Help improving SSTO Space Plane

[davidpsummers]

I made a decent small SSTO and now I'm trying to make a mk 2 sized one that can carry 6. But I'm kinda stuck. I have one that can make a 75 km orbit (barely, without fuel for retro burn) but I need one that can get to my station in a 250 km orbit.

It is pictured below (and below that is the link to the .craft file for those who might want to look at that). I added the second set of engines under the wing, not knowing if they would help or hurt (it seemed underpowered, but weight is weight, it turned out to make little difference), but now I don't know what to try next.

Suggestions?

https://www.dropbox.com/s/2wv7ny3cypm4niq/Icarus%201e.craft?dl=0

Edited August 22, 2015 by davidpsummers

[Scarecrow88]

How the hell do you get that off the runway without knocking it's arse off?

[capi3101]

For a fifty tonne craft, you should be able to get away with four RAPIER engines. You might try replacing a pair of them with something with a higher vacuum Isp...if you reserve them specifically for space use, a pair of Terriers would probably work okay. If you want something you can still use in atmo before switchover, I'd suggest Aerospikes.

My first thoughts when I look at this plane are "it hasn't got enough wing for the length of its fuselage (damn, that's a long fuselage) and it's lacking ailerons". But, if it flies okay otherwise...

You're also probably not doing yourself any favors with all of those radial intakes in the back. One per engine oughta be sufficient - and you can get rid of them completely if you go replacing them with rockets like I suggested. Less mass, more efficient engines - that's two out of three ways to improve delta-V. The third would disqualify this design as an SSTO, of course...

I'm with Scarecrow there - that plane looks like it's begging for a tailstrike. Do you usually just ski jump it?

Edited August 19, 2015 by capi3101

[Slam_Jones]

What exactly is the payload? Just Kerbals for the station? If so, you have way, WAY too much fuel. Below here, I've posted an SSTO that I designed, able to reach orbit and perform a Mun fly-by before returning and landing at KSC, with a crew of 2 as well as 8 passengers (typically tourists). I think the "standard" is about 15 tons per RAPIER. It actually went through multiple revisions with different engine set-ups, and it was even able to achieve it's objective with only TurboRamJets and LV-909's. RAPIERs help a ton, though, due to being able to (usually) go a little higher and faster before losing thrust. I do recommend something like a set of LV-909s as your orbital, high-ISP engines, but ONLY if they don't weigh much. If they're too heavy, they'll really slow down your jets, causing you to lose any dV gains to friction.

One of the big humps to get over is the 200-300m/s "barrier" that occurs with a lot of low-TWR spaceplanes. If you can get through that with no problem, you should be able to get orbit no problem (assuming correct fuel amounts).

You can probably reduce a lot of weight in fuel by really working on your ascent profile. What I typically do is angle up to about 5 or 10 degrees, so that as I'm climbing, my thrust output continues to increase smoothly. During this time I manage the throttle so my TWR never rises past 1.3 (I use Kerbal Engineer for this info). Once I reach about 12km (at which point I'm usually between 300-500m/s, depending on how much attention I've been paying), I crank the engines up to full and pitch down a couple degrees (but still about 5 - 10 degrees above horizon). My intention here is to reach about 1200 m/s at about 20-24km, then switch to closed cycle, pitch up, and complete the burn.

You might also consider adding some more wing area on there. As it stands, it looks like it doesn't quite produce enough lift. The more lift you have, generally the lower TWR you can get away with. In most cases.

I actually made a drone SSTO the other day for use as a miner on Laythe. It's TWR is high enough that I can pitch to basically 45 degrees after take-off, and have my thrust continue to increase. This, I think, might save a lot of fuel, since I'm spending significantly less time in the thicker part of the atmo, losing speed to drag.

SSTOs are extremely difficult at first, but trust me, they get easier with experience. The Laythe-miner SSTO is actually one that I slapped together in all but 5 minutes, and it achieved orbit without revisions. Just practice, and you'll get it

The bigger they get, the harder they are to manage and fly precisely, especially when you move into Mk3 territory

Edited August 19, 2015 by Slam_Jones

[Edax]

I made a decent small SSTO and now I'm trying to make a mk 2 sized one that can carry 6. But I'm kinda stuck. I have one that can make a 75 km orbit (barely, without fuel for retro burn) but I need one that can get to my station in a 250 km orbit.

It is pictured below (and below that is the link to the .craft file for those who might want to look at that). I added the second set of engines under the wing, not knowing if they would help or hurt (it seemed underpowered, but weight is weight, it turned out to make little difference), but now I don't know what to try next.

Suggestions?

http://i.imgur.com/w0bfWtG.pnghttp://i.imgur.com/LYTWPnJ.png

https://www.dropbox.com/s/2wv7ny3cypm4niq/Icarus%201e.craft?dl=0

Perhaps using a 4 turbojet, 2 aerospike engine combo could help improve fuel efficiency. (though I'm not sure 4 turbojets are enough to carry a 50t ship. Maybe 4 turbojets and 2 rapiers if you can manage it?) It's unlikely that you need the thrust from 6 rapiers once in space to reach orbit. It's important to remember that the fuel efficiency of the rapier is very poor.

Edited August 19, 2015 by Edax

[davidpsummers]

How the hell do you get that off the runway without knocking it's arse off?

As long as I don't spam the pitch up key, it works OK.

[GoSlash27]

davidpsummers,

I'll see if I can help you with the design you've created, rather than giving one of mine.

*First of all, I'd recommend ditching the fuselage-mounted RAPIERs, replacing the adapter you've got them mounted to with a Mk1 adapter, and tapering it down to a cone.

*Ditch the XM-G50 radial scoops, and (assuming you have them) replace the ram air intakes with shock cones.

*Also, it won't weigh very much when empty. You should thin the herd on the parachutes.

* All that RCS monoprop is overkill and dead weight. You should get rid of it.

*As a "handling" thing, you should relocate the main gear so they're not angled goofy. It'll track better.

Try that and let me know how it goes.

Best,

-Slashy

Edited August 19, 2015 by GoSlash27

[davidpsummers]

For a fifty tonne craft, you should be able to get away with four RAPIER engines. You might try replacing a pair of them with something with a higher vacuum Isp...if you reserve them specifically for space use, a pair of Terriers would probably work okay. If you want something you can still use in atmo before switchover, I'd suggest Aerospikes.

Well, I thought four should be enough, but with four I got a slow climb (I can do maybe pitched up 30 degrees and, if I spend some time gathering a bit of speed before I climb, 200 m/s), I understand you still want to get out of the lower atmosphere briskly. It was also slow in gathering speed in the middle atmophsere. In the end, the extra two engines seemed to use less fuel getting up to the final push, but then the over all performance wasn't much better. Maybe I'll try going down to five (one in the middle back).

- - - Updated - - -

My first thoughts when I look at this plane are "it hasn't got enough wing for the length of its fuselage (damn, that's a long fuselage) and it's lacking ailerons". But, if it flies okay otherwise...

It seems to, unless I'm missing something. It is sluggist, I may fixe that latter but its not my main problem right now.

[GoSlash27]

^ I second Capi's advice; 4 is more than plenty.

[davidpsummers]

You're also probably not doing yourself any favors with all of those radial intakes in the back. One per engine oughta be sufficient - and you can get rid of them completely if you go replacing them with rockets like I suggested. Less mass, more efficient engines - that's two out of three ways to improve delta-V. The third would disqualify this design as an SSTO, of course...

I don't seem to have any lack of thrust when I switch over to rocket mode. I think if I have too many airbreathing engines, I should just take them off?

- - - Updated - - -

What exactly is the payload? Just Kerbals for the station? If so, you have way, WAY too much fuel. [...]

My intention here is to reach about 1200 m/s at about 20-24km, then switch to closed cycle, pitch up, and complete the burn.

You might also consider adding some more wing area on there. As it stands, it looks like it doesn't quite produce enough lift. The more lift you have, generally the lower TWR you can get away with. In most cases.

[...]

I can reach >1300 m/s at above 25 km before switching over, I just don't have enough dV at that point.

I can try adding more wing and less engine. It doesn't have a bad angle of attack when flying, by maybe that is why I can't climb quickly on four engines.

- - - Updated - - -

Perhaps using a 4 turbojet, 2 aerospike engine combo could help improve fuel efficiency. (though I'm not sure 4 turbojets are enough to carry a 50t ship. Maybe 4 turbojets and 2 rapiers if you can manage it?) It's unlikely that you need the thrust from 6 rapiers once in space to reach orbit. It's important to remember that the fuel efficiency of the rapier is very poor.

I played around with Turbojets. They just didn't seem to give much power, what do the offer over a rapier?

[Scarecrow88]

Right, I actually downloaded it and had a go, but failed miserably to get it into orbit. It suffers from a lot of drag, as has already been pointed out, so in an effort to improve it I removed all the radial intakes, the underwing engines and the parachutes. With only the 2 fuselage mounted rapiers remaining, I also ditched two of the rocket fuel tanks, which considerably shortened the craft, giving slightly less worries about a tail strike. I used 2 shock cone intakes as a replacement for all the radial intakes I removed, and straightened the main undercarriage. In this configuration I managed a 275k x 275k orbit, but it did run out of oxidizer in the process, which meant I couldn't quite dock with my station, though I did get within 750m using RCS.

I'm sure with a few more tweaks you'll be able to achieve your goals, so stick with it.

[davidpsummers]

davidpsummers,

I'll see if I can help you with the design you've created, rather than giving one of mine.

*First of all, I'd recommend ditching the fuselage-mounted RAPIERs, replacing the adapter you've got them mounted to with a Mk1 adapter, and tapering it down to a cone.

*Ditch the XM-G50 radial scoops, and (assuming you have them) replace the ram air intakes with shock cones.

*Also, it won't weigh very much when empty. You should thin the herd on the parachutes.

* All that RCS monoprop is overkill and dead weight. You should get rid of it.

*As a "handling" thing, you should relocate the main gear so they're not angled goofy. It'll track better.

Try that and let me know how it goes.

Best,

-Slashy

Put 4 in-line in the back? I think I know how to do that, but where do I put the intakes? I won't have anyplace to mount anything except radial ones.

Are shock cones better than Ram intakes?

[GoSlash27]

Put 4 in-line in the back? I think I know how to do that, but where do I put the intakes? I won't have anyplace to mount anything except radial ones.

No, no. Delete the 2 in the back and keep the 4 under the wings.

Are shock cones better than Ram intakes?

Yessir.

Best,

-Slashy

[Red Iron Crown]

1. Ditch the radial mounted chutes, they're really draggy.

2. Ditch the radially mounted intakes, they're really draggy, too.

3. Ditch at least one of the probe cores, if not both.

4. Ditch two of the engines. Slashy's advice to ditch the rear ones and replace them with a tapered tail is a good one, or ditch two nacelles.

5. Ditch the monoprop tanks on the engine nacelles. The docking port has plenty of monoprop storage for a single docking.

6. Consider ditching two of the LFO fuselages and using equivalent tanks in the nacelles instead. This will make the plane shorter and easier to handle with the same fuel.

7. Angle the main wing upward a very small amount, so it has a small AoA when level. This helps keep the thrust in a more useful direction.

8. Add small control surfaces to the wingtips for roll control. Horizontal tail surfaces should be pitch only.

9. Use the main wings for LF storage if they are not already.

10. Adjust the main gear so they are perpendicular to the runway. Consider using the smaller, lighter gear bays instead.

11. Most importantly, work on the ascent profile. It should be able to hit 1300+m/s in airbreathing mode by about 21km altitude or so before switching to rocket mode. I strongly recommend using manual mode switching and an action group to toggle modes, this will let it ride the airbreathers up over 25km.

Er, that sounds like I'm slagging on your design but I'm really not. Just sharing some of the design principles I've learned when messing around with spaceplanes in the new aero, and reading what the skilled spaceplane people are doing.

[davidpsummers]

I've tried some incremental changes. (Like I say, it almost achieves orbit in my hands and incremental changes let me see how it works.) A bit less fuel, loose the in-line rapiers.

This brings up two questions.

a) The engines below the wings aren't lined up with the COM, how do I keep them from pitching the plane up?

Do you need to use that silly, long nosed, "tail connector" at the back tip to reduce drag, or will a standard nose done work?

[GoSlash27]

I've tried some incremental changes. (Like I say, it almost achieves orbit in my hands and incremental changes let me see how it works.) A bit less fuel, loose the in-line rapiers.

This brings up two questions.

a) The engines below the wings aren't lined up with the COM, how do I keep them from pitching the plane up?

Bring them up so that the center of thrust is in line with the center of mass.

Do you need to use that silly, long nosed, "tail connector" at the back tip to reduce drag, or will a standard nose done work?

A standard nose cone will work fine. An advanced nose cone will work better. A single shock cone intake will work even better still, but looks kinda dorky.

Best,

-Slashy

[KerikBalm]

I can reach >1300 m/s at above 25 km before switching over, I just don't have enough dV at that point.

I find that surprising... are you talking surface velocity or orbital velocity? I think most people talk about surface velocity while in the atmosphere, I know I do. (which is about 1,500m/s orbital velocity)

but with four I got a slow climb (I can do maybe pitched up 30 degrees and, if I spend some time gathering a bit of speed before I climb, 200 m/s)

I climb my SSTOs at about 20, sometimes 30 initially (I did 30 in 1.02), so that is fine, and it actually seems you may be climbing too fast.

The underwing rapiers give you off center thrust, I don't like to use offset to do part clipping, but you could do that. You could give the wings some slight dihedral, and elevate them a little, so the engines slung underneath are pushing against the center of mass.

Alternately, place Mk2 parts on the sides of your main fusalage, and then place your wings on those (see my SSTO pic below)

As to your question about turbojets: they use less fuel, and have more power up until about mach 3, the rapiers may do better at the very limits of speed and altitude, but you spend so little time there, and its nice to have an apoapsis above where rapiers stop working, rather than limping along at the limits of rapiers. My larger designs use rapiers and turbojets in a 1:1 ratio - I'm not sure its optimal, but it works and is easy to remember.

Definitely remove the radial intakes, they are terrible for high speed craft.

Shock cones are lower drag, nearly the same intake area. Prior to 1.04, they were more prone to overheating than rams, but that is no longer the case (which is a big improvement, as they get a lot of heating at the front)

You'll want control surfaces on the wings.

I know others told you to remove the rear engines, and taper it and put a cone, I suggest putting an aerospike engine or a LV-N. Both get better Isp than rapiers, allowing you to shut down rapiers as soon as you don't need the TWR. I often find I packed too much LF, and having a LF only engine for use in space can be quite nice. Nothing worse than being in orbit with 4000 units of LF, and no oxidizer, no LV-N, and no way to deorbit.

For comparison, here is an SSTO that I made that has many similarities to yours, note, that it is at Mun...

Gets to a low orbit, does a ~850 m/s burn to go to the Mun, the ~300 m/s burn to circularize, more burns to dock.

Then another ~350 to getback to Kerbin.... and it still has a lot less dV remaining. In fact, I've also taken it to Mun orbit, then to Minmus orbit (by a not so optimal trajectory.... but also not so bad), then back to Kerbin, I could probably have landed it on Minmus too, but that gets a bit over the top. I don't want to take SSTOs beyond LKO generally, I even made a crew ferry meant to go from LKO to the moons of kerbin, but I can't be bothered to dock and rendevous with it, and keep it fueled, etc, when my SSTOs can just do the whole thing by themselves.

My "chemical only" (ie, no LV-N or Ions) have a lot less dV than that, but you get the idea....

An aerospike has 340 Isp, a LV-N... 800, so they are left with a lot less

Mk2 cockpit + m2 crew cabin, like yours (except I use the inline, because I like to dock head on)

A pair of Big-S delta wings, like yours.

Unlike yours, my mk2 fusalages are all LF only, the only oxidizer is carrier in the adaptors.

2 rapiers, 2 turboramjets, 1 LV-N.

The rapiers burn the Oxidizer to get a high enough apoapsis, and the LV-N does the rest (sometimes i don't burn all the oxidizer, and I have a longer orbital insertion burn with the LV-N, but then I have oxidizer to make the Mun burn in a single burn with the LV-N instead of perapsis kicking).

I think you are packing way too much LFO tanks, and you are running into the tyranny of the rocket equation... diminishing returns.

I would cut out two of the Mk2 LF long fusalage tanks

I know it exploits the OP reaction wheels and infite restart, completely throttleable with no efficiency loss engines, but I also dock with no RCS, and have no RCS thrusters on my craft (although I did intend to add some to make it easier).

You have a lot of Monoprop on that... if you insist on keeping it for docking, consider adding some monoprop thrusters as an OMS system, so you can at least use it for deorbiting after docking.

Edited August 20, 2015 by KerikBalm

[Vegetal]

Well, I have a Rapier-only spaceplane SSTO that can orbit the Mun and carry a 2.5m lander (and get back too of course). The thing is, it's very underpowered. It's bigger than yours (Mk3) and only has 4 engines, it takes very long to climb.

What happens is that the most engines you have, the better and easier it will be to climb in the atmo, but your delta-V once in orbit will suffer with the extra weight. In my case, the priority was vaccum capability, so I sacrificed atmo performance. You will need to do that a little to your plane.

For reaching a 250km orbit with 6 kerbals, you don't need much. I guess you will be fine with 3 engines, and scaling your plane down a bit, preferably shortening it. I don't think you have too little lift, as the Mk2 parts are lifting bodies.

[Starhawk]

I designed this spaceplane yesterday which is shown in 77x77 km orbit with 261 m/s remaining in the tanks. Enough to take it's 6 kerbals to a rendezvous with a station at 250x250 km.

It has just one rapier. It does slog a bit through the transsonic, and needs a fairly agressive flight profile, but it gets the job done.

It really doesn't take a large craft to do the job.

Happy landings!

[davidpsummers]

Well, I got my plane to my space station. The idea that I had too much drag and didn't need as many engines allowed me to know what to look at.

A question, I was going to just tweak it, but I moved the side engines/tanks to fuselage mounted. (One thing lead to another and it also ended up a lot shorter to keep from having to many tanks, but I didn't have a problem with the length either way.) That lead to wings being attached to the side tanks. Now I am having some problems with wobbly. I assume, even under the new drag rules, a few struts are still in order?

[GoSlash27]

Well, I got my plane to my space station. The idea that I had too much drag and didn't need as many engines allowed me to know what to look at.

A question, I was going to just tweak it, but I moved the side engines/tanks to fuselage mounted. (One thing lead to another and it also ended up a lot shorter to keep from having to many tanks, but I didn't have a problem with the length either way.) That lead to wings being attached to the side tanks. Now I am having some problems with wobbly. I assume, even under the new drag rules, a few struts are still in order?

davidpsummers,

First of all, congratulations!

Now... Struts are murderously draggy, so you want to avoid using them whenever possible. A lot of the time you can make the structure more rigid by reshuffling parts instead of adding struts.

You wouldn't want a load traveling through multiple attach nodes.

Instead of attaching the wings to the outer engine pod, why not attach the wing directly to the fuselage and have the pods directly above or below it? Or instead of having 2 pods on each side, just have 1 that is composed of Mk2-twin Mk1 adapters. That would eliminate an attachment node.

Can we see a pic of what you have right now?

Here are some examples of unorthodox construction techniques (not suggestions for specific design choices)

Best,

-Slashy

Edited August 22, 2015 by GoSlash27

[davidpsummers]

davidpsummers,

why not attach the wing directly to the fuselage and have the pods directly above or below it? Or instead of having 2 pods on each side, just have 1 that is composed of Mk2-twin Mk1 adapters. That would eliminate an attachment node.

Well now. For some reason that hadn't occurred to me... :-). I might even be able to attach them directly to the fuselage above and below wing.

While your suggestion seems the way to go, here is a picture. I'm thinking of replace the lone turbojet with an aerospike. The turbo make a nice assent profile easier, but it occurs to me that I spend a lot more fuel on the push up to orbit...

Edited August 22, 2015 by davidpsummers

[GoSlash27]

Well now. For some reason that hadn't occurred to me... :-). I might even be able to attach them directly to the fuselage above and below wing.

While your suggestion seems the way to go, here is a picture. I'm thinking of replace the lone turbojet with an aerospike. The turbo make a nice assent profile easier, but it occurs to me that I spend a lot more fuel on the push up to orbit...

http://i.imgur.com/lm6ZWVJ.png

Looks like it's coming along nicely.

I'd recommend ditching the turbojet completely and replacing it with a nose cone. I don't think an aerospike is going to have the oomph to push that up to orbital velocity in time by itself, and it's thrust is low compared to the RAPIERs so you wouldn't improve the overall Isp very much.

Now that you've achieved orbit, your next goal is to start trimming your spaceplane back. As you can see, it's way more spaceplane than is necessary for this job. Efficiency tends to build on itself, and it also allows you to build a more rigid and easier-to-fly vehicle.

Good luck!

-Slashy

[juanml82]

I played around with Turbojets. They just didn't seem to give much power, what do the offer over a rapier?

Basically, they can give you the extra punch you need to break the sound barrier.

http://i52.photobucket.com/albums/g13/GoSlash27/Probuildtwo_zpsgpmqo800.jpg

Best,

-Slashy

Wait, wait, how are you making it to orbit with so little fuel

Well now. For some reason that hadn't occurred to me... :-). I might even be able to attach them directly to the fuselage above and below wing.

While your suggestion seems the way to go, here is a picture. I'm thinking of replace the lone turbojet with an aerospike. The turbo make a nice assent profile easier, but it occurs to me that I spend a lot more fuel on the push up to orbit...

http://i.imgur.com/lm6ZWVJ.png

I think that's too much wing. Wings cause both lift and drag, so unless I'm mistaken, you actually want as little wing area as you get away with.

You don't need two probe cores. If you're using them for the torque or the battery, you can use 1.25m parts between the 1.25m fuel tanks. Also, having the four engines set up that way will also increase drag, I think you'll get better results by using a decoupler in the tail to fit two engines.

And if you want to toy with something different, four rapiers and two nervs can also put your six kerbals in orbit and leave you with quite a lot of delta-v to spare, even (or specially) if the rapiers aren't given enough oxidizer to push all the way out of the atmosphere.

[A_name]

Your craft is like three times the size it should be. Cut back on everything. You need 2 RAPIERS at most for a payload consisting in a crew cabin. I would go for cockpit>crew cabin>small rocket fuel fuselage>docking port>small rocket fuel fuselage>Mk2 to 1.25m adapter>advanced nose cone, plus an assembly of shock cone intake>1.25m liquid fuel fuselage>rapier on either side. Once you downsize it to that, you no longer need the big ass wings or landing gear. The old landing gear should do, as should the normal-sized delta wings. Mount your control surfaces on the rear nose cone, lose the parachutes which are very heavy and the radial intakes which you won't need and are terrible for drag, and you're good to go.

- - - Updated - - -

Well now. For some reason that hadn't occurred to me... :-). I might even be able to attach them directly to the fuselage above and below wing.

While your suggestion seems the way to go, here is a picture. I'm thinking of replace the lone turbojet with an aerospike. The turbo make a nice assent profile easier, but it occurs to me that I spend a lot more fuel on the push up to orbit...

http://i.imgur.com/lm6ZWVJ.png

This is still way to big, but it's definitely better than the first one. You're really overestimating the amount of wing and control surfaces you need. Lose the big wings and replace them with the medium delta wings which should provide plenty of lift. The Mk2 parts have lots of body lift on their own, they don't need much wing. Also lose one pair of the side RAPIER assemblies. You don't need that much engine. Once you do that you'll be lighter so you could probably afford to downsize the remaining 2 engine assemblies, I really don't think you need so much fuel. You don't need 2 pairs of horizontal stabilizers at the back either. I would also get rid of the parachutes, although I imagine you probably put them there for "safety reasons". And like Slashy said, get rid of the rear turbojet. Finally, put the docking port between the large fuel tanks which is closer to the center of mass and thus will be easier to dock with.

Edited August 23, 2015 by A_name