Aerodynamics of Supersonic Craft

Supersonic Glider-spacecraftSupersonic 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 ThunderStruck Design and 1-2 size measurementswith 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.

Hyabusa reentry sequencWe 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.

Supersonic Aircraft SpikeThe 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?

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 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?

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 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.

Press Release 2

Jason's CAD picture of ThunderStruck above the earth

Jason’s CAD picture of ThunderStruck above the earth

Thursday 10th Nov 2014

Release Date: IMMEDIATE

Press Release: A New Australian Spacecraft Begins Concept Testing

Sydney, NSW, Australia.

Project ThunderStruck is the brainchild of Australian aerospace entrepreneur Robert Brand. The craft, code-named ThunderStruck is a small winged spacecraft able to re-enter the atmosphere from orbit and land on a runway with a small payload. In fact it is being designed around the premise of being the smallest craft to be stable enough to re-enter and land safely.

The first test is negotiating the transonic phase (the speed of sound) scheduled for April 2015 and it is expected to reach a top speed of over 2,000kph or approaching Mach 2.

The concept testing will be in three phases:

  • Transonic Testing (April 2015)
  • Sounding rocket to space and land (Dec 2016)
  • De-orbiting and landing (5-6 years away)

This is not a rocket and needs to be launched to space aboard a commercial rocket. The craft will be capable of  maneuvering in earth orbit and de-orbiting. It will need an ion engine to go further about the solar system and could service the asteroid miners providing taxi services for returning samples back to earth.

Depending on the outcome of tests and limitations of weight vs size, the payload should be somewhere between 10 to 50kgs. The craft is not expected to be reused if it has been in orbit as the cost of refurbishment of a craft twill likely exceed the cost of a new craft. A craft that has been sent to space on a sounding rocket will not need a heat shield and may be reused.

Project ThunderStruck has support from many aerospace companies and sponsorship will be announced shortly.

The transonic phase will conducted by remote control and it will be a global news event as it will break many world and Australian records. As it will break the sound barrier, sonic booms will be heard. It will need to be launched over a remote area of Australia for the first test and it will have live TV coverage of the event. Cameras on the balloon will show the ThunderStruck aircraft drop on its dive to break the sound barrier. Cameras in the front of the aircraft will display the cockpit view and overlay instruments on the video allowing the pilot on the ground to fly the craft. Missile grade GPS will record and relay the speed of the craft to the ground.

Australia built their own orbital craft back in 1967 and launched it on a spare rocket left over from US testing at Woomera. There has not been a substantial spacecraft built in Australia since that time. There have been cubesats and other small amateur radio craft, but this is a huge departure from just placing small payloads in orbit. This will be the first craft that will be capable maneuvering and the first to have long range capability. There are almost no winged re-entry craft capable of de-orbiting. There is one US military spacecraft and another NASA sponsored craft being built. ThunderStruck is looking to service small payloads and will not compete with other craft.

A mission control centre will be created in Sydney and a backup in another site outside of Australia. The craft will be sold as a service and not a device. It will provide significant employment in the aerospace sector and support companies. At this time most aerospace graduates leave Australia due to poor employment prospects.

Website: http://projectthunderstruck.org

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Contact:   Robert Brand – contact@projectthunderstruck   Australia: 0448881101     Int’l:+61 448881101 – leave a message if not answered.

Photos of Robert Brand on the Project ThunderStruck webpage are available for publication as is the logo and the CAD images of the aircraft.

Robert Brand: Leading Australian space entrepreneur, Senior Adviser for Team Stellar, ex-OTC staff member, amateur radio operator, Public Speaker on Innovation, Social Media and Space with a focus on Australian Space. Proud father of three amazing kids.

Worked on Apollo 11 equipment at 17 years old, supported Apollo missions, Voyager missions, Shuttle missions and ESA’s Giotto mission to Halleys Comet. Several times he was stationed at the Parkes Radio Telescope.

With his son Jason he has launched 21 high altitude balloon mission and recovered all 21 – two of them were in Croatia. He has designed a mechanism to turn a weather balloon into a zero pressure balloon during flight. Many of the balloon flight have been commercial flights for customers.

Balloon Flight with ThunderStruck

Preparing for the Flight of ThunderStruck

Weather balloon burst

What a burst weather balloon should do! Disintegrate

ThunderStruck – Backup Preparations

Jason, our 12 year old pilot is no stranger to having to prepare for the worst and it is what we do every time we send up a payload. Our last flight of a balloon into the stratosphere was a case of just that. Two failures. One on launch and the second on decent. Each problem would be enough to cause most balloon payloads to be lost, but as part of our preparations, we carried two trackers for the one flight. This was a flight in preparation for our project and we are testing. We have had to cover our payload in the video. Our apologies.

Below: An artist’s view of the ThunderStruck aircraft under a zero pressure balloon (more on that another time) at 40km altitude. You may have guessed, I am the artist….. Note that on the ThunderStruck event, we will not be using weather balloons so there will be no unexpected explosions.

Balloon Flight with ThunderStruck

Failure One

The first failure was totally invisible to us. A massive downdraft. The first that we have ever encountered. Uplift-1, our first flight, started in an updraft and it rose at an incredible rate for the first kilometre. In the video below, you can hear me make the comment that there did not appear to be the lift that we knew we had because we had used scales to measure the lift. We could not feel the downdraft pushing the balloon down 15 metres above our heads. I mistakenly thought my lack of “feel” was because of the others also holding the payload. We released the payload and balloon and then our hopes sank as the payload only lifted slowly and then sank back to the ground. We ran to catch it, but it rose again and caught on the edge of the eve of the roof of a nearby wheat silo. It stayed there for only 2 minutes, but it felt like an eternity before it released. It rose quickly as calculated, but one tracker had had its GPS unit disconnected and the other had its antenna twisted 90 degrees effectively lowering the power considerably. None the less we could still track the flight – mostly.

One tracker disabled, but still sending its ID at full power, The other effectively made to look low power. Those GoPro cameras are great. hundred of metres above the ground you can hear (faintly) people talking and a dog barking! They make great gear.

Failure Two

The weather balloons are meant to explode and disintegrate. This one did not. The entire balloon, well over 1Kg fell into the parachute and tangled itself in the chute, effectively making the mass look like more like a tangled flag than a parachute. It slowed the payload in the thick air, but the fall from its maximum height was rapid and the entire fall from 30km only took 15 minutes. This was an average speed of 120kph. Given that the payload probably hit the ground at 30 to 40kph, the initial speed was probably close to 400kph in the thin upper air.

With the tracker only giving us effectively a poor signal, the last track that we received in one of the vehicles headed to the landing site was 2 km above the ground making the landing site potentially one square kilometre.  We also fond out later that the second tracker was never going to give us a signal, because the impact had caused a battery to eject from its holder. We only had one ID every 20 seconds and no GPS location! We used a directional antenna to lead us to the payload, but it was a slow and painful task.

The video below shows the impact and the wooden spars breaking. The camera continued to record! Nothing like a good wiring system to ensure that power kept flowing from the external battery. I did not mention that we use external batteries. The GoPro’s batteries, even with the additional power pack, just do not last for the entire flight if it goes over 2.5 hours and especially if it is taking both videos and stills – The new GoPros are amazing, but need more power for High Altitude Balloon (HAB) flights.

Initially the video above shows the incredible stability of our payload at 30km altitude. The Balloon explodes at the 30 second mark and then plummets and spins at a sickening rate of a  couple of times a second with the disabled chute causing the spin.  At 1 minute 45 seconds, we cut to an altitude of about 3km and it took 3 minutes to hit the ground at 60kph. At the 4:45 mark, the payload hits and spars shatter. The camera keeps recording. By the way, the big tree lined road is the Mid Western Highway. The payload was kind enough to land in a sheep paddock beside the main road. You can’t ask for better.

The Lesson

The lesson here is that if it can go wrong, it will go wrong. Yes, we have recovered every payload that we have sent up, but good preparations both in the payload design and build is important as are the preparations for recovery on the ground. We even carry poles to remove the payload from trees. We can manage 14 metre trees. After that we will have to look at other methods.

Our preparations will be backup, backup and more backup. Redundancy rules over weight considerations where possible. Systems will be over-engineered and more care will be taken than what appears necessary. Project ThunderStruck will fly while the world watches. Delays will be unacceptable. This was UpLift-20 and again we have 100% successful recovery rate. @0 flown and 20 recovered. As our flights become more aligned to the actual shape of the ThunderStruck aircraft, speeds will dramatically increase on decent and the videos will have way more interesting stuff to show, but these lessons were there to remind us not to get complacent.

Stability

Creating Stability Between Supersonic Dive and Subsonic Level Flight

Here is the problem. During the supersonic dive, the weight is ideally forward to ensure that, as an airflow is felt by the aircraft, the drag of the tail keeps the craft oriented in vertical dive. That is assisted bythe drag across the aircraft and a low centre of gravity near the nose. During level flight below supersonic speeds the centre of gravity must be further back and ideally between the wings.

Jet fighter design has all sorts of tricks to alter the centre of gravity (or appear to) to make he changes needed. This can be as simple as changing wing shape or even extend more wing during lower speed flight. Some aircraft even have had swung wings. It is hard to control the major variations between the lift and drag that changes dramatically between subsonic and supersonic flight. None the less they do not need the dramatic changes in the centre of gravity that we are engineering.

ThunderStruck will be essential a poor flier as we are, at this stage, proposing symmetrical wings. The problem is that nothing is perfect and even the subtle differences between the wings can give on more lift than the other and create spin. Because of Bernoulli’s law, you might have supersonic flow on the wings, nose, or any other curvature way before you reach Mach 1. Battling with supersonic airflow below mach 1 is difficult and de-stabilising. We will be experimenting with dropping light airframes with a camera at the nose. Before we reach controlled airspace, we will deploy our parachute and have a reserve one for safety. We will watch carefully to see the effects on stability.

The diagram below is one solution to moving the mass required for stable flight in both modes. The pump must be fast and the liquid must stay “thin” and not become viscus. We will need baffles to slow the sloshing around during the changeover. These divide the tanks into chambers with some small holes joining the chambers to allow them to fill.

Centre of gravity adjustment transitioning from dive to level flight

There are other solutions such as screw thread that will shift the battery and electronics forwards or backwards. Since the flight is short the transition only needs to be one way, the design is thus simplified. I am not a fan of shifting the battery and electronics around. It will take a large movement to have the desired effect and it could cause wired t break if they get caught on something. I personally favour pumping the fluid from forward to back as shown above. Moving it down during horizontal flight creates even more stability by creating dihedral effect between the wings on an otherwise symmetrical aircraft.

Dihedral in aircraft is the inclination of an aircraft’s wing from the horizontal, especially upwards away from the fuselage. in this case it is the centre of gravity that I am measuring it against and this indicates that the weight is below the wings and the aircraft will be easier to fly.

Below is another thought on using systems, but this time we vent the fluid without the need for a pump.

gravity does the work for us and we remove the liquid away from any potential problems within the aircraft. Making it lighter will also make it more controllable once out of the dive.

gravity does the work for us and we remove the liquid away from any potential problems within the aircraft. Making it lighter will also make it more controllable once out of the dive.

Whatever system we chose, we will be writing it up here. we need to fly the craft and we also have access to a wind tunnel for subsonic tests.

Our Aerospace Adviser Asks Questions.

Answering our Adviser’s Initial Questions.

Area_rule_unifilar_drawing.svg

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 auto stability 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 bubble wrap 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 break 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 is the “coke bottle” shape that our adviser mentioned – note the thinner mid section of the central body.