Laythe Planes

[HighFructose]

Well, today I was assembling my Jool-5 kind of mothership, (I'm not actually following the challenge rules, but I'm doing it for fun.) I was building a plane that I can use on Laythe. The idea is to drop from Laythe orbit, then fly around a bit, plant flag on an island, then SSTO to the awaiting mothership. I had a Kerbin SSTO I was going to use, but in the process of modifying it for my needs, it can't SSTO from Keebin anymore. I'm assuming that if it can SSTO from Kerbin, than it can SSTO from Laythe. 

So tl:dr, I'm wondering if a Kerbin SSTO can also SSTO from Laythes surface.

[The Aziz]

Possibly, but remember that Laythe has thinner atmosphere, also it's less dense. Only 0.6 atm at sea level. So jets won't get you as high as on Kerbin, but, on the other hand, space starts 20km lower.

I never did that though, so it's just a guess.

[qzgy]

SSTO-ing from laythe, IIRC is stupidly easy. Slap a rapier on some LF and oxidizer tanks and it should be able to get to laythe without too much trouble. The best thing to do is take advantage of the lower "space" line and use as much of the atmosphere as possible.

Edited August 3, 2017 by qzgy

[fourfa]

The hardest thing, to be honest, is landing.  The lower atmo pressure means a regular Kerbin SSTO will need to land very very fast, and there are no flats anywhere on the islands.  

One option is parachutes for the last few meters. 

The other is to find the flat places - for instance, ice caps on the poles.  And the other 98% of the planet that's perfectly flat...

[qzgy]

8 minutes ago, fourfa said:

flat...

Yes, flat, and will break your fall slightly softer than earth.

Hydrobraking?

[Allocthonous]

2 hours ago, qzgy said:

Yes, flat, and will break your fall slightly softer than earth.

Hydrobraking?

If you have big fins sticking down from the bottom of your plane, this is pretty effective on Kerbin. Get a nice flat glide going and gently dip them in the water, and you slow right down. Should apply to Laythe too, although if the atmosphere is less dense then your minimum glide speed is going to be higher.

[KerikBalm]

12 hours ago, The Aziz said:

Possibly, but remember that Laythe has thinner atmosphere, also it's less dense. Only 0.6 atm at sea level. So jets won't get you as high as on Kerbin, but, on the other hand, space starts 20km lower.

I never did that though, so it's just a guess.

Actually, jets get your higher on Laythe than Kerbin. Perhaps in a nod to real life, the air doesn't "thin out" as fast as it does on Kerbin. The atmospheric scale height on Mars and Titan IRL is higher than Earth - because the gravity doesn't "compress" the atmosphere as much. To get 1 atmosphere of pressure at 0.14 g (Titan's gravity), one needs 1/0.14 = 7x the mass of atmosphere above the surface. KSP does cut off the atmosphere at a lower height, so the resulting comparison is like this before the steep drop-offs to exactly 0 pressure:

If it can SSTO on Kerbin, it can SSTO on Laythe, the only possible exception is if it has trouble taking off on Kerbin, as the atmosphere on Laythe is a bit thinner at ground level, and in addition to higher takeoff speeds, the jet thrust is lower. Also the terrain is generally harder to takeoff from than the grass around kerbin due to lumpyness (and seaplanes have their own challenges).

The jet thrust isn't soo much lower though if you are using rapiers or turboramjets. Because The surface G is 0.8, and the surface pressure is 0.6, if the relationship between pressure and thrust were 1:1, you'd have a 0.6/0.8  3/4 ratio of Laythe TWR to Kerbin TWR.... Now these graphs are not accurate between datapoints, the real graphs are curved, not linear... but...

To get the same TWR, that ratio would need to be 1.33:1 at 0.6 atmospheres... which the wet panther seems to get, and maybe the Juno. I'm guessing a curve properly fit to the datapoints would have the turboramjet and rapier lines below that.... but its not hard to get to orbit of laythe with a panther powered spaceplane (in contrast, its a bit difficult on stock kerbin).

I should also point out that jet thrust is a function of density, not pressure as shown on the graph (although its a fairly good approximation). The MW of the gas on laythe is the same as kerbin, or close enough to the same. The atmosphere is a bit colder which should make it a bit denser for the same pressure (I *think* KSP models this), so you're going to get a bit better than 0.75x kerbin TWR.

But... the main difference is the lower orbital velocity... around 1850 m/s if I recall correctly. On kerbin its more like 2350. The rotation of kerbin gives you basically +200 m/s, whereas the rotation of laythe gives one about 80 m/s. So assume your SSTO reaches 1500m/s surface velocity on air breathers... on kerbin that's about 1700 m/s orbital velocity... and you need another 650 m/s plus some extra for atmo drag and increasing altitude.

On laythe, thats 1580 m/s orbital, and you need another 270 m/s plus extra for air drag and increasing altitude... but you get higher on laythe (see graph above) yet the atmosphere ends sooner, so you don't need to increase your height as much and you'll lose less to drag after airbreathers cut out... so you need much less oxidizer / non-airbreathing dV. Additionally, because you'll be at a higher % of orbital velocity, you can have a lower angle of attack towards the last part of airbreathing thrust/going to orbit, and you should be able to reach a higher airbreathing speed. On top of this, your TWR at high altitude is better than on kerbin: sure the atmosphere starts thinner, but jets cut out at the same air density, so when operating at high altitude approaching their limit, this difference is gone, and terminal airbreathing air density is the same - but the gravity is lower - this factors in with the higher % of orbital velocity, and you can have a much lower angle of attack at high speed and altitude, and reach higher airbreathing velocity.

All this combines to make it much much easier to SSTO on laythe. Liquid fuel only spaceplanes are much easier (ie don't carry oxidizer for the rapiers, carry an LV-N).

The only problems would be take off and landing

 

12 hours ago, fourfa said:

The hardest thing, to be honest, is landing.  The lower atmo pressure means a regular Kerbin SSTO will need to land very very fast, and there are no flats anywhere on the islands.

I wouldn't call it very very fast. The reduction in atmospheric density is less than 1.33x the reduction in gravity. Lets assume that its 1.33 (ignoring temp difference). You would need 33% more lift, due to the V^2 relationship, that means you'd land sqrt(1.33) times as fast: just 1.153x as fast.

15% faster is not "very very" fast IMO.The reduction in air temperature should reduce the difference to lower than that.

There are some pretty flat places with solid ground (but yes to what you alluded at, seaplanes can work too)

Spoiler

^This one is normally not an SSTO on laythe, but I have a rover with LFO tanks and rapier engines that I can drive into the cargobay, dock to the plane, and convert it into a laythe SSTO- its pretty hard to make that work on kerbin, but as I said, its much easier to SSTO on laythe)

^ That one in the background is a seaplane

It was a bit hard to get this off the surface, as pulling up would often have the rear pontoons submerge and slow the craft too much. In the end I needed action groups to deploy surfaces on the hydrofoils and canards, with none of the rear control surfaces deflecting for pitch control (which pushes the back down), but it worked.

 

Edited August 3, 2017 by KerikBalm

[HighFructose]

Thanks everyone for the replies! I think I'm going to redesign my spaceplane. The first one, which I didn't like anyway, even though it's my first SSTO, is broken. I needed to make changes so I will use this data to make the next one better. I'll post pictures of the new one once I build it.