Our Aerospace Adviser Asks Questions. Project ThunderStruck

Area_rule_unifilar_drawing.svgAnswering our Adviser’s Initial Questions

Below is an exchange between our new adviser to the project (to be announced officially soon and myself (Robert Brand). Here are his initial comments and please remember that he has not seen anything yet. Our adviser is a pilot with an aerospace engineering degree.

Our Adviser  Hi Robert, Here are a few questions and thoughts.

1. Propulsion

At a first glance you may think you don’t have a propulsion problem, because the thing is falling down.
The fact is, you do. The basic forces and their components (lift, weight, thrust and drag) are always in balance as long as the aircraft is not accelerating in any axis.

This is valid the other way around as well: The aircraft will accelerate as long as the forces are not in balance.
For your case, you need to have the capability to accelerate beyond the sound barrier.

The problem is that the parasitic drag increases exponentially as you approach M=1 and because you are going at a certain angle towards the ground, a certain component of this force, or all of it if you dive vertically, adds to your lift. Once your lift becomes greater than your weight, you will start to slow down.

If this happens before M=1, you will never reach supersonic speed. If it happens after M=! you can further accelerate, because the drag drops after transonic. Transonic is the worst place to be. I order to be supersonic, you must achieve M=1 ASAP, before the air becomes dense.

If you drop from 33km, forget it, because at 30km you can already feel the effects of atmosphere.
The first thing you need to do is apply total surface design, or coke-bottling. The total surface of your craft must be consistent, so at the place where you have wings, your fuselage must be narrower. This dictates your fuselage to be in a shape of a coke bottle. This will reduce drag significantly.

Also, center of lift on the wings changes in supersonic flight and you need to cope with that. There are two strategies, variable wings or variable centre of gravity. I have a very original idea how to solve that.

2. Stability

Any object going through a fluid tends to assume a low drag position. Sometimes this low drag position means rotating and spinning.
You can solve this problem by active control (unless you have f-16 engineers on board, forget it) or aircraft design.
I would suggest delta wings, high swept. Delta wing has an inherent autostability feature and high sweep angle to reduce drag and effect of the wings.
Accept it, your aircraft can be designed either for high speeds or low speeds, unless you have flaps or variable wing geometry.

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My Response

I look forward to how he views this and I will report back soon. I expect that I will have allayed most of his fears:

Firstly we are already applying the constant area rule. Even the A380 has aspects of the rule in the design. I lectured at Sydney Uni on the subject only a few weeks back. I understand the rule and some other rules to do with supersonic flight, although their effects are much less than the constant area rule.

Yes, the wings may very well be more swept back than in the image on the site. We will do drop tests to a certain the best wing shape and we have access to a wind tunnel.

The wings will be symmetrical (top and bottom)– ie zero lift. They will be therefore not an issue at supersonic speeds. The elevator will provide the “lift” with speed at lower altitudes. Yes, it will land “hot” – we may use “flaperons” ie combined flaps and ailerons. It should be noted that these are less effective as ailerons when they are biased down as flaps, but they will be bigger than needed. They will be symmetrical also. Flaperons are really ailerons  that are mixed with the flaps signal on the transmitter to bias them both “on” as flaps/ The ailerons do not work with the same efficiency when they are both biased down, but they do work. We may use separate flaps, we may not use flaps. Testing will determine the stability and best options.

Below is a video that shows how they mix the signals in the transmitter of radio controlled models to adjust the various control surfaces. This is a third party video

The spin will be counteracted by the large ailerons even in low air, the trick is to stop the spin in the first place by making the craft very symmetrical and test that aspect.

Our novel answer to controlling the need for different centres of gravity: We will have serious control of the centre of gravity in the craft and we will be able to move the batteries and electronics with a screw mechanism back and forward in the fuselage. This will keep the craft from being unstable at supersonic speeds. Once it goes back to subsonic, we will begin moving the centre of gravity back as we begin to level out the flight and slow the craft.

At slower speeds, we have air brakes that will slow the craft if needed

The supersonic spike at the front of the aircraft is used to create the shock wave with a pin point device ahead of the fuselage and ensure that the biggest part of the shock misses the wing entirely. A shock wave over the wing creates massive drag and this is why many pilots in the early days, tried to break the sound barrier and failed. The spike doubles as a VHF / UHF antenna

Three weeks ago we launched a payload mainly of wood, covered in bubblewrap for the electronics and, with the parachute deployed, it reached 400kph. For the event we will be using a Zero Pressure Balloon to get to over 40Km altitude. If the 9Kg of the payload are not enough, we will increase the weight and size of the craft. We will brake the sound barrier, but need to show it is a fully working aircraft after the dive.

In World War II bombs from high altitude aircraft regularly broke the sound barrier. We will shift the centre of gravity well forward and act like a bomb. We should be able to punch through that barrier with a lot to spare – even Felix Baumgartner broke he sound barrier for his jump altitude of 39Km. He was not very aerodynamic. We expect to terminate supersonic flight at around 31Km
Yes, transonic is a bad place. We do not intend to allow the craft to stay there! Punch through while the air is super thin and keep accelerating!

Will we make Mach 1.5? – it depends on our launch altitude. We will achieve Mach 1 – the sea level speed of sound is our target. About 1200kph.

Area_rule_unifilar_drawing.svgThere is much more, but I expect that I have answered most of your questions in this email. We will be using ITAR controlled GPS units for supersonic tracking and also we will be using radar transponders to warn other aircraft. The Jason and I will be testing a lot of aspects of the flight with drop tests from balloons. I will be launching another balloon in a week’s time.

The picture above shows the constant area rule – efficiency is gained by the cross-sectional area of the aircraft being constant along its length. The fuselage gets thinner where the wings are as there area has to be accounted for. This rule is important as aircraft get close to the sound barrier and this is why Boeing 747 aircraft were so efficient.

Note the light blue area has to be the same as the dark blue area, including the area of the wings. This id the “coke bottle” shape that our adviser mentioned

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.