SSTOs, need some help

[Rune]

So, you don't like to rely in jets. Then I guess it's good the game makes rockets SSTO not that hard to build. Basically, go the way Talon did, and build a rocket SSTO that has jets and wings as a payload. What does that mean, exactly? Well, lots of thrust in rocket mode at the best T/W possible, and a boopload of rocket fuel. The jets become only an aid to get to ~10kms, afterwards you put it on a 45º heading and light the rockets at T/W>1.5 with enough fuel, and you will go to orbit today, just like a rocket. Check the X-33 in my thread (shuttle section): if you take out the jet engines and the aviation fuel... it remains a SSTO, since it has >4.5km/s on rocket power and T/W>1.1.

I can't tell you exactly what parts to build it with, that's your choice, but using the rocket equation (or a mod like kerbal engineer or mechjeb that does it for you, in case you dislike math), just give it some similarly high delta-v. Also, a very typical rookie mistake is to assume the aerospike is a good engine on a SSTO. That is very wrong, because the only advantage the aerospike has is a higher sea level isp. But the whole point of using jets is to not light the rocket at sea level, right? So you waste that and end up with an engine that just has a horrible T/W. By the time you light the other engines, they already give 90% of their vacuum isp, so the LVT-30 is actually the best 1m engine for SSTO's if you don't need the gimballing, since it has the higher T/W. So you can put a lot of fuel in front of it.

Just remember to keep the weight of the systems to fly on atmo small so you can retain a good payload fraction, and fly it like a rocket once you switch to rocket engines. That means vertical speed is important, and T/W means how much delta-v you waste to gravity.

Rune. Then again, payload fraction to orbit will be ****ty, compared with any airhogger. Prepare tankers to go other places.

[fcdrifter13]

I made this this morning. All stock using parts clipping and not using far, I forgot what a challenge it was trying to get a SSTO up without it. It uses 2 turbojet engines with 4 intake a piece on them with a pair of lt-v30s. The only down fall was it is so slow to take off and its a bugger to get up the speed you need, it wants to leave atmo at 1k m/s so I had to keep dipping back down and gaining more speed.

I would like to do the same plane with stretchy tanks. both side tanks would be just liquid fuel, and a major portion of the center would be just oxidizer would make the plane have almost double the dv with a very slight gain in weight.

[Captain Sierra]

I'd like to use FAR, but over half the planes I build are ripped apart by FAR. FAR brings me back to my jet fighter sim days. Oh wow I enjoy flaying it, I just can't build for it.

And to Rune, I have the jet ascent profile down to a science so I want to keep the jet ascent phase but then I need some advice on how to fly the rocket portion. I'll definitely look at switching the engine. I have a 1.25m linear aerospike that is lighter, less efficient, but packs one hell of a wallop. Would that be worth trying?

Also, thanks to everyone giving advice. I might have this moved to tutorials when I get better so that other SSTO noobs can have these tips.

EDIT: RLA Stockalike "Cutter" Linear Aerospike: 210kN thrust, Atmo ISP 340, Vac ISP 342, Weight 1.5 tons. That engine is the way to go.

Edited September 29, 2013 by Captain Sierra

[Hodo]

I'd like to use FAR, but over half the planes I build are ripped apart by FAR. FAR brings me back to my jet fighter sim days. Oh wow I enjoy flaying it, I just can't build for it.

And to Rune, I have the jet ascent profile down to a science so I want to keep the jet ascent phase but then I need some advice on how to fly the rocket portion. I'll definitely look at switching the engine. I have a 1.25m linear aerospike that is lighter, less efficient, but packs one hell of a wallop. Would that be worth trying?

Also, thanks to everyone giving advice. I might have this moved to tutorials when I get better so that other SSTO noobs can have these tips.

EDIT: RLA Stockalike "Cutter" Linear Aerospike: 210kN thrust, Atmo ISP 340, Vac ISP 342, Weight 1.5 tons. That engine is the way to go.

I love FAR, but found you should strut more. Some of my best SSTOs are using FAR, my first SSTO was built using only FAR and stock parts. After that I got the B9 parts and had to relearn what works with those, now all I can build is SSTO planes.

Some are easier than others to get into orbit some are just down right a pain. I also follow the no more than 3 to 1 intake to engine ratio. And try to make them look aesthetically pleasing. I have posted pics of them dozens of times already in the SSTO thread, so no point in dragging them here. Later I may design a simple SSTO using only stock parts that works in FAR and put it up just to show you can do it without airhogging.

[-Velocity-]

It's not hard to make a space plane once you get the formula down. Personally, I usually depend more on rocket engines than jet engines. So I put enough jet engines (maybe TWR = 2) to get me to around 19km and maybe 500-700 m/s, and then I kick on the rocket engines to take me the rest of the way up. My SSTO space planes contain very little jet fuel.

[Captain Sierra]

wow, you run a really rocket heavy profile. I go 23-24km @ 1350m.s before I lose jet power and kick in the rockets, then again, my rocket profile SUCKS so I burn way too much fuel.

[Van Disaster]

Basic single seat near-stock spaceplane built for FAR:

Just using TT inline wheels. If you wanted to move a couple more kerbalnauts around, can just stick a firesplitter Mk1-sized cabin behind the cockpit, any more than that would need a bit more care in wing design.

I don't generally intake spam - occasionally I'll add extra intakes if I'm radially mounting rockets, but only if there's an obvious mount point rather than stacking them or attaching them on their own. I will try and make it to 30km using reduced throttle, but I'll switch lower down if that's not looking effective.

[Captain Sierra]

Let's get off FAR. Here is where I am at now. I am still sucking at the rocket powered phase of the ascent. That aside, I am getting better with building them.

[Vector]

If you want to get near orbit with jets, there is a fundamental limit with number of intakes vs. weight. When you're leaving the ground, TWR is what matters, but once you start feathering the engines to prevent flameout, the ratio that matters is intakes to weight. Thrust is proportional to intakes x air density, and drag is proportional to mass * velocity^2 * air density. Your peak velocity will be proportional to velocity^2 = intakes/mass.

I can appreciate your desire to keep some realism and not just cram overlapping intakes into the same space. Even with your self-imposed constraint, the intakes/mass ratio still holds, meaning if you want to limit to 4 intakes per engine, you may need to add extra engines (not necessarily extra fuel) to get the intakes/mass ratio up. Your TWR will be crazy high at takeoff, which seems like a waste, but it's worth it because overall payload fraction will be much better if you are able to reach about 30k @ 2100m/s and don't have to burn a ton of rocket fuel to reach orbit.

[Captain Sierra]

The problem with that is twofold.

1) not enough rocket thrust. Too much plane to bump into orbit, though I have started using more powerful and more efficient non-stock aerospikes.

2) no enough lift. I have to start paneling the body in wing surfaces to get skyward but my aircraft don't like this and tend to want to flip out.

[Vector]

Have you tried this before jumping to these conclusions?

1) If you do it properly, you can get apoapsis outside the atmosphere with purely jet engines, and then you don't need high rocket TWR to bring your periapsis above the atmosphere. If you start your rocket burn low/slow, then yeah you are fighting a ton of drag and you need high TWR to even maintain the same speed.

2) If you get going fast enough, the centrifugal force counteracts gravity so you don't even need wings at all. I'll admit most of my SSTOs are vertical launch instead of horizontal launch, but even so, with enough intakes it's not a big deal to get apoapsis outside the atmosphere with no wings and no rockets.

[pa1983]

Yea its pretty easy to slingshot out in space on jet on Laythe. Never tried it in kerbin atmo but my crafts are a bit big for that atm so I haven't really designed them for that. Ascent profile is everything any way. Its almost impossible to design a good craft if you dont know a good ascent profile and its almost impossible to get a good craft to orbit if you dont know how to fly it with a good ascent profile. They go hand in hand.

[Captain Sierra]

Yes and yes. I have tried. My jet ascent profile as as good as I can make it but my jets just won't do that. not enough power. these are 1 man SSTOs so I am trying to use a ratio of 2 jet engines and a single rocket. Best ascent profile I can get thusfar is up to 24km going 1350m/s. I cannot in fact blast myself sub-orbital on just jets. My aircraft don't have the power.

pa, I know ascent profile is everything. Once I go rocket, I have a hard time getting it right. Care to help out with that final section of it?

[pa1983]

Your ascent profile is not over just because you go rocket engines. You should fly level at the highest altitude your jet engines can operate at 100% throttle to maximize acceleration and minimize drag. If there not at 100% your just hulling dead wight around that dont do any work. Use lots of wings to climb, that will allow the craft to use the jets to accelerate and not fight gravity. The wings are for lift not the engines. Most people building spaceplanes rely on there jets and not wings for lift. Its more of a Jet rocket then a spaceplane.

Most of my spaceplanes have TWR less then 1:1 (actual trust during takeoff not maximum engine numbers). My latest has 2:3 at take off and it still can climb efficiently at 45 degree angle. In theory they might have more but turbofans only deliver full trust during specific circumstances.

You can click on the bottom link in my signature. There is a craft,"Falcon VIB" in the 3:e post. It has 3 intakes per jet, 15 intakes and 5 jets if I recall. It is capable of going to Laythe and back with no refueling. But its an old outdated design. Think I used it in 0.18 or something the last time it flew. So its definitely possible to make a modern version with similar capability's.

You could also check my latest design the Kosmos. It has 12.5 intakes per jet but its fuselage can hold a ton of fuel so a hybrid of that could work. Its also safe from flame outs, can fly with at least on engin off with out a hitch and thats important when you push the limits. Probably what I would do if I had a 4 intake per jet engine limit and I would create a similar ascent profile but with a lower sealing for the first step.

But in the end it takes like 1/10 the fuel going from 2000m/s @ 32km compared to going from 1300m/s @ 24km altitude. Savings are enormous due to the LOW drag that starts at 32km altitude thats why I go for that. After that I just sling shot the craft to 44km costing it up on jets before going rockets.

So you need to do something similar. Once at top speed you need to start climbing again, throttling down just enough to keep flame outs away and you need to go 100m/s or more vertically on jets for as long as possible. Having lots of lift will once again help you here getting the most out of the jets and later rocket engines. Drag is a problem but its proportional to wight so wings realy dont have that much drag like engines and fuel tanks. If you can you can run both jets and rockets to maintain best efficiency. Best don by grouping jet engines so you can run one 1 or two jets in combination with the rocket engines.

Edited September 29, 2013 by pa1983

[Captain Sierra]

So I tried 4 clipped ram intakes to an engine for 4 jet engines and a single lv30. I could get 27km, no higher. I was still accelerating past 1850m/s but that was the altitude cap. Even with that level of intake spammage. Once I finally did break 32km, it was almost nothing to finish making orbit. I am starting to see why yall make these massive air hogging jets. Because then you can use NERVAs to make orbit.

[Vector]

So I tried 4 clipped ram intakes to an engine for 4 jet engines and a single lv30. I could get 27km, no higher. I was still accelerating past 1850m/s but that was the altitude cap.

Are you not reducing throttle below 100%? It is very important to reduce throttle and go past the flameout altitude because you can get a lot more power out of those engines at partial throttle until the edge of the atmosphere.

Either use MechJeb or place one jet in the center and deactivate the jets symmetrically until you have only one in the center. Even when the air is so thin that you have to throttle down to 5% you can still raise your apoapsis and periapsis.

[arq]

Building an SSTO to get to orbit is one thing. Getting one that can go all the way to Jool is much tougher. However, it can be done. Your ship will need to be reasonably big, because it needs to lift at least one LV-N into orbit along with it, and a lot of fuel. Small ships have a hard time doing this, because the LV-N is so darned heavy.

I recommend you try the Procedural Wings mod. They are stock-balanced and make it much easier to make large aircraft.

I wanted to try to build an SSTO around a mainsail transfer stage, because of the mainsail's very high TWR. Really I might have done better with a handful of LV-T30s (or maybe not?), but that didn't sound like as much fun. This is the Atlas 2 SSTO:

It masses 144t at launch and can reach LKO with over 44t of rocket fuel remaining, even without babying the throttle back on the jets. This leaves it with over 5100m/s of dV. It still has around 1/4 of its jetfuel left for landing shenanigans. The intake ratio is 31/10 intakes/turbojets and it uses a mainsail and two LV-N's to reach orbit.

When I get a window I'm going to try to send one to see if it can do a Laythe landing and return. Though it also makes a decent tanker, 44t is a decent bit of fuel.

[Rune]

I'd like to use FAR, but over half the planes I build are ripped apart by FAR. FAR brings me back to my jet fighter sim days. Oh wow I enjoy flaying it, I just can't build for it.

And to Rune, I have the jet ascent profile down to a science so I want to keep the jet ascent phase but then I need some advice on how to fly the rocket portion. I'll definitely look at switching the engine. I have a 1.25m linear aerospike that is lighter, less efficient, but packs one hell of a wallop. Would that be worth trying?

Also, thanks to everyone giving advice. I might have this moved to tutorials when I get better so that other SSTO noobs can have these tips.

EDIT: RLA Stockalike "Cutter" Linear Aerospike: 210kN thrust, Atmo ISP 340, Vac ISP 342, Weight 1.5 tons. That engine is the way to go.

Actually, that engine is still worse than the LVT-30 for a SSTO. At switchover conditions, the LVT-30 gives you 210kN, at ~369s is, for a mere 1.2mT. But lets center on your rocket climb. First, vertical speed matters. You want to get out of the atmosphere as fast as possible to minimize drag losses. So try to run out of air with at least 100m/s of vertical speed. Then, increase it with the rocket. You need a beefy rocket, T/W 1.4 or thereabouts will make things easier and give you much less gravity losses. That usually means you have MUCH more rocket thrust than jet thrust. The designs we get to orbit with tiny engines transition at close to orbital speeds, and then gravity drag is insignificant, but when you are switching below ~1,800m/s, you need much more thrust to overcome it. You will know you are doing things right when you can follow these instructions without having to point your plane too far away from prograde, and the climb to orbit on rockets is short. That is the hallmark of an efficient rocket insertion, actually, that it is fast. The longer and farther from prograde you have to burn, the higher gravity losses, and in direct proportion.

Wings won't help you much by that point, not enough air, they are only useful for the jet climb. But they are MUCH lighter than jet engines, so you want to save "dead" orbital weight and make jet T/W lower than one with a ton of wings for efficiency.

Or you know, give up completely and start airhogging. My favourite intake ratio is 6-1, that will get you over 2km/s. But SSTO's on 1-1 intake ratio are indeed possible, they just have higher delta-v requirements on rocket power (and as a result, lower payload fraction).

Rune. Now I am getting the itch to build a SSTO on standard jets and 1-1 intake ratio to figure out the right engine/tanking/payload proportions.

[bsalis]

Ok, so let me get this right, you want an...

* SSTO Spaceplane

* 100% Stock

* VTOL ability

* Water landing/takeoff capable

* No airhoggin (less than 3:1 ratio)

* No clipping

* No LT-N

* and I suppose you want to do a transfer to Jool from LKO after refuelling too eh?

Well, wait no more! It's possible, and here it is... (craft file)

Clicky pic for a demo video of the older model. This version is basically the same, a little better actually.

(RCS system purchased separately - but be a man and dock without it)

Have plenty of experience on Laythe with this craft. Works brilliantly...

There is also a variation with additional rocket VTOL engines... (craft file)

But wait, there's more. Land and sea refueling kit available too...

[Captain Sierra]

Bsalis, that is awesome but well beyond the scope of what I need. Rune, thanks for your tips on the rocket profile. I am throttling down to about 60% on my most recent. I can probably get as low as 50% before I start accelerating more slowly. I will try. And Rune, that was what I have been missing, that sharp jet climb right before flameout.