**Can SpaceX Really Make a Tesla Fly?**

Tesla has always promised a flying (!) ‘SpaceX’ version of
the Roadster. Most assume it’s just an idle dream, but occasional updates
suggest not, with the latest coming from Tesla’s design chief in early 2022.

So is this even possible? Could SpaceX
technology really make a Tesla Roadster fly (apart from on the end of Falcon
Heavy)?

No journalistic opinion is required, it’s just a basic
Physics and rocketry problem. So I thought I’d whip
out my digital envelope and scrawl some calculations on the back to see if I
could get a more quantitive answer. Here goes …

First of all, what does ‘fly’ mean here? Probably just short
hops, like across a car park. But even this is pretty sci-fi, so how would it
work?

No fire-breathing rocket engines for sure. Small cold gas
thrusters underneath the car would blast out compressed air stored in a
composite overwrapped pressure vessel (COPV).

This is the same basic tech that SpaceX use to adjust the
attitude of their returning Falcon 9 first stages (those puffs of gas you see below).
But instead of pre-stored gas, the SpaceX Roadster would presumably use a pump
to gradually refill the COPV. The SpaceX Roadster would still be a pure EV!

*Now, to find out if all
this is really feasible, we first need to figure out the amount of air required
by the thrusters to keep the Roadster hovering and compare that with the
capacity of a suitable COPV.*

To get a Tesla to hover is going to require upward force
(thrust): approximately the thrust required to just overcome the force of
gravity (bit more going up, bit less going down). That force is given by the
familiar equation:

F = Ma

In this case, to just support the car against gravity, that
becomes:

F = M_{Tesla}_{
}g

Where:

F is the force required in Newtons

M_{Tesla} is the Mass of the Tesla Roadster

g is the acceleration due to Earth’s gravity: 9.8 ms^{-2}

Now to calculate the mass flow through those cold gas
thrusters, we can use the specific impulse equation:

F = g I_{sp}
m

Where:

F - the force calculated above

I_{sp}_{ }- the specific impulse for cold gas thrusters

m - the mass flow rate through the thrusters

Substituting one equation into the other, we get

M_{Tesla}_{ }g = g I_{sp} m

Simplifying and rearranging, the mass flow, m, is then just:

m = M_{Tesla}
/ I_{sp}_{}

Wikipedia quotes the maximum specific impulse for a nitrogen
thruster as 76s. Air is mostly nitrogen, so let’s just assume a round 70s (a
figure SpaceX quoted for its nitrogen thrusters in a Starship presentation).

Plugging these numbers back into our equation, we get:

m = 2000 Kg / 70s = __28 Kg/s__

*Those SpaceX cold
gas thrusters will need to emit very roughly 28 Kg of compressed air every
second to keep our Roadster hovering.*

Now let’s figure out how much air the COPV might hold.
Looking at the dimensions of a Roadster and given the fact that the COPV will
replace the rear seats, I reckon a cylindrical COPV with hemispherical ends of very
roughly 2.0m x 1.0m should fit. That would conveniently have a volume of a bit
more than one cubic metre.

To find out
how much air the COPV could hold, we need to know its maximum pressure. My
first version of this article estimated a max COPV pressure, based on old NASA
tests, of around 2000 PSI. However, Elon has said recently (Feb 2021) ‘around
10,000 PSI’ – SpaceX has been doing lots of development work on COPVs.

From graphs
I found online, air at a pressure of 10,000 PSI has a density of at least 500
Kg per cubic metre (depending on temperature). This sounds a lot, but is around
a quarter of the mass of the whole car, so seems feasible.

If the COPV
can hold about a 500 Kg of air then a Tesla Roadster might hover for about fifteen
or twenty seconds at 28 Kg/s mass flow through the thrusters, as calculated
above, depending on the size and max pressure of the COPV.

Some modest
acceleration and deceleration for a drift in the hover wouldn’t take much extra
thrust. A nice safe ~10-20 m/s (~20 – 40 mph) would allow hops of a few hundred
metres.

Control
would likely have to be fully automatic across the whole ‘flight’ envelope, given
all the obvious risks (hitting things, running out of gas, losing control etc)
of manual control.

This would
use the car’s cameras (existing Teslas have eight
already) to map the flight area for suitability and would need to be very
conservative. Tesla’s Full Self Driving technology already creates detailed 3D
maps on the fly that would work well for this.

Think a
twenty second ‘flight’ doesn’t sound much? Try visualising it! In fact, it’s
probably sufficient for those short hops I conjectured. Whether issues such as
flying debris and noise would make this impractical, I can’t say. I also can’t
say whether it might ever have functional value beyond some serious not-on-public-roads
fun!

*No legacy
automaker could do this. A flying Roadster would be the ultimate pure-EV halo
car and an incredible marketing coup.*

*In conclusion: yes, SpaceX
CGT tech really could make a Tesla Roadster fly, allowing 15-20 seconds of
hover time and a slow drift across a car park.*