Why Break the Sound Barrier with a Small Aircraft?

Supersonic Glider-spacecraftThe Sound Barrier is a Major Steppingstone

As I announced in my last post, Jason, my 12 year old son, will attempt to break the sound barrier. Above I mention that this is actually a steppingstone. “A steppingstone to what?” you may ask. The simple answer is “to build a spacecraft”. So why to we need to break the sound barrier? Well we want to test transonic flight. Not on the way up, but on the way down! ie slowing from supersonic speeds above the sound barrier (Mach 1 and higher) to subsonic speeds )below Mach1

Reentry

This is the hard part for any craft that I may build in the future. We can always buy a ride to space on one of the many well known rockets such as ESA’s Ariane rocket or SpaceX’s Falcon9. So what is the grand plan?

Personally, I see the future of any craft that I build (within an aerospace company) as being a reentry vehicle to return samples from space. This will mean transiting a number of challenging areas in its return to earth. Two of the critical areas are

  • the initial intersection with the atmosphere that will cause massive heating of the exposed portions of the craft – this often requires either:
    • an ablative shield – one that wears away as it heats, carrying the heat away
    • a strong insulator such as the tiles used on the space shuttle
  • crossing the sound barrier – that is the transonic area of flight. This is from Mach 1 to Mach 0.75 – the speed of sound down to 75% the speed of sound.

Hyabusa reentry sequencIf we were using a capsule like the Japanese Space Agency’s (JAXA) return capsule, Hyabusa, transonic regions would not be a problem, but I believe that the future for me is in building an aircraft-like reentry glider that will allow up to 20Kg of payload to safely transit to earth.

The picture to the right is  the landing sequence for JAXA’s Hyabusa that landed in the centre of Australia. It is not complicated, but you do have to know what you are doing and the downside is that it lands whether the winds take the parachute.

I want to fix that problem. I would love to be able to direct the returning spacecraft to a point on the map that allows us to land it without having to recover it from an unknown place in the desert.

Supersonic Aircraft SpikeThe picture at the top of page is somewhat like the expected end product. I expect that the spike will not be on the spacecraft, but it will be on the transonic test vehicle.

The picture at right is a test vehicle with a spike. There are many supersonic aircraft that either have a spike of a very sharp nose well ahead of the wings.

Returning from space the spike would be a liability in the heat of reentry. It will also not be an asset in slowing down a craft. We only need to have the spike to help lower the Resistance to breaking the sound barrier for our tests. In our tests we will use gravity to accelerate the test craft to way past the speed of sound, but shock waves (pressure waves) would slow us down and limit our top speed. We would probably still break the sound barrier dropping the craft from around 40km altitude, but the quicker we transit the sound barrier the higher our top speed.

So what does the spike do?

supersonic shockwaves in a windtunnelAs I said a sharp nose is the same as a spike and the image to the left shows the effect of the spike as it moves the shock wave to the point and away from the wings. A sharp point is a very low area of shock and in the image you can see the shock waves from the wings as very low level compared to the shock from the tiny front of the aircraft. So long as the wings are tucked in behind the initial shock wave than the resistance to flight is lowered.

Now I may have been a bit simplistic here, but none the less, the spike is important to supersonic flight. Since we are wanting to slow down, we can actually round the nose of the returning spacecraft after we conclude the test flights.

So Why Didn’t the Shuttle Need One?

WPointy nose and shockwaves at mach 6.ell it did need to slow down and so you might think that a blunt nose is a good thing, but that is not the reason. But wouldn’t a sharp nose be good for takeoff, spike or no spike? Well yes, but the shuttle had wings that were very wide and a spike could not be placed that far forward. The resulting shock waves on takeoff and especially re-entry would be a bit problem as they would hit the wings.

Re-entry would be the biggest problem. The shock wave from a pointy nose would hit the wings and further heat the air. You would be adding thousands of degrees to the heat that it is already being generated on the leading edge of the wing – not a good idea!

The image above right shows a pointy nose model in a mach 6 airstream. You can see the shock waves hitting the wings midway along their leading edge.

So What Happens with a Blunt Nose?

The image to the right says it all. The blunt nose acts as a ram and pushes the shock wave way to the side. This misses the wings by a long way. The blunt nose does add to drag so that is another benefit, but a minor one.

What Else Protected the Shuttle from Shock?

Ever consider the orange main fuel tank? Where was the shuttle positioned relative to its nose. It had a point, but was really broad.

What effect did that have during launch at high speeds. The shock wave that resulted missed the shuttle entirely. It is important that the top of this tank was far enough forward to protect the shuttle. The whole design and shape of the combined modules on the launch vehicle was super critical and not just a random bunch of sizes. Minimizing shock waves means being able to both protect the vehicle and increase the payload as you have less drag.

In other words, if the main tank had needed less fuel and had been smaller, then it would still have needed to be as high to push the shock waves aside.

Each and every part of an aircraft that changes its size or sticks out causes shock. You must account for it or suffer the consequences.

The image at right clearly shows the  shock wave of the jet disturbing the water. You do not have to be traveling at supersonic speeds to produce shock waves, but the faster you go, the more power is lost and the stronger the shock wave.