Spaceplane design advice

[eagle92lightning]

20 minutes ago, Combatsmithen said:

EAGLE YOU NECROED YOUR OWN THREAD!

 

Also what you see in my picture is the Kerbin shuttle. Capable of carrying a medium payload to LKO, taking off from the runway and landing back at the runway.

I am sure what that means but I think I set a record.

[ajburges]

On 10/28/2016 at 10:08 AM, AeroGav said:

1300 m/s is a pretty moderate speed.    Some folks post screenshots of 1600,  I think they're mad.

The higher up you are, the faster you can go without roasting because the air is thinner.   So above 20km 1300 is ok.   Above 1600 things can get pretty warm if you're not at 30km altitude or more.

Also, it helps if you use an inline cockpit.    The further back something is from the front of the plane, the less heat it gets.   Also, put the basic Communotron 16 antenna on the tip of your nose cone , that extends the shock wave further forward protecting the rest of your plane.

How fast should you go?  1150 is mach 3.8, the speed at which Rapier produce max power in airbreathing mode.   You want at least this, or you're not getting the most out of them.    900 is mach 3, peak for Whiplash.

At mach 4.5 , the Rapier is still producing 80% of it's peak power, so the loss is gradual if you go a little bit too fast.  After mach 4.5 the curve takes a sharp turn downward however, by mach 6 thrust will have fallen to zero.     If you can get that fast airbreathing, you probably brought too many jet engines (extra weight to carry to orbit), have too little wing and too much thermal protection.  

I think that is a little low for optimal speed.

SSTO anerobic dV is a function of mass ratio, but max aerobic speed is a function of thrust vs drag (assuming sufficient wing loading).

The fuel mass ratio as a function of dV for rocket mode is exponential. Assuming 1000 m/S dV for a 1600 m/s switchover, switching 100 m/s earlier requires an 12.0% increase (4.7% of total mass) in vessel mass dedicated to rocket fuel when you toggle! That ignores the extra drag losses making up that 100 m/s as well

Conversely, matching force of drag to force of aerobic thrust scales better for fuel because of the combo of high Isp and low thrust. This assumes you can enter the low thrust region of your engines at all though. RAPIER designs typically can due to the requirements of braking the sound barrier which is also a thrust vs drag comparison.

High aerobic speed is the goal. You get there with low drag to thrust ratio and flight plan. I don't know where peak efficiency lies, but I believe you are overlooking relevant trade-offs.

[AeroGav]

2 hours ago, ajburges said:

I think that is a little low for optimal speed.

SSTO anerobic dV is a function of mass ratio, but max aerobic speed is a function of thrust vs drag (assuming sufficient wing loading).

The fuel mass ratio as a function of dV for rocket mode is exponential. Assuming 1000 m/S dV for a 1600 m/s switchover, switching 100 m/s earlier requires an 12.0% increase (4.7% of total mass) in vessel mass dedicated to rocket fuel when you toggle! That ignores the extra drag losses making up that 100 m/s as well

Conversely, matching force of drag to force of aerobic thrust scales better for fuel because of the combo of high Isp and low thrust. This assumes you can enter the low thrust region of your engines at all though. RAPIER designs typically can due to the requirements of braking the sound barrier which is also a thrust vs drag comparison.

High aerobic speed is the goal. You get there with low drag to thrust ratio and flight plan. I don't know where peak efficiency lies, but I believe you are overlooking relevant trade-offs.

Yes that's an old post i no longer quite agree with.  The communotron 16 trick no longer works in 1.2.2.

Theoretically you can air breathe to mach 5.5 (1650m/s), because that's the speed where the RAPIER's thrust multiplier drops to the same that it was at mach 1.  So if you had enough to go supersonic you should in theory be able to do 1600.     In practice few of my designs get that fast.  I'm usually 1400-1500.

I often angle the wings with a few degrees of incidence, and tend to fit  a fairly generous amount of wing area.   This results in nice easy takeoff and landing speeds, cooler re-entry and allows you to climb to a decent altitude where the air is quite thin before making a shallow dive through the sound barrier.  Also this will be an altitude where NERVs are close to their vacuum ISP rating, and can assist with this manuver if needed. So you can get supersonic on a very low TWR ship.     Finally, once you exceed air breathing top speed, the incidence and generous wing area means you get better lift drag ratio and can climb to orbit with NERV power alone, also this means you rise through the atmosphere quickly when the speedrun ends, due to lift, which keeps things cooler.

But, the downside is it can produce too much lift during the speedrun, causing the ship to climb even with prograde set at 22km.  Unless you fly with a negative AoA, but that's very draggy.