Official FAR Craft Repository

[Wanderfound]

Hi guys, I got a question about spaceplane engine choice

I am wondering if 4 RAPIER are the best choice for 70-80T spaceplane under FAR.

Here is mine: http://kerbalx.com/gilflo/FAR-Avenger-3

I am trying to find a combination giving the best DV option with enough TWR for orbit.

I need 4 turbo to be able to take off from runway, so 4 turbojets + another combination of Closed cycle engine will add more weight and I need more than 0.5 TWR to make orbit from 20.000m where turbojets become inefficient.

A combination of 2 turbo, 2 rapier and 2 nuclear or 2 other engines.... i am not sure that the weight added gives me more DV on final.

Then once in vacuum if you want to transit on Mun you can light 2 or 4 engines, you don't get more DV, you just do it faster or lower.....

Apart from a few special cases, RAPIERs will generally beat turbojet plus rocket. But as a general theme...you don't need a lot of engine if you keep your drag and mass under control.

Some examples...

One jet:

Javascript is disabled. View full album

Two jets:

Javascript is disabled. View full album

Three jets:

Javascript is disabled. View full album

Four jets:

Javascript is disabled. View full album

Most of those are, if anything, overpowered (especially the three-jet one; that's a stupidly fast sportscar of a thing). Most of 'em would work perfectly fine even if you took one of the jets away.

At the size of plane you're building, I'd probably go for a RAPIER/Nuke/RAPIER combo (like the second example above). A fair bit of streamlining and weight reduction (lose some tanks, lose some wings) to make that work, however.

What speed are you taking off at?

Edited June 9, 2015 by Wanderfound

[gilflo]

Taking of speed is 150, TOW is 73T. It can reach a 250 km orbit with 29% left in tanks.

The longer version with 4 Rapier got a long cargo Bay plus a small cargo bay. Inside is a big 5T scansat satellite with 3000Dv . No more tanks

Same take off speed, same wings, and TOW 80T. Reach also 250 km orbit with 25% left in tanks.

Got a docking port on both for refuelling in orbit.

In fact i think i don't needthat extra intakes air, but i put them because in 0.90 version, under FAR RAPIER with 1 B9 sabre air intakes each were working up to 30000m on aircycle, so i thought the more air they got, the higher i could keep enough power before switching on closed cycle.

Here i switch between 24000 and 25000m at mach 4.

I tested the ratio 20T by Rapier that is the maximum, more than that and you are unable to accelerate over mach 1.4 at 10-12 km.

Mach 1.4 seems to be kind of barrier.....once it's passed, acceleration grows quicker and quicker and you need to keep an eye on temperature to optimize acceleration and altitude gain.

Edited June 9, 2015 by gilflo

[FourGreenFields]

Is there a bug with the Mk 3 to 2.5m adapter slanted? Every time I reach roughly Mach 2.2 the thing fails, even at an AoA of less than 0.5.

[Wanderfound]

What happens if you try to take off at a lower speed (110m/s or so)? 150m/s is a bit on the high side.

Does the nose refuse to lift, or does the nose lift but the plane stays grounded?

Are you after something with unrefuelled interplanetary ability, or is it intended to be refuelled before leaving LKO? Either way can be done, but the second option is inevitably going to be a more efficient and "sporty" ship than the first. If you're taking the second option, you can afford to lose a fair bit of tank capacity.

- - - Updated - - -

Is there a bug with the Mk 3 to 2.5m adapter slanted? Every time I reach roughly Mach 2.2 the thing fails, even at an AoA of less than 0.5.

The Mk3 adaptors have stupidly low joint strength. Stock Bug Fix strengthens them a bit, KJR strengthens them a lot.

Edited June 9, 2015 by Wanderfound

[FourGreenFields]

The Mk3 adaptors have stupidly low joint strength. Stock Bug Fix strengthens them a bit, KJR strengthens them a lot.

Well, thing is, I have KJR... And I am pretty sure it is installed right.

[gilflo]

What happens if you try to take off at a lower speed (110m/s or so)? 150m/s is a bit on the high side.

Does the nose refuse to lift, or does the nose lift but the plane stays grounded?

Well, in FAR, in SPH, Take off speed is around 150 (mach 0.42 at 0km) with AOA 12.5, so 140 is my rotation speed, 150 is lift off.

Before 140 the nose won't lift and if i place the main gear to help the nose lifting before 140 i stall. There's 87m2 for wings for 73T lo lift so that's quite not a glider....

Are you after something with unrefuelled interplanetary ability, or is it intended to be refuelled before leaving LKO? Either way can be done, but the second option is inevitably going to be a more efficient and "sporty" ship than the first. If you're taking the second option, you can afford to lose a fair bit of tank capacity.

Second option is easy. I tested docking and refuelling, not a problem.What is more challenging is the interplanetary flight, with refuelling on planet, that's why i am looking for engine optimization to get more DV left in orbit

[Wanderfound]

Well, in FAR, in SPH, Take off speed is around 150 (mach 0.42 at 0km) with AOA 12.5, so 140 is my rotation speed, 150 is lift off.

Before 140 the nose won't lift and if i place the main gear to help the nose lifting before 140 i stall. There's 87m2 for wings for 73T lo lift so that's quite not a glider....

Second option is easy. I tested docking and refuelling, not a problem.What is more challenging is the interplanetary flight, with refuelling on planet, that's why i am looking for engine optimization to get more DV left in orbit

Bad stall or minor stall? A slight stall during takeoff (particularly on canards) is fairly normal. So long as your airframe design is stable, you can generally hold a minor stall for quite a while (e.g. long enough to gain sufficient altitude that you can afford to drop the nose and clear the stall) without it being much of a problem.

Maximising fuel in orbit is as much about flight profile as shipbuilding; what speed and altitude are you hitting before lighting the rockets?

- - - Updated - - -

Well, thing is, I have KJR... And I am pretty sure it is installed right.

I'm running with both; might be worth a try.

[gilflo]

Bad stall because it will not be fliable. But what is the matter, because as long as i got enough runway lenght a higher take off speed gives a higher manouvreability and better climb angle.

I light rockets at 24.5 km.

[Wanderfound]

Bad stall because it will not be fliable. But what is the matter, because as long as i got enough runway lenght a higher take off speed gives a higher manouvreability and better climb angle.

I light rockets at 24.5 km.

The reason I'm asking about takeoff is because it's the chokepoint on jet thrust; the minimum number of jets required is basically defined by "what do I need to hit takeoff speed before the end of the runway?".

If you can get it off the runway and into the air, you can generally get it to orbit. So, minimising your takeoff speed may allow you to drop an engine or two, which will in turn reduce the dead weight you're pushing through space after you've reached orbit, increasing your range.

My usual design when I'm trying for a maximum range ship is basically "one nuke, plus as much LF tankage as I can get into the air on two RAPIERs, with as little wing as possible and just enough oxidiser for the apoapsis boost". But there's a balance involved there, as there's an inverse relationship between thrust and lift required at takeoff: high TWR ships can get away with tiny wings, low TWR ships tend to need more lift.

Edited June 9, 2015 by Wanderfound

[FourGreenFields]

I'm running with both; might be worth a try.

Tried it - didnt help.

[Wanderfound]

Tried it - didnt help.

Assuming that there isn't a bug somewhere: screenshot? There might be something about the design that is causing unusual stress on the tail.

[FourGreenFields]

Assuming that there isn't a bug somewhere: screenshot? There might be something about the design that is causing unusual stress on the tail.

Used it on the front half. More aerodynamic in this case.

It looks like it's generating an awfull load of negative lift. About as much as a delta wing. I suppose that might lead to some stress... wouldnt have expected it to generate this much...

http://imgur.com/axVUWKJ

[gilflo]

The reason I'm asking about takeoff is because it's the chokepoint on jet thrust; the minimum number of jets required is basically defined by "what do I need to hit takeoff speed before the end of the runway?".

If you can get it off the runway and into the air, you can generally get it to orbit. So, minimising your takeoff speed may allow you to drop an engine or two, which will in turn reduce the dead weight you're pushing through space after you've reached orbit, increasing your range.

The minimum speed you can fly is defined by your wingload and the lenght of runway needed for take off depends on your TWR as well as your take off speed. Minimising your weight by removing engines and keeping same wing surface is not necessarily minimising runway length as you will get better wingload and lower take off speed but also lower TWR.....

My usual design when I'm trying for a maximum range ship is basically "one nuke, plus as much LF tankage as I can get into the air on two RAPIERs, with as little wing as possible and just enough oxidiser for the apoapsis boost". But there's a balance involved there, as there's an inverse relationship between thrust and lift required at takeoff: high TWR ships can get away with tiny wings, low TWR ships tend to need more lift.

So what is the maximum weight you can send on orbit on one Rapier and what make the difference between 2 Rapier and 2 Rapier plus one nuke? At 24km when you switch your Rapier on rockets, Do you really need the small thrust of the Nuke or is there a kind of "frontier" before reaching orbit where you will switch off rockets and rely only on Nuke, that will save some fuel?

Another question: when building spaceplane in SPH can you really see TWR and DV capacity with KER or Mechjeb under FAR?

[Wanderfound]

The minimum speed you can fly is defined by your wingload and the lenght of runway needed for take off depends on your TWR as well as your take off speed. Minimising your weight by removing engines and keeping same wing surface is not necessarily minimising runway length as you will get better wingload and lower take off speed but also lower TWR.....

Yup. The point I was getting at wasn't about reducing your takeoff run per se; it was about reducing post-takeoff (and especially post-atmospheric) mass in order to maximise fuel efficiency.

Reducing the number of engines will reduce your runway top speed, but it should also reduce the speed required for takeoff (by reducing wing loading). So even a seemingly large reduction of thrust often does not have as much of a negative effect on takeoff ability as might be expected.

There's nothing inherently wrong with having lots of jet thrust (look up the Kerbodyne Goblin sometime...), but I did want to make the point that it's often counterproductive, and is also much less necessary than it may appear.

As with rockets, you can go with brute force and strap on moar boosters, or you can shave your payload down, streamline and try for efficiency. They're both valid approaches. But I do realise that I'm banging on about this a bit, so I'll try to leave it there.

So what is the maximum weight you can send on orbit on one Rapier and what make the difference between 2 Rapier and 2 Rapier plus one nuke? At 24km when you switch your Rapier on rockets, Do you really need the small thrust of the Nuke or is there a kind of "frontier" before reaching orbit where you will switch off rockets and rely only on Nuke, that will save some fuel?

Another question: when building spaceplane in SPH can you really see TWR and DV capacity with KER or Mechjeb under FAR?

Answering in reverse order:

Yes, although they tend to give meaningless numbers for the ÃŽâ€V of the air-breathing engines (and the TWR figure is a bit dodgy too, given how much jet thrust changes with speed and altitude).

With a RAPIER/nuke ship, my normal pattern is to light the nukes and kick the RAPIERs over to rocket mode once the jets lose their enthusiasm, shut the RAPIERs down once the apoapsis hits 60,000m and do the rest on just the nuke. A particularly low TWR ship (e.g. a big Mk3 with only one nuke) may require a bit of extra RAPIER thrust during circularisation, but you still want to do as much as possible of the burn on the hyper-efficient LV-N. Nukes are unnecessary and counterproductive for LKO lifters, but if you're having to do 1,000m/s+ ÃŽâ€V transfer burns after you hit orbit the isp advantage pays off heavily.

Maximum weight? No idea. But the single-engine ship shown in my earlier post is comfortably within the limits; notice how much runway it has to spare. If you want to push the limits and don't care about elegance, you can always just ski-jump off the end of the runway; so long as you start climbing before you crash into the ocean, you're all good.

The top speed of an aircraft is the point at which thrust is balanced by drag; mass is irrelevant in that calculation, except for the fairly minor factor that more mass -> higher wing loading -> higher AoA -> slightly more drag. Higher jet TWR has relatively little effect on top speed; what you gain from extra engines is primarily acceleration rather than speed. Acceleration is fun, and you need enough to enable takeoff, but if you're going for range you want to be focussing on top speed rather than acceleration.

Edited June 9, 2015 by Wanderfound

[Wanderfound]

A long-range demonstrator:

Javascript is disabled. View full album

Craft file at https://www.dropbox.com/s/lc3abgswoipa73u/Kerbodyne%20Muttonbird.craft?dl=0

[Kitspace]

What is the blue line on the transonic design overlay graph in the new dev build?

Thank you!

Edited June 10, 2015 by Kitspace

[Kitspace]

Is it possible that a classic subsonic airframe with a swept wing that is on the center of mass is stable without an actual stabilizer at the back of the plane?

It of course has no control authority without a stabilizer but it is stable in that condition and adding one even if it is very very small just for the elevators placement makes the yellow pitch line on the graphs uncomfortably steep.

It always happens this way with planes designed by me but does not seem very realistic.

[FourGreenFields]

Is it possible that a classic subsonic airframe with a swept wing that is on the center of mass is stable without an actual stabilizer at the back of the plane?

It of course has no control authority without a stabilizer but it is stable in that condition and adding one even if it is very very small just for the elevators placement makes the yellow pitch line on the graphs uncomfortably steep.

It always happens this way with planes designed by me but does not seem very realistic.

If you have swept wings, and the middle of those wings is exactly at the CoM I'd expect it to be stable (or at least neutrally stable). Airflow moves to wingtips on swept wings, which leads to more lift there, and it might be possible that that also shifts the CoL back when at an AoA.

However, such a tail-less plane does have elevator authority. No idea where you'd get that idea from.

[Littlerift]

Is it possible that a classic subsonic airframe with a swept wing that is on the center of mass is stable without an actual stabilizer at the back of the plane?

It of course has no control authority without a stabilizer but it is stable in that condition and adding one even if it is very very small just for the elevators placement makes the yellow pitch line on the graphs uncomfortably steep.

It always happens this way with planes designed by me but does not seem very realistic.

With swept wing, tailless design where the CoL was directly over the CoM I would imagine it would be stable to some degree, although assuming the elevators were on that wing you would have issues with control. In such a case, the plane would pivot around the combined CoM/L, and with the elevators so close this position the authority of the elevators would be minimal. It is this pivot effect that leads to the typical design of either tail/canard mounted elevators and ailerons mounted toward the tip of the wings; the further the control surfaces are from the CoM/L the more output you gain from your input. With the CoL directly over the CoM, however, you would likely be better off having the control surfaces closer to the CoM/L, as the plane will be very twitchy if the elevators are too far away. I've found with FAR that more than ever you have to consider that lower control can be more beneficial. Most of my planes have only around 8 degrees maximum of elevator deflection, because with more deflection they exceed controllable AoA.

Subsonic, however, the issue could easily be the swept wings, as they simply don't offer many bonuses at subsonic speeds over a straight wing.

[FourGreenFields]

With swept wing, tailless design where the CoL was directly over the CoM I would imagine it would be stable to some degree, although assuming the elevators were on that wing you would have issues with control. In such a case, the plane would pivot around the combined CoM/L, and with the elevators so close this position the authority of the elevators would be minimal.

On the other hand, you can easily build a tail-less plane to be only barely stable, which means you need less force to move around, which can counter the lower elevator efficiency.

Also, instead of moving control surfaces closer to the CoM, it is prob better to decrease max deflection (as stated later in your post). That should lead to slightly less drag when using that control surface.

[Kitspace]

I am not trying to build a tailless plane.

There are delta wings fuselage wings and other stuff for that.

I am trying to build a classic plane. Appropriate to call it an airliner style airframe?

That is supposed to be a heavy subsonic transport plane. Not a spaceplane not even a supersonic one.

Just need it to have a considerably good range which is not a problem with stock planet sizes and carry lots of cargo.

But the question was not about that.

I said that it is uncontrollable without stabilizer because then there is no sane place for elevators. It is certainly not impossible but not sane either.

The question is that in theory a plane of such design would just tuck nose down without a stabilizer.

In game however it has a balance point at a particular angle of attack and there is a stable slope on the line.

It would not be that much of a problem actually but adding a proper stabilizer just increases the slope of the yellow line further and the pitch up authority becomes insufficient.

Any way to figure out where the mid air chord is located to check against that?

It feels like the center of gravity is way far forward. The trick is that it does not look very forward relative to the wing position at all.

So that is what seems unrealistic.

Thank you!

[kcs123]

I am not trying to build a tailless plane.

There are delta wings fuselage wings and other stuff for that.

I am trying to build a classic plane. Appropriate to call it an airliner style airframe?

That is supposed to be a heavy subsonic transport plane. Not a spaceplane not even a supersonic one.

Just need it to have a considerably good range which is not a problem with stock planet sizes and carry lots of cargo.

But the question was not about that.

I said that it is uncontrollable without stabilizer because then there is no sane place for elevators. It is certainly not impossible but not sane either.

The question is that in theory a plane of such design would just tuck nose down without a stabilizer.

In game however it has a balance point at a particular angle of attack and there is a stable slope on the line.

It would not be that much of a problem actually but adding a proper stabilizer just increases the slope of the yellow line further and the pitch up authority becomes insufficient.

Any way to figure out where the mid air chord is located to check against that?

It feels like the center of gravity is way far forward. The trick is that it does not look very forward relative to the wing position at all.

So that is what seems unrealistic.

Thank you!

It is all about how far is peak or majority of pressure from center of mass. Latest dev build of FAR gives you in transonic design just that.

New blue line in area ruling graph represents preasure on each length point of graph.

Sorry, I didn't have time to edit pictures and upload them that could gives more info about it.

But, yes, plane with swept wing without additional vertical stabilizers on tail is possible to create.

Recently I wass messing around with some small Rescue drone craft that is without tail but still stable, maneuverable by design.

It is not SSTO, need Shutlle style placement to orbit, but have plenty of dV for those orbital randevous with stranded Kerbals in orbit.

Quite possible to land this on Mun or Minimus too. But main purpose is to have more recoverable pieces when you get from orbit.

50% of rocket parts that was used to place it in orbit is recoverable too, I will post some pictures as soon as I optimize design some more, to give you better clue about it.

[FourGreenFields]

It feels like the center of gravity is way far forward. The trick is that it does not look very forward relative to the wing position at all.

So that is what seems unrealistic.

Thank you!

So, just to check I got that right:

-You are trying to build a plane with swept wings

-The middle of your wings is roughly where the CoM is

If that is so, then your plane will already be stable afaIk. Swept wings (as said in my last post) produce more lift on their wingtips -> CoL moves back -> stable.

So you are adding tail wings to a plane that is already stable -> possibly more stability than you'd want.

Try using a canard + normal elevator design, or move your swept wings further forward (or add weight to your tail).

[Kitspace]

So with the airliners the center of mass range is not behind the wing root?

Then where is it located lengthwise actually?

It would be nice to visually understand where the mid aerodynamic chord of the wing actually begins to compare its relative position to the center of mass to the real life airplanes numbers?

Also what kind of wing profile shape the mod actually uses for the physics calculations?

Is it symmetric and how thick is it actually?

[Titov]

Hi all.

I have serious problems to design a MK3 shuttle with FAR :

For now, it could climb to orbit without too much problems, but the re-entry is a big mess.

When the pitch is a bit over the prograde vector, it goes frontside rear, upside down, etc.

As far as I know, the space shuttle was rentering with a rather high pitch up, wasn't it ?

Here is my shuttle, not that FAR from a known working design :

Might you please help me to understand what I do wrong ?

Shall I add more weight to the nose ?

How to proceed in that case ?

Thanks !

Edited June 11, 2015 by Titov