Bloodhound testing

[wobblywalrus]

The friction coefficient between the wheel and the track surface is what keeps the car from being blown sideways and provides the ability to steer the thing.  A high coefficient means it is easy to stay on track and to steer.  The opposite happens with a low value.

Sandy soil can provide a reasonable friction coefficient in its compacted and undisturbed condition.  The soil particles are interlocking with each other and they resist displacement.  Compressed air trying to get under the wheel or a subsonic shock wave ahead of the wheel can disturb the soil particle interlock.  The frictional coefficient drops.  The car is skating on an aeriated surface with little ability to control it.

What to do.  First, compare the particle size distribution, particle specific gravity, and particle angularity to the last known good surface, the Black Rock Desert.  Finer size, rounder particle shape, and lighter specific gravity can all cause problems.  A soil scientist can be a help here.  Chemicals were applied to fine grain non-cohesive soils on many of the construction sites I worked on to keep the soils in place.  Something similar might be needed on the pan.

Second, look at the wheel surface.  A single central circumferential rib might be needed to get down below the aeriated layer to reach more consolidated material.  Slots or holes in the wheel surface may relieve air pressure under the wheel.  This may have been done.  I have not seen the wheels.

This is posted here as some things for experts to consider.   

[MX304]

Does anyone have a picture of the wheels?

   

[TrickyDicky]

Does anyone have a picture of the wheels?

Best I can find is at https://www.bbc.co.uk/news/science-environment-49184375.  But you have probably seen that article.  Key paragraphs:

The first thing to test is the high-speed desert wheels.

Each wheel weighs 95 kg and is forged from solid aluminium, without any tyre on it.

At 1,000mph, the wheels experience 50,000 times the force of gravity trying to tear the wheel rim apart, so it has to be solid metal; nothing else will cope with the extreme loads. So far, so well understood. Now we get to the bit that we don't know - how will these wheels behave on the desert surface?

Metal rims running on the hard mud surface of Hakskeenpan will have very little grip due to friction.

Normal road cars rely on tyre grip for their stability and safety, and tyre companies spend a huge amount on developing the right rubber compounds for maximum grip.

None of that helps us, as 50,000-g would destroy any rubber tyre, so we are working with the unusual (and poorly understood) dynamics of solid metal wheels.

We have given the metal wheels some lateral grip on the desert surface by making them a shallow "V" profile.

As the car runs along the track, the wheels cut ruts in the mud surface, providing the sideways grip that we need. Unfortunately, the faster we go, the shallower the ruts become - at slow speeds (200mph), they will be 10-15 mm deep, but at supersonic speeds the wheels will be making tracks less than 5mm deep, which will provide almost no sideways grip.

There is some good news at supersonic speeds, as the aerodynamic grip will be huge, so that car will get pretty much all of its directional stability from the supersonic airflow.

This should also give the car some very lively steering at high speeds, with the front wheels acting like rudders in the supersonic airflow, producing very rapid steering responses.

Now for the bad news. As the car accelerates, the mechanical wheel grip goes down quite quickly, but the aerodynamic forces (which depend on the square of the speed) build up much more slowly.

This means that at "medium" speeds (somewhere between 300mph and 500mph), there is very little surface grip from the wheels and there is very little aerodynamic response.

This is where controlling Bloodhound may well feel like driving on ice at 400mph.

I'm going to have to learn how to control the car as it accelerates from "normal" wheel grip below 200mph, through a period of almost-no-grip-at-all-oh-help-it's-all-over-the-place-like-driving-on-ice, to above 500mph where the steering is becoming super-fast.

Just to make things more complicated, we also need to assess the lateral stability as we increase the speed, so I need to learn how to control the car and try to measure its stability, all at the same time. Luckily, I love a challenge.

I do know they put effort into assessing different profiles, including some that had quite sharp ridges/edges.  Looks like they ended up with quite a subtle shape.

[TD]

A bit more:

https://www.bbc.com/news/science-environment-28737132

A few notes (PDF) about manufacture and materials:

https://www.innovaltec.com/wp-content/uploads/2015/08/BLOODHOUND-SSC-Innoval-Technology-involvement-March-15-v2.pdf.

AA7037 aluminum.

A video on spin testing:

https://www.youtube.com/watch?v=WIpVwBVTwCY

A static image from what looks like the spin test rig, from https://www.bbc.com/future/article/20141023-the-worlds-fastest-wheels, posted in 2014:



I don't know if this shape was that used in South Africa.   The lid of the test rig weighs 10,200 kg, presumably in case of what the SpaceX guys call a 'rapid unplanned disassembly' (RUD). 
 

[wobblywalrus]

Thanks for posting that info.  The wheel shape looks good.  It would not take a tall ridge to make a big difference for the better, is my guess.  Maybe 12mm tall?

[wobblywalrus]

Oops, I forgot the attachment.  Titanium might be a good material for the ridge.  It expands less than aluminum when heated.  The ridge ring would lock onto the wheel tighter when the assembly heats up.  Also, Ti is strong and ductile, especially at elevated temps.

[WOODY@DDLLC]

Mayoman presentations to the Ancient Aviators: https://www.dropbox.com/sh/okrhyxnrc1mlzvr/AACfpRdJ3Q79hhP8YL830P46a?dl=0
Not quite the same without his narration but good stuff just the same! 

[wobblywalrus]

The idea of a titanium ridge ring in the post before last is not a good idea.  Impact with a big rock just under the surface could smash the ring enough so it deforms and stretches out to a larger diameter.  It would come off of the wheel and interesting events would follow.  A ridge would need to be incorporated into a new one-piece wheel.

It seems the vehicle transitions from wheel dependent steering to aero based directional stability at higher speeds.  How does the car be steered at high speed?  It seems moveable surfaces such as rudder flaps are prohibited by the  regulations.     

[MAYOMAN]

What regulation prohibits augmenting the two wheel steering (yaw control) with aerodynamic rudder control? Likewise, there isn't a prohibition of aerodynamic pitch control (canard fins) either. I suppose at some speed the aerodynamic rudder provides more yaw control than the two wheels. When  I spoke with Andy Green several years ago about the rear wheel steering on the Thrust SSC (it is dynamically unstable) he said the aerodynamic steering controls saved the project.

[Malcolm UK]

Whilst not saying that moving control devices have to be used, the FIA make sure that they are permitted - abstract is from Appendix D - 2019.

D2.3.3 Category C: Special Automobiles.
D2.3.3.a These Records may be subdivided according to the type of engine used (jet, rocket, etc.).
D2.3.3.b The use of moveable aerodynamic devices is permitted.

[TD]

I may have missed it but I haven't seen anything in the Bloodhound design or photos of the realized car which makes me think the vertical tail can move.

Ditto for Thrust SSC and Aussie Invader 5R.

On AI5R and on Bloodhound it seems like the front canards may be adjusted for angle of attack.  Whether that's dynamic or not isn't clear.  I think Thrust SSC had a means to adjust rear ride height.

Tim

[MX304]

I may have missed it but I haven't seen anything in the Bloodhound design or photos of the realized car which makes me think the vertical tail can move.

Ditto for Thrust SSC and Aussie Invader 5R.

On AI5R and on Bloodhound it seems like the front canards may be adjusted for angle of attack.  Whether that's dynamic or not isn't clear.  I think Thrust SSC had a means to adjust rear ride height.

Tim

Bloodhound does not have canards. Some early renders showed them, but they were never installed.

[MX304]

What regulation prohibits augmenting the two wheel steering (yaw control) with aerodynamic rudder control? Likewise, there isn't a prohibition of aerodynamic pitch control (canard fins) either. I suppose at some speed the aerodynamic rudder provides more yaw control than the two wheels. When  I spoke with Andy Green several years ago about the rear wheel steering on the Thrust SSC (it is dynamically unstable) he said the aerodynamic steering controls saved the project.

Thrust SSC did not have any sort of aerodynamic steering controls.

[J79]

If moveable rudders and canards make driving at high speed safer, why would it be prohibited? Sounds like the rules need to be updated to today's standards.

[wobblywalrus]

One time, years ago, I asked a guy how they steer a rocket in space.  It was in reference to the Saturn 5 rocket that launches the astronauts.  He said the nozzles where the flames exit the back end can be adjusted so the center of thrust can be varied to steer the rocket.

The sonic shock wave in front of the wheels might be disturbing the sand so the Bloodhound is skating on top of a layer of non-cohesive material.  That can explain its squirrely behavior at speed.  Some sort of steering that does not depend on the wheels might need to be installed before the next visit to Africa.