Space X Option Package

[Yggdrasill]

@ Yggdrasill don't focus on propulsion, focus on downforce and center of pressure.

The motor has more than enough power, the issue is grip for acceleration, braking distance and cornering. Grip is the limitation, why they can't accelerate/brake faster.
The tires have a friction coefficient and you can add downforce to improve on that.
A normal car decelerates at 1g, a Formula 1 car does it north of 5g and the bulk of that is due to the downforce.
Sucking air from under the car does quite a lot, a diffser does that and is plenty useful but add fans to suck air and it get crazy.
Aero features can give you downforce while adding minimal drag, Active aero is even cooler as it gives you flexibility and you can even add drag when needed. This is, no idea how to call it, let's say super-active aero.

Downforce scales with the square of speed just like drag so it's very relevant at high speeds but very little at low speeds.
And this is where the jets can help, at low speed more than at high speed. They can help a little at high speed too but at high speed you can create downforce with the air intake and other aero features. At low speeds they are more useful as downforce doesn't help you much at all so it's a neat trick. They likely use them mostly in bursts or they would need to either be able to compress air in real time or have large air "tanks".
For example, a normal car decelerating at 1g needs almost 700m for a full stop from 400km/h. A Bugatti Veyron can decelerate at 1.3g from that speed and stops in under 500m. An F1 car likely does it in towards 100m.

So to recap, the car doesn't need extra propulsion, it has a huge amount as it is, the hard part is how to apply it to the road.

May be that the thrusters could be used to also improve downforce, but I don't think the math works out. The 45 kg air system I did some math on earlier could do 7350N for 3 seconds. If the 2020 Roadster weighs 2000 kg loaded, the 7350N could be used to increase downforce by 37.5% for 3 seconds. And at that point, forget about replenishing the air supply in the near term. This was 45 kg at 700 bar - it might take hours to compress that much air.

Using propane to provide propulsion seems like the route to go.

 

[J X 3]

Musk already confirmed air and any other gas is not practical.
Can't comment on your math and what can be achieved (would be curious to learn if you can share) and I suppose how the car's overall drag and lift is impacted matters too as the jets change the flow of air around the car- that's an additional opportunity i suppose.
For the weight of the car, with M3 batteries i think the battery pack would be 1000 kg if light materials used for the frame, maybe 10% more if not. If they got better batteries by then, we'll see. My hope is that they keep it around 1700kg, excluding the Space X package.

I could be less than objective as from the day they announced it i was hoping they actively suck air from under the car but this is my thinking right now, the key at high speeds is the air intake and downforce to improve grip as the 3 motors provide enough power.

 

[Yggdrasill]

Musk already confirmed air and any other gas is not practical.

No, Musk specifically said they were going to go a lot further than merely using cold gas thrusters.

 

[J X 3]

I don't interpret it that way, I think he means it's not just about 10 fixed air nozzles so it's also about air intake and the ability to direct the nozzles, maybe about how it changes the airflow around the car. About battery cooling maybe and with that, charging rates too.
So i think he means yes they use compressed air but it's not a simple dumb system.
He suggests flight and likely that's an exaggeration with this car but a fixed nozzle wouldn't work so pretty sure the flexibility in this area is certain.
Any other gas is not practical because it's not renewable, you run out of it quickly and then what. Cost and pollution too ofc but if they eject a mass, got to be one that if not finite and , in this case, it can only be air.
BTW what is your math on mechanical volume for that 45kg system?

Edit; thermal management is really important too and beyond that, maybe a really evolved system could eject both hot and cold air.

 

[bay74]

Any chance it's just boundary layer momentum injection to delay separation?

 

[Yggdrasill]

I don't interpret it that way, I think he means it's not just about 10 fixed air nozzles so it's also about air intake and the ability to direct the nozzles, maybe about how it changes the airflow around the car. About battery cooling maybe and with that, charging rates too.
So i think he means yes they use compressed air but it's not a simple dumb system.

I don't read it like that. My impression is that he and SpaceX looked at the cold gas thruster option, determined it was workable, but that it didn't go far enough with regards to capabilities, and instead went a different route. But it's certainly possible I'm wrong.

He suggests flight and likely that's an exaggeration with this car but a fixed nozzle wouldn't work so pretty sure the flexibility in this area is certain.

I don't think there's anything to the flight talk. But if there are ten thrusters and each of them are capable of 2000N, that could theoretically lift the car. So he could be referring to this.

Any other gas is not practical because it's not renewable, you run out of it quickly and then what. Cost and pollution too ofc but if they eject a mass, got to be one that if not finite and , in this case, it can only be air.

Air will of course be a large part of the exhaust gas. Using propane, 30 kg turns into 90 kg of CO2 and 49 kg of H2O, consuming the oxygen from the air, and then you have an additional 365 kg of nitrogen, argon, etc. So, basically the 30 kg of propane results in 504 kg of ejection mass. Or put differently, 94% of the ejection mass is air.

Obviously, there's still need for a significant fan system to compress air and inject it in the thrusters, probably with a buffer tank, which could also be why Musk gave partial credit to the cold gas thruster idea.

BTW what is your math on mechanical volume for that 45kg system?

I assumed a 50 liter tank, which will weigh around 35-40 kg, rated at 700 bar and made from carbon fiber (based on the specs of the Mirai tanks).

Now, I just did my calculation for air mass on the back of an envelope, but the right way is to use the ideal gas law. From it, we can get the number of moles: PV / RT = (70 MPa x 0.05 m^3) / (8.3144598 x 293K) = 1437 mol

1437 mol times 28.97 grams/mol = 41.6 kg

 

[J X 3]

@ Yggdrasill

Was thinking about what you meant by 45kg and figured air density at 1.225kg/m3 and ok 700 times that ends up close enough.

Then was considering your statement that it would take hours to "reload".
If you have a 0.1 square m opening so 1m by 0.1 m, the air flow at 400km/h (or 111.11m/s) would be 11.11 cubic m of air per second.
And 50l is 0.05 cubic m so at 700 times that, 35m3 of intake air needed. That's 3.15 seconds so gets to 3 seconds at max speed if max speed a little over 400km/h.
What it takes to compress that air, not sure and ofc the air volume scales with speed but even is the Roadster does 10 times less at 400km/h, that wouldn't be too bad.
And I have no idea how you calc the force you get so got to figure that out, unless you help me out.

 

[Yggdrasill]

@ Yggdrasill

Was thinking about what you meant by 45kg and figured air density at 1.225kg/m3 and ok 700 times that ends up close enough.

Then was considering your statement that it would take hours to "reload".
If you have a 0.1 square m opening so 1m by 0.1 m, the air flow at 400km/h (or 111.11m/s) would be 11.11 cubic m of air per second.
And 50l is 0.05 cubic m so at 700 times that, 35m3 of intake air needed. That's 3.15 seconds so gets to 3 seconds at max speed if max speed a little over 400km/h.
What it takes to compress that air, not sure and ofc the air volume scales with speed but even is the Roadster does 10 times less at 400km/h, that wouldn't be too bad.

700 bar is really really high pressure. Even if the Roadster receives a sufficient volume of air though it's air intake, compressing it to 700 bar in seconds would be quite energy intensive and you'd need a huge compressor.

The formula for calculating stored energy is here: Compressed air energy storage - Wikipedia

70 MPa x 0.05 m^3 x ln (0.1 MPa / 70 MPa) + (70 MPa - 0.1 MPa) x 0.05 m^3 = -19,4 MJ = - 5,4 kWh

So, best case, it takes 5.4 kWh to fill the 50 liter tank with compressed air at 700 bar. (100% efficiency.) If you're going to to that work in 3 seconds, you need a 6.5 MW compressor...

The one thing I'm quite unsure of is how much of this energy can be utilized in the thrusters. I went for such high pressure simply to reduce volume.

And I have no idea how you calc the force you get so got to figure that out, unless you help me out.

You can get the force from the specific impulse of the thruster, and the mass flow rate. Specific impulse - Wikipedia

When I assumed 45 kg air being ejected over 3 seconds and an Isp of 50, I got 15 kg/s x 50s x 9.8 m/s^2 = 7350N.

 

[ecarfan]

When I first read Elon’s tweet I thought he was joking. But there is too much detail, and then he tweeted a followup with more details. So maybe he’s not joking (but the “flying” part is certainly a joke).

These “rocket thrusters” (his words) are certainly not rockets in the SpaceX sense: starting and maintaining a chemical reaction with fuel and an oxidizer to produce thrust. He uses the word “engines” which I believe he should not have used as that means combustion is taking place and the resulting energy is being used to perform work, in this case producing force in a desired direction. I cannot imagine a street legal Tesla containing multiple rockets with hot exhaust exiting the bodywork. Yes I know ICE cars have exhausts. But the mission of Tesla is to advance the use of sustainable energy. Burning any amount of fossil fuel in a Tesla is not going to happen. @FredericLambert tweeted “So what you are sayin is that it won’t be an all-electric car? What kind of fuel are those thrusters going to burn” but Elon didn’t reply.

Elon mentions a COPV (Composite Overwrapped Pressure Vessel, every Falcon 9 has several of them filled with helium for tank ullage*) suggesting that the “rockets” are some kind of cold gas thruster (no combustion involved, just high pressure gas), but that is in response to a tweet by @Djenkins6 and then Elon says “But we are going to go much farther”. Does that mean no COPV with cold gas thrusters but instead something different? I don’t know.

Electrek article: https://electrek.co/2018/06/09/elon-musk-tesla-next-gen-roadster-spacex-package-rocket-thrusters/

* Ullage. See https://space.stackexchange.com/que...lcon-9-require-a-helium-pressurization-system

 

[J X 3]

700 bar is really really high pressure. Even if the Roadster receives a sufficient volume of air though it's air intake, compressing it to 700 bar in seconds would be quite energy intensive and you'd need a huge compressor.

The formula for calculating stored energy is here: Compressed air energy storage - Wikipedia

70 MPa x 0.05 m^3 x ln (0.1 MPa / 70 MPa) + (70 MPa - 0.1 MPa) x 0.05 m^3 = -19,4 MJ = - 5,4 kWh

So, best case, it takes 5.4 kWh to fill the 50 liter tank with compressed air at 700 bar. (100% efficiency.) If you're going to to that work in 3 seconds, you need a 6.5 MW compressor...
The one thing I'm quite unsure of is how much of this energy can be utilized in the thrusters. I went for such high pressure simply to reduce volume.
You can get the force from the specific impulse of the thruster, and the mass flow rate. Specific impulse - Wikipedia

When I assumed 45 kg air being ejected over 3 seconds and an Isp of 50, I got 15 kg/s x 50s x 9.8 m/s^2 = 7350N.

I'll have to check the specific impulse link properly but for now I wonder why you are using 50 Isp and also wonder why the energy in does not equal energy out so the energy used to compress = thrust. And the pressure in the tank drops as you eject air so doesn't seem quite right.
And for the energy needed to compress the air, from the pumps perspective the air comes at the car's velocity so there is energy there.

 

[lance]

I'd hate to be a bicyclist behind these thrusters whilst in traffic

 

[bhzmark]

Would you do it if you had a cop car behind you?

Many places have exhibition of speed laws: Exhibitions of Speed Laws | LegalMatch Law Library

Other places would consider it reckless driving.

on-ramps -- I have a favorite for doing luda demos. no problem.

 

[dhanson865]

Like this from Bosch but made by SpaceX instead.

 

[Daniel in SD]

May be that the thrusters could be used to also improve downforce, but I don't think the math works out. The 45 kg air system I did some math on earlier could do 7350N for 3 seconds. If the 2020 Roadster weighs 2000 kg loaded, the 7350N could be used to increase downforce by 37.5% for 3 seconds. And at that point, forget about replenishing the air supply in the near term. This was 45 kg at 700 bar - it might take hours to compress that much air.

Using propane to provide propulsion seems like the route to go.

Wouldn't using the thrusters to provide directional thrust be even more ineffective? Could there be a way to use them to both provide downforce and lower the pressure underneath the car providing additional downforce? Venturi effect?

 

[Model S M.D.]

The more I think about it, the more genius it seems. Quickly releasable, powerful short bursts of electrically powered compressed air would allow Tesla to push beyond the physical limits of traction and would definitely embarrass any gas powered car even further. And as he states, it would also help with cornering and braking... The next gen Roadster will redefine what a supercar is--very exciting stuff.

 

[EV-lutioin]

May be that the thrusters could be used to also improve downforce, but I don't think the math works out. The 45 kg air system I did some math on earlier could do 7350N for 3 seconds. If the 2020 Roadster weighs 2000 kg loaded, the 7350N could be used to increase downforce by 37.5% for 3 seconds. And at that point, forget about replenishing the air supply in the near term. This was 45 kg at 700 bar - it might take hours to compress that much air.

Using propane to provide propulsion seems like the route to go.

Space X has been working on new rocket engines that use methane instead of the standard kerosene-based rocket fuel, so I suppose that technology could find it's way to cars, but it would really go against their zero emission goals. I'm more inclined to think they would use water heated by electricity.... assuming this is not all in jest. It would essentially be a vape car.

 

[Gwgan]

Maybe the jets clean the cameras.

 

[slipnslider]

Rocket thrusters is perfect example of why some investors want to put some limits on Elon's power. Some inventions may be possible with enough time and money, but that doesn't mean they're worthwhile. Those resources would be much better invested elsewhere in the company.

 

[J X 3]

Rocket thrusters is perfect example of why some investors want to put some limits on Elon's power. Some inventions may be possible with enough time and money, but that doesn't mean they're worthwhile. Those resources would be much better invested elsewhere in the company.

Quite the opposite , this is a major asset with autonomous vehicles as it increases safety dramatically.
It also enables higher peak and average speeds so that is a plus with car as a service where time is money for both the customer and Tesla.
If done right, this triples Roadster revenue while boosting margins so the return on this investment is quite high even if you don't factor in the long term upside and this being in every car.
The fact that others aren't doing it is great and Tesla has an advantage on the tech side that helps here.
The marketing upside is huge too.
So all in all, this might end up being the biggest Tesla news in a long while, even if some folks might need 5 to 10 years to get it.

 

[gregd]

So I see from his tweets that Elon did mention performance as the goal, but perhaps there's a possibly secondary opportunity in improving safety? Consider that the Russian Soyuz spacecraft use a set of thrusters to slow the spacecraft moments before landing back on Earth. They're effectively an airbag or cushion to decelerate the craft, in place of the big splash that occurs with US spacecraft during a water landing. Neither is pleasant, from what I have read, but also neither life threatening.

Couldn't the SpaceX thrusters be used by the Roadster milliseconds before an impending impact, to decelerate the car a bit, effectively extending the crumple zone? Or, if aligned (aimed) at the impacting vehicle, slow it down a bit too? No idea if they'd be powerful enough, but that's the thought anyway.

Extra points for (re)moving anybody who's intent on harming the car in other ways...