Ferram Aerospace and Real Solar System scale Spaceplanes..

[AeroGav]

So I've spent all these years doing easymode stock space planes,  but I want to get a feel for what it might take to create something like that in real life.

So I installed Realistic Progression 1 modpack which scales the planet up to earth size (orbital velocity now something like mach 21 instead of mach 7) as well as Ferram Aerospace.

I am not too hung up on 100% reusability and am ok with dumping off empty tanks and any engines that are cheap enough to miss, so long as the majority, preferably 70% or more, of my craft goes to space and comes back again.    

What is really the essential thing for me is that the wings actually play a part in getting us to space... rather than just for coming back down.

In the stock game, I create craft with Lift/Drag ratio of 3 to 1 or better so I can get to orbit on fuel efficent jet and nuclear engines,  not being tied to a thrust weight ratio > 1.

The problem is when you combine Ferram and RSS ...   you can get a lift to drag of 7 to 1 or so at mach 1.4,  once you're though the high drag  transonic (mach 0.9 to 1.3) region.   This is about twice what i get in stock.   But, with Ferram the L/D inexorably declines as we get faster.  By mach 5 it's heading below 2:1,  and that's not even a quarter of the way to space.      

With the stock aero model,  L/D ratio us unchanged from about mach 1.4 onwards.  I've built SSTO spaceplanes for RSS with stock aero that were still doing  L/D 3 to one at mach 20,  albeit at a very high altitude.  And yeah things were kinda toasty by this point.

Is there any way to improve my L/D at super high mach, super high altitude flight ?    Would drooping outer wing sections like an XB-70 Valkyrie, create compression lift, or trying to make a waverider planform ?   Or do i just need to do more of what makes a good mach 2 airplane,  sweep the wings back at an even sharper angle,  reduce span, lengthen the fuselage,  try to improve  area ruling even further ?

Of course,  do too much of those things and it won't be able to lift off the runway before the tyres explode,  which is a separate issue..

[jimmymcgoochie]

You're running into exactly the same problems that have vexed aerospace engineers for decades. Building a plane that can fly to orbit means balancing paradoxical requirements- it needs enough lift to be able to take off fully laden with propellants and payload, yet without producing excessive drag at higher speeds; to carry the necessary fuel to make it to orbit, yet be light enough to actually make it to orbit and have a meaningful payload mass; to cope with a wide range of aerothermodynamic regimes, from near sea-level flight to hypersonic to orbital re-entry, yet be stable in all of them with and without the mass of payload and propellants. All the wings and control systems needed to make it fly in the atmosphere are dead weight in space, while hauling your empty fuel tanks around wherever you go will reduce your delta-V and make re-entry more difficult too with a larger area to try and shield from the heat.

The only orbital spaceplanes to ever fly are the Space Shuttle and Buran (once), both of which were more like rockets that happened to have wings for coming back down- the Shuttle discarded its external tank with every flight while the solid boosters could be recovered and reused and the shuttle itself brought the main engines ('first stage') and its OMS system ('second stage') back down with it, and Buran was more of a side-mounted recoverable second stage on the Energia rocket which was fully expendable; grand plans of making the four boosters return and land propulsively a la SpaceX, and indeed Buran itself, ended with the collapse of the Soviet Union with a single uncrewed test flight completed successfully.

The closest thing to a true spaceplane that flies to orbit and back is probably Skylon, which needs several revolutionary technologies- the SABRE air-breathing rocket engines, air pre-coolers capable of chilling superheated hypersonic air to cool subsonic air for said engines to breathe, a skin that can handle the full heat of re-entry with a lot more finesse than Starship's belly-flop technique, using light and efficient (but not very dense) liquid hydrogen as fuel and getting a lot of oxidiser for free from the air. It's still a long way from flying, but just look at the design required to theoretically let it put a few tons into orbit per flight:

To put it simply, spaceplanes are HARD. KSP doesn't really explain that- reaching 1600m/s on air-breathing RAPIER power means you're about 3/5 of the way to orbital velocity, while doing the same with Skylon's SABREs means you're only about 1/6th of orbital velocity and have a lot more altitude to gain to get out of the atmosphere too.

Spaceplanes in RP-1 are feasible as upper stages to send crew around in Earth orbit and possibly even to the Moon, but it's incredibly difficult to build a usable SSTO spaceplane or even one that can throw parts away. There's also little incentive to build one as by the time you're far enough along in the tech tree to be able to reap the benefits of a reusable launch system, you're probably past the point where the small payloads a spaceplane can manage are useful. Maybe if you build a winged rocket and use airlaunch to put it at a fairly high speed and altitude on 'launch' then it might be possible?

[AeroGav]

1 hour ago, jimmymcgoochie said:

You're running into exactly the same problems that have vexed aerospace engineers for decades. Building a plane that can fly to orbit means balancing paradoxical requirements- it needs enough lift to be able to take off fully laden with propellants and payload, yet without producing excessive drag at higher speeds; to carry the necessary fuel to make it to orbit, yet be light enough to actually make it to orbit and have a meaningful payload mass; to cope with a wide range of aerothermodynamic regimes, from near sea-level flight to hypersonic to orbital re-entry, yet be stable in all of them with and without the mass of payload and propellants. All the wings and control systems needed to make it fly in the atmosphere are dead weight in space, while hauling your empty fuel tanks around wherever you go will reduce your delta-V and make re-entry more difficult too with a larger area to try and shield from the heat.

The only orbital spaceplanes to ever fly are the Space Shuttle and Buran (once), both of which were more like rockets that happened to have wings for coming back down- the Shuttle discarded its external tank with every flight while the solid boosters could be recovered and reused and the shuttle itself brought the main engines ('first stage') and its OMS system ('second stage') back down with it, and Buran was more of a side-mounted recoverable second stage on the Energia rocket which was fully expendable; grand plans of making the four boosters return and land propulsively a la SpaceX, and indeed Buran itself, ended with the collapse of the Soviet Union with a single uncrewed test flight completed successfully.

The closest thing to a true spaceplane that flies to orbit and back is probably Skylon, which needs several revolutionary technologies- the SABRE air-breathing rocket engines, air pre-coolers capable of chilling superheated hypersonic air to cool subsonic air for said engines to breathe, a skin that can handle the full heat of re-entry with a lot more finesse than Starship's belly-flop technique, using light and efficient (but not very dense) liquid hydrogen as fuel and getting a lot of oxidiser for free from the air. It's still a long way from flying, but just look at the design required to theoretically let it put a few tons into orbit per flight:

To put it simply, spaceplanes are HARD. KSP doesn't really explain that- reaching 1600m/s on air-breathing RAPIER power means you're about 3/5 of the way to orbital velocity, while doing the same with Skylon's SABREs means you're only about 1/6th of orbital velocity and have a lot more altitude to gain to get out of the atmosphere too.

Spaceplanes in RP-1 are feasible as upper stages to send crew around in Earth orbit and possibly even to the Moon, but it's incredibly difficult to build a usable SSTO spaceplane or even one that can throw parts away. There's also little incentive to build one as by the time you're far enough along in the tech tree to be able to reap the benefits of a reusable launch system, you're probably past the point where the small payloads a spaceplane can manage are useful. Maybe if you build a winged rocket and use airlaunch to put it at a fairly high speed and altitude on 'launch' then it might be possible?

I appreciate the effort you put into the post.   I am however fully aware that spaceplanes are hard, and have gotten into some bitter spats with people on this forum who seem to delight in gloating about this fact.  Obviously,  the folks I was arguing with could always just point to the indisputable fact that in real life, the only way to put a man in space is with a fully expendable rocket.

The stock game is balanced so that both ways of getting to space are viable, at least in sandbox mode - but I am frustrated by the fact Ferram effectively blocks off the non-rocket path by clamping L/D below 2 above mach 4.     This would be justifiable if it were based on a law of aerodynamics,  but it appears to originate from Kucheman's equation he devised by extrapolating the trend of supersonic aircraft development in the 1950s :

 

http://www.aerospaceweb.org/design/waverider/equations/eq08.gif

Even then,  Ferram appears to apply the cut more aggressively than the above formula implies, creating balance issues when combined with RSS.

http://www.aerospaceweb.org/design/waverider/figures/fig12.jpg

I've looked through a few dev posts from the folks who maintain this mod :   It appears that "all" it does is area rule your craft in real time and use that to apply an amount of drag at mach 1.  Which is quite an achievement given the need to not kill a person's PC in the process.   It is a significant improvement in realism on stock aerodynamics though which admittedly is a low bar.   It neatly solves the problem of a player combining two delta wings to make a square one - without their Voxel based method to test the end result,  the game will not know whether a low speed or high speed airframe was the outcome.

However it doesn't appear to simulate other aerodynamic effects,  so you can cheese the game and make unrealistic aircraft with Ferram too  :    for example,  it doesn't seem to mind tandem wing designs ,   and it appears you can make a supersonic biplane if you want, so long as both wings are swept !

From a few test shapes created in SPH,  it appears that aspect ratio and taper don't affect lift drag ratio at all, and drooping the wingtips like an XB-70 Valkyrie makes your L/D worse,  which confirms a dev post who said that Ferram doesn't model compression lift. 

The only things that appear to influence high mach L/D are the proportion of your craft that is wing - like in stock game,  the more wing area,  the higher max L/D you can get, because the fuselage makes only drag, no lift - and transonic drag.

So I'm guessing I need to make the skinniest possible fuselage that holds only Kerbals and Engines,   and then attach the largest swept wings I can, and fine tune the sweep angles to minimise transonic drag.    And put all my liquid hydrogen in those wings because surface area to volume ratios don't matter for boiloff apparently !   Well,  if I had a nuclear thermal engine that could switch between operation on Ammonia and hydrogen available, i wouldn't cheese that so hard.

As you can guess I've been reading on http://www.aerospaceweb.org/design/waverider/design.shtml  whilst dreaming up these designs.

Whilst sites like this lure the prospect of better performing waverider designs,  they also indicate that these numbers are only achieved at the design mach number. 

What I'm hoping to do is get to mach 5 or 6 on ramjets,   then switch to NTR and hopefully have a L/D > 2 for a couple more machs.   After that,  supplement with some conventional rocket thrust and for the final leg of the journey our TWR will be good enough to  brute force things with most of the propellant gone...

[DA299]

10 hours ago, AeroGav said:

I appreciate the effort you put into the post.   I am however fully aware that spaceplanes are hard, and have gotten into some bitter spats with people on this forum who seem to delight in gloating about this fact.  Obviously,  the folks I was arguing with could always just point to the indisputable fact that in real life, the only way to put a man in space is with a fully expendable rocket.

The stock game is balanced so that both ways of getting to space are viable, at least in sandbox mode - but I am frustrated by the fact Ferram effectively blocks off the non-rocket path by clamping L/D below 2 above mach 4.     This would be justifiable if it were based on a law of aerodynamics,  but it appears to originate from Kucheman's equation he devised by extrapolating the trend of supersonic aircraft development in the 1950s :

 

http://www.aerospaceweb.org/design/waverider/equations/eq08.gif

Even then,  Ferram appears to apply the cut more aggressively than the above formula implies, creating balance issues when combined with RSS.

http://www.aerospaceweb.org/design/waverider/figures/fig12.jpg

I've looked through a few dev posts from the folks who maintain this mod :   It appears that "all" it does is area rule your craft in real time and use that to apply an amount of drag at mach 1.  Which is quite an achievement given the need to not kill a person's PC in the process.   It is a significant improvement in realism on stock aerodynamics though which admittedly is a low bar.   It neatly solves the problem of a player combining two delta wings to make a square one - without their Voxel based method to test the end result,  the game will not know whether a low speed or high speed airframe was the outcome.

However it doesn't appear to simulate other aerodynamic effects,  so you can cheese the game and make unrealistic aircraft with Ferram too  :    for example,  it doesn't seem to mind tandem wing designs ,   and it appears you can make a supersonic biplane if you want, so long as both wings are swept !

From a few test shapes created in SPH,  it appears that aspect ratio and taper don't affect lift drag ratio at all, and drooping the wingtips like an XB-70 Valkyrie makes your L/D worse,  which confirms a dev post who said that Ferram doesn't model compression lift. 

The only things that appear to influence high mach L/D are the proportion of your craft that is wing - like in stock game,  the more wing area,  the higher max L/D you can get, because the fuselage makes only drag, no lift - and transonic drag.

So I'm guessing I need to make the skinniest possible fuselage that holds only Kerbals and Engines,   and then attach the largest swept wings I can, and fine tune the sweep angles to minimise transonic drag.    And put all my liquid hydrogen in those wings because surface area to volume ratios don't matter for boiloff apparently !   Well,  if I had a nuclear thermal engine that could switch between operation on Ammonia and hydrogen available, i wouldn't cheese that so hard.

As you can guess I've been reading on http://www.aerospaceweb.org/design/waverider/design.shtml  whilst dreaming up these designs.

Whilst sites like this lure the prospect of better performing waverider designs,  they also indicate that these numbers are only achieved at the design mach number. 

What I'm hoping to do is get to mach 5 or 6 on ramjets,   then switch to NTR and hopefully have a L/D > 2 for a couple more machs.   After that,  supplement with some conventional rocket thrust and for the final leg of the journey our TWR will be good enough to  brute force things with most of the propellant gone...

I haven't built a lot a spaceplanes in RSS with FAR, but I have built a few so here's my experience:

 

1) First of all, FAR doesn't really model compression lift. IMO this would be a huge addition for spaceplane guys like you and myself, increasing L/D ratios to well over five at even Mach 7+. As it is, we're stuck with about 2 - 2.5 L/D at higher than Mach 5 speeds. I'd love to build a wave-rider in KSP, but Ferram is unfortunately  busy IRL and there are  no plans whatsoever to model compression lift in FAR(It was too computationally expensive, the last time I asked.)

2) The thing in FAR that is different from stock aero, is that you need to have a lot of altitude for high L/D. At 30 kms, and Mach 5, it is possible to get >2.5 L/D with a properly designed plane. And by properly designed I mean as close to a Sears-Hack Body as you can get. You also need tiny wings with very high sweep, and this brings along its own set of challenges like very high takeoff speed(my designs lifted off at about 150 m/s with flaps, slats, the whole shebang.). 

3) Of course, I don't think you can get to 30 kms with the stock rapiers. I remember that I used scramjets from a mod for this very purpose, and with those, it is possible to reach ~40km(In RSS) and Mach 10 on airbreathers alone, at which point you need to switch to NTRs. No matter what you do, the highest L/D you'll get at above Mach 10 is about 2. But this doesn't really matter, as the drag is very little above 40 kms, and because you have a lot of speed(Mach 10 is still half of orbital velocity, so your ballistic trajectory greatly reduces the amount of TWR you need), it is possible to get to orbit with roughly 0.3-0.4 rocket mode TWR. This will increase your dry mass by a lot though, as you basically need 3 different engines to reach orbital speeds. So while it is possible to reach orbit, the payload fractions are almost non-existent.

4) KSP Interstellar is by far the the best mod for engines if you really want a RSS SSTO. It has a lot of nuclear thermal nozzles, that can use either air or onboard propellant. It's very easy to make an SSTO with those mechanics. Almost a easy as making a stock SSTO on Kerbin.

 

[AeroGav]

13 hours ago, DA299 said:

2) The thing in FAR that is different from stock aero, is that you need to have a lot of altitude for high L/D. At 30 kms, and Mach 5, it is possible to get >2.5 L/D with a properly designed plane. And by properly designed I mean as close to a Sears-Hack Body as you can get. You also need tiny wings with very high sweep, and this brings along its own set of challenges like very high takeoff speed(my designs lifted off at about 150 m/s with flaps, slats, the whole shebang.). 

The design point is the transition to NERV power after the airbreathers quit..   so I'd say mach 6 plus.      This is where our TWR will be at its worst and so our L/D needs to be as good as it can be for us to gain energy.     Over time, we'll get lighter as propellant is used and our TWR will improve.    As we start getting up past 50% of orbital velocity gravity will start to reduce from orbital freefall which again will help us.

The challenge is I am targeting a higher mach number than you are,  but on the other hand i can fly as high as i want , whereas you have to stay low enough to get air into your RAPIERs.

 

[AeroGav]

I'm really not used to the interface of the B9 Procedural Wings mod..   I've spent ages perfecting a design, only to mangle it beyond repair with a single misclick.   I'm also at bit confused about what the "leading edge" and "trailing edge" sweep numbers meant.   Though they did make me realise i'd put the wings on upside down, forcing another start over.

Anyway, before i lose all sanity,  i made a quick comparo of 3 different wing planforms.   Yeah unfortunately I wasn't able to change only one parameter, because i don't know what i'm doing with PRocedural parts.

Previously, I  would use multiple wing parts so get an increasing sweep angle as you move outboard... since that looks like the waverider shape that keeps as much of the wing as possible inside the bow shock

http://www.aerospaceweb.org/design/waverider/figures/fig14.jpg

If the above link doesn't work,  suffice to say that planform looks a lot like a "starfleet logo".    Not sure it's worth the hassle though as bow shock is not modelled.   Arrgh..  for science i should really build such a wing and attach it to this body and see how the numbers look.

Anyway,   first iteration.    Apparently the "mid chord sweep angle" is 73 degrees.    It has a taper of 0.3 and a mass of 3.18 (tons?).

This design is best for wave drag and cross sectional area.   Unfortunately, while i'm optimising my area ruling to four decimal places, i've failed to notice this wing doesn't hold much liquid hydrogen.  We've got such a good TWR (1.27) that the wing isn't really needed,  but the burn time is only 3 min 41 seconds with a Vac dV of 3276

 

Next i tried playing with the sliders to try get a 60 degree sweep.  I ended up with 65 degrees and a slightly  larger wing overall. It's mass is 3.92 instead of 3.18, and the taper is less, at 0.2.    It has about the same mach 1 wave drag area though the cross sectional area is twice as much.   This version of the craft holds a lot more LH2.   Our TWR drops to 0.98  but the burn time is now 6 min 26 seconds,  with 4757dV.   Still seems under-fuelled.

Finally I tried to make a data point at 50 -ish degrees of sweep.    This results in a "taper" of only .024 - which i find odd as the wings have almost zero chord at the tip,  and are almost the whole length of the fuselage at the root - and a truly enormous wing of 7.84 tons.     The max cross sectional area hasn't improved much, but our mach 1 wave drag area has about doubled.   Then again i haven't spent a lot of time optimising this version but i'd be surprised if you can improve much with such a vast wing.       The good news is,  that wing holds a lot of fuel and we have 6662dV and a TWR about 0.49.   

 

Now comes the (not) fun bit - making screenshots of the L/D plot then counting pixels with MS paint to see if one peaks higher than another.     All are plotted as an AoA sweep from 5 to 10 degrees at mach 6.

The gigantic 50 degree swept wing does ok here, with a peak L/D 44 pixels above the zero line.     For some reason,  the intermediate 60 degree wing is rubbish, with an L/d only 38 pixels tall.     The smallest wing does best though,  with 58 pix.  However it doesn't hold enough fuel and in the process of writing this, noticed that i also married that wing to an earlier, smaller version of the fuselage too which may account for some of the differences.

edit - further experimentation with the 50 degree wing...   reveals that a finely tapered leading and trailing edge profile actually worsens performance.   The best airfoil profile appears to be a rectangular  slab.  Increasing or decreasing thickness over quite a large range does not seem to matter at all.        Leading edge sweep appears to have a wide sweet spot,   and the trailing edge sweep is best when it is about a third of the leading edge sweep angle.    A wing chord that drops to zero at the tips is desired..