SSTO limitations and interplanetary spaceplanes

[diegzumillo]

Hi all

I've been trying for a while to make an interplanetary ssto space plane with no success. I'm disconsidering refueling in orbit, of course. My best designs are able to make a round trip to mun and minmus (including landing) but I never managed anything remotely close to going to another planet.

There seems to be an upper limit to spaceplanes, they simply don't scale up. When I try to make a bigger spaceplane based on a successful smaller version it takes so much fuel to get it to orbit that it gets there with the same amount of delta v as the previous version, approximately.

Anyone has any thoughts on it? ever succeeded in making an interplanetary spaceplane in one stage or maybe a non-space plane ssto? again, without refueling.

[FishInferno]

I personally never understood the need for an interplanetary spaceplane. I would just take the payload to LKO, release it from the spaceplane, dock it to a interplanetary tug, and send it on its way. This way you only need to pay for the trip to LKO. Just my 2 cents.

[GoSlash27]

^ There's that, but I'll set it aside.

Solid spaceplanes scale.

What is going wrong with your spaceplanes that they're not scaling? What difficulties are you running into with big spaceplanes that you're not seeing in little ones?

-Slashy

[diegzumillo]

It's a nice challenge.

[diegzumillo]

^ There's that, but I'll set it aside.

Solid spaceplanes scale.

What is going wrong with your spaceplanes that they're not scaling? What difficulties are you running into with big spaceplanes that you're not seeing in little ones?

-Slashy

All I can really say is that they get much harder to maneuver. So I don't really know if they are scaling and I'm messing up the ascension or if they really don't scale well.

[GoSlash27]

We're getting somewhere

Are you still hitting the same speed before you have to go to rockets, or are you going to rockets earlier? Are you losing yaw control and spinning out?

[diegzumillo]

It usually can't pick up the same amount of speed. Do you think I may be scaling the mass more than the thrust? now that I typed that sentence it started making some sense...

[numerobis]

You have to scale the thrust to the mass. At low altitude, that means more turbojets. At high altitude, that means more intakes.

[GoSlash27]

If you're not suddenly losing yaw control, then it's not the problem I figured you were dealing with.

But yeah, not scaling the wings, engines or intakes linearly would cause you to run out of steam early.

Having to go to rockets early will erase any gains you might have from scaling.

Best,

-Slashy

[diegzumillo]

And how does the delta v scales?

[GoSlash27]

It doesn't.

Ignore DV for your jet- powered portion of the ascent. You just want 150 units (one LV tank) of jet fuel per turbojet engine.

For the insertion portion, you will need to budget 150 m/sec from your LV-N, which is the engine you will be using for an interplanetary spaceplane regardless of size.

Best,

-Slashy

Edited December 11, 2014 by GoSlash27

[diegzumillo]

It doesn't.

Ignore DV for your jet- powered portion of the ascent. You just want 150 units (one LV tank) of jet fuel per turbojet engine.

For the insertion portion, you will need to budget 150 m/sec from your LV-N, which is the engine you will be using for an interplanetary spaceplane regardless of size.

Best,

-Slashy

Oh, I know it doesn't matter for the ascent, but my hope was to get into orbit with a bit more dv left than the smaller version. You're saying it won't? D:

[liweih]

The air intake is your best friend.

[GoSlash27]

Oh, I know it doesn't matter for the ascent, but my hope was to get into orbit with a bit more dv left than the smaller version. You're saying it won't? D:

No, it will, if done right. Doubling the mass of the spaceplane means doubling the fuel for your LV-N. Actually more than doubling it, since a lot of your core components like the cockpit and LV-N don't get doubled.

But before any of that can happen, you have to be able to double the mass of a spaceplane without losing your gains.

You double the wings, double the engines, double the intakes, and double the payload... but almost all of that additional payload is fuel for your LV-N.

Best,

-Slashy

Edited December 11, 2014 by GoSlash27

[Wanderfound]

Single Stage to Jool, no refuelling: http://s1378.photobucket.com/user/craigmotbey/Kerbal/Challenges/SSTYC/story

Nukes make it fairly simple. A solid but not overweight airframe, just enough jets to take off, an LV-N and about twice as much fuel as you'd put on a normal spaceplane that size.

Edited December 11, 2014 by Wanderfound

[diegzumillo]

That's fantastic! So I am messing the ascent, I can't put anything that size and mass with only 2 jet engines. Is there any catch on your design, like 57 overlapping air intakes?

Edited December 11, 2014 by diegzumillo
fixing engrish

[GoSlash27]

Wanderfound is "the man" for interplanetary spaceplanes.

No, you don't have to intake- spam to make it work. Really, you can lift spaceplanes at any scale economically into orbit with just .015m^2 of intake per engine.

You have engineering challenges that come with making bigger spaceplanes such as asymmetric starvation, floppy wings, etc, but other than that, it all scales linearly in my experience.

Best,

-Slashy

[Wanderfound]

That's fantastic! So I am messing the ascent, I can't put anything that size and mass with only 2 jet engines. Is there any catch on your design, like 57 overlapping air intakes?

FAR. Making spaceplanes work in stock soupmosphere is more trouble than it's worth.

But, apart from that, no part clipping or other trickery. If it can hit 130m/s before the end of the runway, it can go to space.

Have a poke at the design tutorial at http://s1378.photobucket.com/user/craigmotbey/Kerbal/Tutorials/Hangar%20to%20Landing/story as well at the first few posts in the Kerbodyne​ thread.

[diegzumillo]

Ah, FAR explains it. I did make some interplanetary space planes with FAR too. It's harder in some ways but a bit overpowered in others. I enjoyed messing with this mod for a while but overall I think it's less fun than stock.

[Wanderfound]

Ah, FAR explains it. I did make some interplanetary space planes with FAR too. It's harder in some ways but a bit overpowered in others. I enjoyed messing with this mod for a while but overall I think it's less fun than stock.

The same flight principles apply in stock, however; build and fly right and you hit orbit with less than a minute of rocket power, and if you've brought an LV-N with you you can probably go interplanetary.

[Overfloater]

When increasing the overall weight, did you also increase the amount of intakes by the same percentage?

How many intakes your small plane had?

If it had (for instance) 5 intakes per 10 tons, then so should your bigger plane if you want it to perform the same.

The more intakes you have, the less rocket fuel you will use to circularize.

I use 10 intakes per 13 tons, that's just enough to circularize on jets alone.

edit: by the way, the smaller you craft is, the less intakes per weight it will need - because with stock aerodynamics less parts means less drag.

Edited December 11, 2014 by Overfloater

[diegzumillo]

Hm, I didn't consider I had to increase the air intakes with *mass*. I was increasing the air intakes with the number of additional jets only.

[Stratzenblitz75]

Use this trick. By placing your intakes and jet engines one after another, you can eliminate asymmetric flame-out and make the most out of your air intakes. Seriously, I performed some tests on this and it turns out that a craft using the previously mentioned trick performs MUCH better than a craft that doesn't. You can see my results here(see the bottom two albums). Also, since you don't have to deal with asymmetric flame-out, its very easy to scale up the design. I made a series of Jet-abusing heavy-lifting SSTOs based off of this concept.

Also, 10 intakes per 13 tons is ridiculous. You can get away with much less. My SSTOs used about 1 RAM intake per 4 tons and they made it orbit almost purely on jet power. (They only needed a small 50 m/s insertion burn at Apoapsis)

[cicatrix]

Aside from their 'kool' look, is there a real practical niche for space planes? Any 2 stage to orbit vehicle will have greater payload/weight and cost effectiveness than a spaceplane. Spaceplanes take longer time to ascent too - I think of those tedious minutes that I waited for my SSTO to claw its way through the atmosphere... No, I stopped building them. I'm on a tight budget and money do matter. Rockets are cheap and reliable.

[Overfloater]

Also, 10 intakes per 13 tons is ridiculous. You can get away with much less. My SSTOs used about 1 RAM intake per 4 tons and they made it orbit almost purely on jet power. (They only needed a small 50 m/s insertion burn at Apoapsis)

You can, especially with very low part count. A high-part-count craft will be flying at about 27,000m accelerating just barely, while the fuel burns at full throttle. A bigger portion of the fuel will be needed for the jets, this can compromise the interplanetary capability.

If you're going for less intakes try to avoid putting dead weight like girders, i-beams, tail cones, NCS adapters...

Engines are dead weight too: If you're going to Laythe you don't need high TWR in space, so minimize the rocket engines.

Edited December 11, 2014 by Overfloater