Those silly jets

[VegasBlue]

To quote from the John Stapp article..

"By June 8, 1951, a total of 74 human runs had been made on the decelerator, 19 with the subjects in the backward position, and 55 in the forward position. Stapp, one of the most frequent volunteers on the runs, sustained a fracture of his right wrist during the runs on two separate occasions, also broke ribs, lost fillings from his teeth and bleeding into his retinas that caused temporary vision loss; in one run he survived forces up to 38 g."

Dentist: What brings you in today?
Roadster owner: my fillings fell out.
Dentist: How on earth did that happen?
Roadster owner: The light turned green, the space X thrusters fired, but a pedestrian walked into the cross walk so I had to stop.

 

[D.E.]

Not to mention you will have to periodically drain the tanks if they still have air in them to prevent corrosion in the tank. These tanks are nothing to play around with Watch at the 1:30 mark

That was a low pressure compressor and tank, 120-150 PSI. Compressed gas cylinders run 2250 PSI. Some scuba tanks run 3000 PSI.

 

[bolosky]

How is there a weight to air?

Not trying to be facetious; I am seriously confused by this.

Air has mass, obviously. Ordinarily, it has no weight, however, because it's floating in the atmosphere (that's why all the air doesn't just fall to the bottom of the room). (In case you're not up on physics, mass is how much stuff you have, weight is the net downward force of gravity. Unless something has very low density they're linearly related, and so people often use them interchangeably, but this is one case where the difference matters.)

When you compress air, then there's a lot more mass in the same volume and it isn't floating anymore, hence it has weight. If you compress it enough, then it also turns into a liquid, which is much more dense than gas.

 

[mongo]

Air has mass, obviously. Ordinarily, it has no weight, however, because it's floating in the atmosphere (that's why all the air doesn't just fall to the bottom of the room). (In case you're not up on physics, mass is how much stuff you have, weight is the net downward force of gravity. Unless something has very low density they're linearly related, and so people often use them interchangeably, but this is one case where the difference matters.)

When you compress air, then there's a lot more mass in the same volume and it isn't floating anymore, hence it has weight. If you compress it enough, then it also turns into a liquid, which is much more dense than gas.

Weight = mass * gravity.
Air always has weight, and all the air is trying to fall to the ground. That is why there is 14.7 psi of pressure at sea level.

You also cannot liquify oxygen nor nitrogen at any temperature normally found on Earth. Regardless of pressure, a gas must below the critical temperature to liquefy.
Air: -140.52C
O2: -118.6C
N2: -147.0C

Critical Temperatures and Pressures for some Common Substances

 

[bolosky]

Weight = mass * gravity.

Not exactly. Weight = mass * gravity - volume * surrounding medium density.

That is, you have to subtract the buoyancy force. So, since air has the same density as surrounding air, it has zero weight. Air under water has negative weight, so it goes up. Helium is less dense than air, so it has negative weight and goes up.

So a hot air balloon (the whole system) has roughly zero weight, but lots of mass.

 

[mongo]

Not exactly. Weight = mass * gravity - volume * surrounding medium density.

That is, you have to subtract the buoyancy force. So, since air has the same density as surrounding air, it has zero weight. Air under water has negative weight, so it goes up. Helium is less dense than air, so it has negative weight and goes up.

So a hot air balloon (the whole system) has roughly zero weight, but lots of mass.

Yeah, for the weight of an object in a medium, however we were taking about the medium itself.
What you wrote:

Ordinarily, it has no weight, however, because it's floating in the atmosphere (that's why all the air doesn't just fall to the bottom of the room).

Set a bucket of water on your hand.
Now put your hand in the buck on the bottom.
In the first case, your hand feels the weight of the water, the second, it doesn't (in a way you can easily perceive, if you have a glove on, it could).
However, in both cases, the water has weight. Air also has weight.

The Earth has an atmosphere because the air is being pulled down. Air pressure is the direct result of air having weight.

A piece of wood in water floats, but still has weight (and displaces the weight of water equal to it). The hot air balloon has a maximum altitude determined by its weight and the decreasing density of the atmosphere.

 

[whttiger25]

You could put 5 million horsepower in a car and it wouldn’t necessarily make it faster because of
Limits in coefficients of friction between rubber and asphalt. If you could create extra downforce without increasing weight, then you can increase acceleration. Vertical jets would allow faster acceleration, as the dont add weight in the direction of acceleration (horizontal) but increase friction between the tires and road

 

[mongo]

You could put 5 million horsepower in a car and it wouldn’t necessarily make it faster because of
Limits in coefficients of friction between rubber and asphalt. If you could create extra downforce without increasing weight, then you can increase acceleration. Vertical jets would allow faster acceleration, as the dont add weight in the direction of acceleration (horizontal) but increase friction between the tires and road

Yah, tires limit acceleration.

Mass of the nozzle and air system is the same regardless of the direction it's pointing and is not directionally dependent.

Unless the tires have a coefficient of friction greater than one, using the air to generate a forward force directly will produce more acceleration than a down force which loads the tires.

 

[Saghost]

Yah, tires limit acceleration.

Mass of the nozzle and air system is the same regardless of the direction it's pointing and is not directionally dependent.

Unless the tires have a coefficient of friction greater than one, using the air to generate a forward force directly will produce more acceleration than a down force which loads the tires.

Sustained acceleration and braking on performance street tires on good unprepared surfaces is around 1.4g I believe, implying a similar coefficient of friction.

Racing slicks on drag strips go way past that.

So down is lighter for the same acceleration, assuming infinite power/torque available - and that’s without the theory tossed out above about using vacuum under the car to build downforce beyond the inertia of the air moved.

The other thing is, jets to the rear blow directly on any cars behind you, and if it’s enough to make a difference in acceleration times, it won’t be a gentle breeze. The risk of damaging the next car, either directly or by kicking up FOD, makes blowing to the rear unwise.

 

[mongo]

Sustained acceleration and braking on performance street tires on good unprepared surfaces is around 1.4g I believe, implying a similar coefficient of friction.

Racing slicks on drag strips go way past that.

So down is lighter for the same acceleration, assuming infinite power/torque available - and that’s without the theory tossed out above about using vacuum under the car to build downforce beyond the inertia of the air moved.

The other thing is, jets to the rear blow directly on any cars behind you, and if it’s enough to make a difference in acceleration times, it won’t be a gentle breeze. The risk of damaging the next car, either directly or by kicking up FOD, makes blowing to the rear unwise.

Agree with those points. I was reacting to "Vertical jets would allow faster acceleration, as the dont add weight in the direction of acceleration (horizontal)" and gave an (over?)simplified response.

 

[whttiger25]

Agree with those points. I was reacting to "Vertical jets would allow faster acceleration, as the dont add weight in the direction of acceleration (horizontal)" and gave an (over?)simplified response.

I agree you need a >1 coefficient of friction for downforce to allow greater acceleration than horizontal force.

I do think with the power and torque you can create from electric motors the friction is the bigger limiting factor vs power, and that's what I was trying to address.

It is also interesting to note that Musk is talking about using space-grade cold air thrusters, which I assume are not only incredibly expensive, but also very weight efficient.

I also think I like the idea of these thrusters being more about handling than raw acceleration.

 

[mongo]

I agree you need a >1 coefficient of friction for downforce to allow greater acceleration than horizontal force.

I do think with the power and torque you can create from electric motors the friction is the bigger limiting factor vs power, and that's what I was trying to address.

It is also interesting to note that Musk is talking about using space-grade cold air thrusters, which I assume are not only incredibly expensive, but also very weight efficient.

I also think I like the idea of these thrusters being more about handling than raw acceleration.

Oh yeah, the tire can't cash the checks the motors write!

My concern with increasing down force is that it still relies on the tires for grip. Using the thrusters directly makes the additional maneuvering ability immune to changes in road surface and grade. Car would handles a well on ice as a groomed track (I thought there was Musk comment to that effect, but I can't find it at the moment).. Downforce also increases the load the tires need to handle along with requiring stiffer suspension.

SpaceX makes a lot of their own parts, including valves, so the cost might not be that much (depending on the raw material used).

 

[bolosky]

There's a wonderful thread on this very forum from a few years ago where someone does an engineering study about using the jets to reduce the pressure under the car and so improve grip/acceleration that way. After seeing it, I've been convinced that this is what Tesla is going to do. Direct thrust, either from the top or the back, doesn't accomplish nearly as much.

Check it out, it's worth the time.

Speculative Aerodynamic Model of Thrusters

 

[ThomasD]

Keeping the thrusters clean and damage free while under the car maybe difficult

 

[Brando]

You will have to have some sort of dryer system to keep moisture out of the lines If the valve becomes damaged it can turn the cylinder into a missile. There is no way Tesla will take on the liability of this setup

carbon fiber tanks, no rust, failure mode is to tear open and leak. - my opinion.