"Good" payload fraction for a space plane? And general advice for d/v and fuel calculation.

[Cairol]

Normally I go with rockets, because I'm not very expierienced with space planes.

But for fun I decided to build a space plane to transport 3 communication satellites to the Muns orbit.

Technically it's an SSTO, but that was not a "must" for me in this case.

The payload is only 3t total.

My space plane has a mass of 43,600 kg, which leads to a payload fraction of 0.07.

And the ship does not even have enough LFO in the current form to get to the Mun and back. It barely brings me to LKO without circularization.

Means: I even have to stick more tanks on it, making it heavier than it already is, probably leading to sticking on even more engines, and so forth...

Before you ask: My ascent profile is this: Climb at 40° to 9-11 km, level out and speed up to 1000 m/s, climb at about 25° till 22 km, switch to closed cycle and accelerate to 1600 m/s,

climb till reaching target AP. Don't know if this is efficient at all...

And now I wonder what payload fraction is considered "good" for a space plane.

And if someone could tell me a rule of thumb how much d/v and how much LF and Ox I need

in total and in relation to each other to get such a space plane into orbit, it would help me a lot

with my building phase. A good video tutorial on that would also be helpful.

I like flying space planes to orbit, because it's much more "work" to do than just using a rocket.

But I obviously need more expierience on how to create efficient designs.

[mhoram]

According to the Stock Payload Fraction Challenge, some people managed to get payload fractions above 45% for ssto spaceplanes.

[SpaceplaneAddict]

Hey, I just wanted to give you some tips, here ya go. Sorry I can't give a "rule of thumb"

- Replaced some of those LF tanks with LOx

- You don't nead those Lv909's, you already have RAPIERS

-Before you say you nead the extra TWR, I doubt it, as lond as you climb shallower, you'll be fine

Other than that, you're fine

Hope I helped!

SPAddict, out.

P.S Maybe some more wing too.

[The_Rocketeer]

Considering the amount of xenon/RCS you're bringing, why bother taking the spaceplane to Mun? Just put the sats in LKO and then ION-drive them out to mun.

[kStrout]

Is there any reason you need it manned? I see you have a drone core there, and if the only point is to launch satellites, then you should switch out the nose for a 1.25 to mrk 2 adapter with a tail connector on front, more fuel less drag similar mass! Also that design looks like you should only need 2 rapiers, try getting the side tanks down to mrk 1.

[RexKramer]

Comments above have trended towards removing stuff you don't need. I'll continue that with advice that two Rapiers would be more than enough. Lose the 909s, as well as two of the Rapiers.

[Cairol]

Is there any reason you need it manned?

I run remote tech and I have no connection on the backside of the Mun. That's why I fly kerbaled and that's why I bring those satellites to the Mun in the first place.

The drone core is...well, just for emergencies and for fun in this case. I can test my orbital network with it after deploying all sats. (two more in polar orbit will follow)

Considering the amount of xenon/RCS you're bringing, why bother taking the spaceplane to Mun? Just put the sats in LKO and then ION-drive them out to mun.

I want to deliver them by the space plane to be able to place them precisely in 120 degree angle to another in orbit.

For me, thats best done with a deploy vehicle in Mun orbit that has a third of the orbital period of the satellites. So I can drop one satellite each time my ship is reaching the AP marker

and only do the fine tuning with the sats after deploy.

I've done that a couple of times now on other bodies to get my orbital network up and running.

The Terriers were intended for slightly more efficient space burns. I don't need them for the ascent.

But I guess it does not matter much on that short trip. So I'll discard them.

I tried it with only two Rapiers. But my TWR was too low to get to space before running out of fuel.

I'll see how I can cut down the mass of my craft.

[The_Rocketeer]

I want to deliver them by the space plane to be able to place them precisely in 120 degree angle to another in orbit.

For me, thats best done with a deploy vehicle in Mun orbit that has a third of the orbital period of the satellites. So I can drop one satellite each time my ship is reaching the AP marker and only do the fine tuning with the sats after deploy.

I've done that a couple of times now on other bodies to get my orbital network up and running.

You could achieve the same thing by making the hohmann transfer with the sats still in a stack, using just the rearward engine. Get the whole stack into a 1/3 orbit, then just separate from one end and do your fine tuning. I did a similar thing when I deployed a series of probes around Jool's moons.

[GoSlash27]

Cairol,

for a Mk2 spaceplane 25% payload fraction is considered good. For a Mk.3 30% or so.

I have a Mk.3 that does 41%.

I only ever build spaceplanes to go to LKO and back. That is what they're best at.

From what I'm seeing from your description, your main problem isn't your ascent profile, but rather your mindset. Spaceplanes aren't perfect when you can't add anything more to them, they're perfect when you can't take any more off of them.

Always look for what you can remove, not what you must add.

Best,

-Slashy

- - - Updated - - -

By way of demonstration

This is an example of what you can orbit with 4 RAPIER engines. Clearly you've got more than you need in that department. This tanker transfers 40t of payload in LKO.

Conversely...

This plane is in your weight class and orbits nearly 9 tonnes on a single RAPIER.

Aerodynamically clean, no wasted mass, and perfectly balanced supplies. This is what makes an efficient SSTO spaceplane.

Some specific pointers for your spaceplane:

-Mk2 bicouplers are draggy. Try not to use those for mounting paired intakes.

-Struts, external RCS quads, and external RCS tanks are draggy.

-Too many control surfaces reduce your efficiency and make your handling twitchy.

-solar panel arrays are draggy. A couple panels mounted to the tops of the wings are low- profile and clean and will do the job.

-Extra engines are mass that you have to carry to orbit. a single RAPIER can do this job.

-You can save a lot of drag by setting up the plane with some wing incidence (front of the wings higher than the rear) so that it creates lift while the fuselage is aligned with the airflow. Mk2 fuselages are clean when they aren't crooked.

Best,

-Slashy

Edited September 13, 2015 by GoSlash27

[Vegetal]

Ditch the Terriers, ditch 1 or 2 Rapiers.

You also have 2 big and draggy RCS tanks on the outside. First - you don't need that much RCS. Second - if you really want them, put them inside the cargo bay, beside the probes. Drag is king.

[GoSlash27]

Assorted good advice... Drag is king.

^ This. If you take nothing else from this thread, take this. Drag is king. If it looks draggy, it *is* draggy and drag is the #1 source of inefficiency in spaceplanes.

Best,

-Slashy

[RexKramer]

Here's an example of what I accomplished removing drag. I like twin engine designs, which for the mass I'm lifting (similar to the OP's goal) are overpowered. I had a good design with twin Rapiers mounted to the side of a MK2 tube. It was able to get to LKO with about 2/3 of the payload I wanted.

I removed the side tanks and engines, and went with two engines off the back of the MK2 tube via bicoupler. The result was a 50% increase of payload to LKO, using less fuel to get there, with an extra 600m/s over what I had before. All this just from removing the drag of laterally mounted engine pylons.

I realize the bicoupler isn't the greatest. However, just by streamlining my vessel by removing everything mounted laterally except the wings, I was able to vastly increase the performance.

[GoSlash27]

I realize the bicoupler isn't the greatest. However, just by streamlining my vessel by removing everything mounted laterally except the wings, I was able to vastly increase the performance.

RexKramer,

Bicouplers are actually very good for that job. It's just when people turn them backwards to use them to mount intakes when they turn piggy. If you want to mount intakes and engines parallel to the main fuselage (this is often a necessity), it's better to use Mk1 parts. That way the shock cones occlude the entire stack.

Best,

-Slashy

[Vegetal]

^ This. If you take nothing else from this thread, take this. Drag is king. If it looks draggy, it *is* draggy and drag is the #1 source of inefficiency in spaceplanes.

Best,

-Slashy

On that subject, I started experimenting with giving wings a little bit of incidence angle. Aircraft behavior got stranger and less predictable, but the gains in performance were so absurd that I applied this to most of my spaceplane designs.

Why? Well, if your aircraft is perfectly pointed towards prograde when level, all the fuselage drag comes to a minimum, and even Mk2 planes benefit from this. In some extreme cases you can ditch one or two engines with this method, ALL BECAUSE OF DRAG!

All hail Drag! Sacrifice things to please the Drag god!

[GoSlash27]

if your aircraft is perfectly pointed towards prograde when level, all the fuselage drag comes to a minimum, and even Mk2 planes benefit from this. In some extreme cases you can ditch one or two engines with this method, ALL BECAUSE OF DRAG!

All hail Drag! Sacrifice things to please the Drag god!

^ Yup.

Especially with Mk2 fuselages. They try to create lift and when they do it's extremely inefficient for drag. If you can get it set up so that you're aligning the fuselage with the slipstream, you need very little thrust to get it supersonic and beyond. Less thrust means less engines, which means less of everything else that's not payload.

This cascades and propels spaceplanes into super- efficient territory. It's all about the mindset of eliminating waste instead of adding stuff. All praise the drag Gods!

Best,

-Slashy

[Cairol]

Some really good tips here.

I'll see how I can reduce drag and become more efficient.

The second SSTO shown by Slashy shows clearly that my design is far from optimized...

Never could have imagnied that one Rapier could bring three times the payload to LKO than my design. Óó

According to the wiki, all small solar panels have the same drag. Does KSP evaluate the exposure or form of external parts and differenciates

for example between wing-mounted solars and side mounted ones?

I didn't attach that much importance to those small parts (drag-wise) annyway. Guess I should have.

I'll definitely try that "incidence angle" thingie for my wings.

Edited September 13, 2015 by Cairol

[GoSlash27]

According to the wiki, all small solar panels have the same drag. Does KSP evaluate the exposure or form of external parts and differenciates

for example between wing-mounted solars and side mounted ones?

I didn't attach that much importance to those small parts (drag-wise) annyway. Guess I should have.

I'll definitely try that "incidence angle" thingie for my wings.

Cairol,

It used to be that way, but it's been changed since 1.0 and the wiki hasn't been updated to reflect it yet.

KSP models the drag and mass of these parts, but the single panels are treated as "physicsless"; their mass and drag is added to their parent part.

Also, their drag cannot be occluded by other parts in front of them or clipping them into their parent. They are exposed to the airflow even when they look like they aren't unless they are inside a fairing or cargo bay.

The specific drag numbers for each type of panel mounted for minimal drag:

Type/Exposed area/Drag coefficient/slipstream "side drag"

single panel/.006/.738/.534

1x6 shrouded/ .107/.869/.177

1x6 simple/ .058/.873/.519

Gigantor / .104/.814/1.24

flat plate area that induces roughly the same drag (subsonic/supersonic)

single panel/ 4.6mm/4.4mm

1x6 simple/5.2cm/5.1cm

Gigantor/9.1cm/8.5cm

1x6 shrouded/9.4cm/9.3cm

^The gigantor XL actually generates less drag when mounted longitudinally than the shrouded solar panel arrays and you would need to mount 20 surface mount panels to have the same drag as a single shrouded array.

Best,

-Slashy

Edited September 13, 2015 by GoSlash27

[imcute]

On 9/13/2015 at 12:50 AM, Cairol said:

Climb at 40Ã

aerogav would yell at you