Supersonic Shock Waves
by Robert Brand
Phase 1 Test Craft
As you all know by now, Jason, my 12 year old son, will attempt to break the sound barrier mid year – he will be 13 by then. This is the first test of a high drag system that should limit our airspeed at supersonic speeds. It is the Phase 1 testing that we talk about in our documents. We need to go fast for this experiment, but returning from space we need to slow down. Our transonic tests will be with a very different looking craft than our future spacecraft.
To the right, you will see where Jason’s design started. As you may have noticed, the ThunderStruck’s current design looks nothing like the image to the right. At each stage he has had to modify the craft to achieve the goals of going supersonic and get up to around 2,000kph with full stability. Only then will we deploy the experiment and hopefully slow the craft dramatically. As you are aware, it now looks more like the craft to the left. These are massive differences and we will explore the choices he made in another post. Right now we are looking at how we get rid of the major shock waves and also how our Phase 2 test aircraft may look. The main differences in the 2 craft are:
- No supersonic Spike
- No central tail
- Winglets above and below with a ganged rudder
- Delta wings that have two angles of protrusion from the fuselage
- Elevators and ailerons are at the rear of the wing
- The wing extends to the rear of the craft
Phase 2 Test Craft
In Phase 2 the craft that we design will need to travel straight up into space on a sounding rocket. We will separate from the rocket and continue our climb (momentum) to apogee (top of the flight) and then fall back to earth. Apogee may be as high as 200km. The air is so thin that we can conduct what we call weightlessness experiments for several minutes. Once the air starts to create drag, the experiments will end as the craft will slow. At this time we do not want the craft to accelerate further, but it will. Unless we feather the wings(like Virgin Galactic’s Spaceship 2) or create massive drag in some other way (our Phase 1 experiment) we will go too fast for our craft’s well-being. We need to go slow as possible. We do not want to have to slow with unusual braking as this may de-stabilise the small craft.
We could use a capsule, an ablative heat shield and a parachute like JAXA’s Hyabusa, but we are creating a winged vehicle, although the capsule will always be another option. I guess the cat’s is out of the bag. We will have a couple of configurations possible with capsule or winged reentry as an option. The ThundrStruck craft will be a modular design in the style and electronics. 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 where ever the winds take the parachute.
I want to fix that problem for those customers that need a precision landing or effectively or a smooth landing. 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.
The picture at the top of page is where we started. I expect that the spike will not be on the spacecraft and it is also now unlikely to be on the transonic test vehicle, but it is important to understand why we see them on supersonic craft. Sometimes a very long sharp nose can also produce the desired effect.
The picture at right is a NASA 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. Why? We discuss this after the following paragraph.
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 as an option to help lower the Resistance to breaking the sound barrier for our tests. At the time of posting, Jason has gotten rid of the spike and opted for wings tucked back behind the shock wave.
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 and the better the results from our experiment.
So What Does the Spike Do?
As I said a sharp nose is the same as a spike and the image to the left shows the effect of the nose/spike as it moves the shock wave to a point well ahead of the main body of the craft 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 drag is lowered considerably. The reason that it was so hard to break the sound barrier was simple. The craft used had their wings in the high drag area caused by shock waves.
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 in Phase 2 tests returning from space via a sounding rocket, we can actually round the nose of the returning spacecraft and still get the supersonic shock to clear the wings
So Why Didn’t the Shuttle Need a Spike?
Well it did need to slow down and so you might think that a blunt nose is a good thing to create drag, but that is not the reason. Wouldn’t a sharp nose be good for takeoff, spike or no spike? Well, in some ways, 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 sharp 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! See the image above right. This would be a poor design for such a craft. The image 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 sides. This misses the wings by a long way – and the tail of course. The blunt nose does add to drag so that is another benefit to slowing down, but a minor one. It is the additional heat caused by the shock wave over the wings during re-entry that had to be eliminated
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.