use jet fuel in space so just pack lots of rocket fuel. That makes designing your space plane easier and allows a greater range of travel.

During flight keep an eye on your intakes, if you are still at full throttle when it goes below .20 then there is a great chance of flame out. The key is NOT to go directly to 20,000 unless you have an insane amount of intakes. Instead, level off a bit before that so that you can start building speed. The faster you go the more the intakes will draw and the easier it will be to break orbit. I would suggest something that goes at least 1800 m/s as a minimum. That isn't that hard to attain because when you get above 25,000 feet you can throttle down to almost nothing and still gain speed and altitude. You are going to be spending time at altitude, more than anywhere else in the flight, because you have to keep your ascent slow to gain the speed you need to escape orbit easily.

[tavert]

The turbojet and the atomic rocket are by far the two best engines for small sstos.

LV-N and "small" do not belong together in the same sentence. Unless you need thousands of m/s of delta-V or are pushing 10+ tons of payload, the LV-N is too heavy and you'd be better off using a smaller engine (most often the 48-7S). See http://forum.kerbalspaceprogram.com/threads/45155-Mass-optimal-engine-type-vs-delta-V-payload-and-min-TWR

[barrenwaste]

The atomic weighs in at about 2 and 1/4 but has double the thrust and more than double the ISP of the 48-7s. Yeah, the 48-7s weighs only .1, but it doesn't provide enough thrust and uses far to much fuel in comparison with the Atomic. If you are going small, then a single LV909 is a much better choice than a 48-7s. Either way, two 8-7's or a single 909, you aren't going to get the performance range that you do from the atomic.

[tavert]

Did you read the chart(s) I linked to? There's a region in TWR-dV-payload space where the 48-7S gives better performance (lower total craft mass), and a region where the LV-N gives better performance. Despite the lower specific impulse of the 48-7S, for small craft the extra 2 tons of fuel mass often gets you more delta-V from the same total craft mass. For orbital insertion on spaceplanes, you don't need high TWR or thousands of m/s dV.

And as far as the LV-909 goes, it is very very rarely the best choice. Its optimal region is a very narrow band between the 48-7S and the LV-N, at low TWR.

Edited January 3, 2014 by tavert

[barrenwaste]

Yup, read your charts. I think they are wrong. I think you failed to take into account the changes in values that occur during flight and pilot error. Maybe I'm wrong on that, myself. What I do know is that when I fly with the atomic I get a much larger operational envelope than with the smaller, lighter rockets. I also know that I build small sstos. Those two things tell me that either your math is off or you didn't take into account some of the things the game does.

Edit: Edited because previous version was to confrontational and I want a discussion not an argument or miffed people.

Edited January 3, 2014 by barrenwaste

[tavert]

Glad I didn't see the first version then? Anyway...

Can you be more specific, explain exactly what you mean by "changes that occur during flight," "operational envelope," and "some of the things the game [takes into account]"? As you burn fuel, your mass decreases so your TWR increases. But this happens faster with lower Isp, so if two craft with the same initial TWR execute the same maneuver, the lower-Isp craft will end the maneuver with a higher TWR (and in less time) than the higher-Isp craft.

My charts are combining the rocket equation with the stats of the stock engines and fuel tanks, despite the visual complexity of the results there's not much more to them than that. You should be able to verify the results in-game with KER or MechJeb readouts of TWR and Delta-V.

Maybe "small SSTO" is too generic. We can work with some concrete numbers if you propose an example craft.

Edit 1: or here, I put together a simplified comparison with Desmos: https://www.desmos.com/calculator/o1too2zrat

This compares the vacuum delta-V (y axis) vs total craft mass (x axis) for 1x LV-N (purple) vs 2x 48-7S (green) vs 1x LV-909 (blue). The LV-909 gives a little less thrust than the other two options (50 kN vs 60 kN), but still it's a worse choice for all but a tiny range of masses/delta-V's. Play with the slider for p to modify Payload mass, which is everything on the craft except the bipropellant tanks, fuel, and engines. This includes any disabled jets, your cockpit, wings, RCS, leftover jet fuel and tanks, etc. Aircraft landing gear are massless in flight so they don't add anything here.

For small masses/delta-V's, a pair of 48-7S engines is the better choice. You can get better dV in exchange for lower TWR by using only one 48-7S instead of two, but I didn't plot that option here. I'm also assuming you're only using FL-T100 or larger fuel tanks in this simplified plot, the more detailed original set of charts considers combinations using the smaller tanks as well.

Edit 2: and here's the solution for the intersection point between 1x LV-N and 2x 48-7S: http://wolfr.am/1dsHaIL

Solution plotted with payload on y-axis, dV on x-axis: http://wolfr.am/1cqzen0

Below the line a pair of 48-7S is better, above the line an LV-N is better.

Edited January 3, 2014 by tavert