FAR spaceplane questions (amount of motors and wings)

[tseitsei]

How to estimate or calculate roughly how many engines and how much wing area I need for a spaceplane with certain weight?

[jarmund]

Bumping for interest. While I can normally eyeball a decent TWR, I have no idea if my Lift is enough.

[Van Disaster]

Err, that's awkward because it's a bit shape dependent. I would attempt to aim for a max of 0.5-0.6t/m^2 of static wing loading and until you get used to things don't go below 0.45-0.5 Jet TWR. To get the wing loading go into the static derivatives tab & push the button, it'll show the wing area - you'll have to get the craft mass elsewhere. Just divide the mass by the wing area to get the loading. L/W in flight is the opposite way round & a dynamic figure which is presumably there to help you not rip your wings off...

Edited April 11, 2015 by Van Disaster

[Crzyrndm]

Instead of a wing loading, I'd go with the level flight AoA in the static analysis being reasonable

Edited April 11, 2015 by Crzyrndm

[tseitsei]

Err, that's awkward because it's a bit shape dependent. I would attempt to aim for a max of 0.5-0.6t/m^2 of static wing loading and until you get used to things don't go below 0.45-0.5 Jet TWR. To get the wing loading go into the static derivatives tab & push the button, it'll show the wing area - you'll have to get the craft mass elsewhere. Just divide the mass by the wing area to get the loading. L/W in flight is the opposite way round & a dynamic figure which is presumably there to help you not rip your wings off...

Ok. thanks

Another question:

How much rocket TWR I should have to efficiently reach orbit?

[UnusualAttitude]

Your rocket TWR should be as high as possible: pack as much rocket thrust as you can without compromising a decent fuel load. My stock-part FAR spaceplanes with the best dV remaining in orbit have a rocket TWR of more than 1.5.

While it is certainly possible to get to orbit with a rocket TWR of less than 1, it will be less efficient (unless you're using the nuclear engine) as you will spend more time fighting drag and gravity in the upper atmosphere.

So yeah, low jet thrust/high rocket thrust is the way to go.

[Wanderfound]

It's not a question that has a formulaic answer.

At a minimum, you need a combination of thrust and wing that is sufficient to get airborne before you run out of runway. Above that limit, anything goes.

For a spaceplane, you want as little wing as you can get away with [1] and enough thrust to get up to Mach 4+ at 25-30,000m before you kick in the rockets. But exactly how much is enough is a complicated question. Sporty performance calls for more engines, high-G turning ability calls for more wing, etc.

[1] Within reason. Too much wing loading and the increased drag caused by the increased AoA will start negating the mass saving of less wing. Highly-loaded wings require higher mass wing tweakables, too.

[tseitsei]

OK. One last question. When I switch to my rocket engines how much I should pitch up my plane for maximum efficiency? 45 degrees? As much as I can without my horizontal velocity to start decreasing? or something else?

[Van Disaster]

If I can pitch up I usually just go 25-30 *AoA* at most; the idea is to start bumping vertical speed without raising AoA too much, as that will cause a lot of drag. Supersonic "stalling" is only really a thing when you pitch so much you can't pitch down again, but high AoA is really draggy. Once you're up at 45-50km you can start pitching forwards again to start your circularization, unless you're really high TWR & you can do that all outside the atmosphere. I would go for a really low AP and raise your orbit once you're there, burning for high AP inside the atmosphere is not optimal. You can treat the last 10km of the atmosphere as space, your actual drag there is minimal.

If I'm super heavy like my largest craft get I'll just leave it burning at 15 deg pitch - those things have massive wings so they do actually get significant lift up high once there's enough speed. In general lower the TWR - more wing area you need.

[Wanderfound]

Keep it as flat as you can while building jet speed. When you switch to rockets, pitch up as steeply as you can while maintaining control.

[capi3101]

OP, you might be interested in the thread I started that chronicles my own shenanigans with FAR planes. Start with Page 4 of the thread - the first three pages are just me mulling over the decision to switch.

Now, I will say that FAR does use formulas. Stock aero, the math uses algebra - you can use addition, subtraction, multiplication and division to solve them. FAR uses calculus - you can solve the mathematics with a high-end Cray supercomputer and a doctorate in aeronautical engineering if you've got a few years to devote to such endeavors.

Okay - it's really not all that bad. But there are no hard answers when it comes to FAR. Just guidelines. I can give you the ones I've been told repeatedly not to use (by the same guys who've already posted the most excellent advice you've received on this thread) and still use anyway; just realize they may not work for every situation.

Here's what I go with -

* One RAPIER engine for every five tonnes of mass in your main fuselage, or one Turbojet for every six; this translates to about one RAPIER per 9.5 tonnes of total plane or one Turbojet per 10.5 tonnes of total plane. Accompany each Turbojet with a rocket or rocket cluster that outputs about 100 kN of thrust equivalent (so for a pair of Turbojets, you want an LV-T45 or thereabouts).

* For each RAPIER, you want the equivalent of 1 Engine Nacelle and 1 FL-T800 worth of fuel. If you go with Turbojets, include an extra Engine Nacelle's worth of fuel - this works out to an equal amount of Liquid Fuel and Oxidizer, about 440 units for each engine.

** With the maximum mass for the suggested engine configuration and fuel load outs, that should give you just enough rocket delta-V to get you to a 70k orbit with a TWR of roughly 0.55 on the Runway. Delta-V is the limiting factor - you can raise more mass by adding more fuel, but try not to get under a TWR of 0.45 until you've got more experience.

* Each jet/RAPIER will need not much more than 0.1 units of Intake Area. A Shock Cone intake and Engine Nacelle's worth of air is plenty.

* I've found that wing dimensions should be based solely on the dimensions of the Main Fuselage - if you've stowed your fuel and engines in outboard tanks, you'll need to ignore them for determining wing. The figures I go with roughly match those of RL supersonic craft (the SR-71 and Concord), and are as follows:

** The wing root chord should be 50% of the length of the Main Fuselage. Tip chord should not be much more than 20% of the root chord, and it's perfectly fine to have a triangle-shaped delta wing (i.e. no discernable tip chord at all).

** The total wingspan should be 45% of the length of the main fuselage only (ignore side tanks - if you don't you'll wind up with a very short, heavily loaded wing and your roll authority will be nil). Be sure to subtract out the width of the main fuselage when determining the span of an individual wing.

** With the wing area figure determined (which FAR will tell you if you momentarily go into your stability analysis at this point), you want the tailplane area to be 40% of the wing area and the fin to be 20% of the wing area). These will need to be more heavily swept than the wing.

**All wing and control surfaces should be set to anywhere from 0.3-0.5 normal strength to save on mass. The fin/rudder in particular can be tuned down to 0.3.

* Go into FAR analysis. Check the combination of 0k/Mach 0.35 first. Look at the u₀ stat - this will be your craft's take-off speed, and the AoA should be directly below it. u₀ should be somewhere in the vicinity of 125 m/s for most designs and the AoA should not be greater than 10 degrees. A takeoff speed of 100 m/s or below generally indicates excess parasitic drag later in the flight; 150 m/s or above is probably going into the drink if you try to take-off.

**If you have any red numbers - red is bad - go the simulator and put a value of 5 into whichever value is in the subscript of the red figure and mash go (e.g. if M_w is red, test w). You can test more than one figure at once if you need to. The specifics of the simulation graph doesn't matter - what matters is its shape. If the value oscillates up and down, but the height of the up-and-down decreases from left to right, you've got an instability that will correct itself with time. You can live with that. If it oscillates but the waves stay the same height, you've got a neutral instability situation; you can live with it but you'll have to be careful not to overdo things while you're flying. If the oscillation increases from left to right, or it just shoots off from the get-go (i.e. it doesn't oscillate at all), your plane is dynamically unstable as it is; flying the plane is going to be a chore and you should expect to not go to space today unless you're going to fix it.

*Test your plane at 5k/Mach 0.85, 10k/1, 15k/2, 20k/3 and 30k/4. Check your level flight AoAs as you go. If you get greens and/or whites on everything, groovy. Reds should be investigated, as should any situation where the AoAs get much above 20 degrees.

Hopefully that's useful to you - but like I said, these are guidelines at best. Use them at your own risk.

Edited April 12, 2015 by capi3101