Speculative Aerodynamic Model of Thrusters

[MESM]

This links to a video showing my aerodynamic models of the Roadster 2020 with SpaceX options and how it might perform. It is of course speculative.

At the end of the video I compare my model with Elon's cryptic tweets.

I have 'test driven' the video against a range of people in an attempt to make sure that the explanation is clear, however comments, criticism, and improvement are welcome.

It will be fun to see if I am right or wrong on the day that it is announced (possibly later this year).


I have a an extra PowerPoint slide set with annotated simulation results http://www.mbtrusts.co.uk/DropBox/Roadster%20Thrust%202018%20July%2020/Post%20Video%20Details.ppsx

 

[Brando]

Paper lifts because of "higher" air pressure under the sheet of paper. This is how wings lift air planes.
Higher pressure "under the road" ?? no, if you could do that the road would tear apart. right?

In other words, this method does not create downward pressure to hold the roadster to the "paper road" but creates lift to lift the "paper road". I think you get it. But for James? I think Q could figure out something to help defeat evil.

Elon being ridiculous? Well, we know that never happens! Except with the Boring Company or him becoming the Emperor King of Mars or never mind. You have a good point. [dwunken elon on teslakia]

great video clip
you've got talent
keep working on content

 

[Joerg]

Twitter this to Elon, lets see what happens.
My guess is you pretty much nailed it.

 

[MESM]

@Brando - Another way of measuring suction could have been to insert a pressure-measuring tube from the top of my second draft car and glue its opening so that it opened into the space under the car (in the centre of the car). I could have measured the suction using water in a U bend.

In the end the I thought the paper looked better on video because you can see the suction - you do not have to infer it.

Yes! I think Elon would send a team of engineers to the set, and Q would get some crazy toys!

@Joerg - Thanks Joerg. It's going to be amazing if it works out!

 

[ggr]

Paper lifts because of "higher" air pressure under the sheet of paper. This is how wings lift air planes.
Higher pressure "under the road" ?? no, if you could do that the road would tear apart. right?

Wrong, I'm afraid. A small amount of the lift comes from higher pressure under the wing, but most of it comes from lower pressure above the wing. I give this demonstration to scouts doing their aviation badge. Take a sheet of paper. Hold it against your lower lip, and blow. The paper lifts.

 

[LargeHamCollider]

This system is going to have a regulator, they're not going to be running at constantly decreasing pressure. Also they're going to have standard converging/diverging nozzles with high exit velocities, likely angled up and over-expanded to avoid the high energy pebble problem

 

[Brando]

Wrong, I'm afraid. A small amount of the lift comes from higher pressure under the wing, but most of it comes from lower pressure above the wing. I give this demonstration to scouts doing their aviation badge. Take a sheet of paper. Hold it against your lower lip, and blow. The paper lifts.

It is the difference in air pressure, right? measure how you will But we still don't get down pressure, do we? we get lift, right? not down forces.

@MESM - it isn't suction, it is the lower air pressure so relatively the other side of the tube has "higher" air pressure , right? so that pushes the water up. Well, I guess you can pick your frame of reference but it is the difference that does the work. Few absolutes, most things are relative.

"absolute zero" - I'd I to do some reading to try and explain why.

 

[MESM]

@Brando Hi, apologies for taking some time getting back to you, these are my first few videos (my video editing is getting faster). I've made another video with some extra explanation:

Hope this helps.

@LargeHamCollider Hi - thanks for your comment. The reason I do not favour the idea that he is using the rockets as conventional thrusters is that you would need insane velocities. Anything that can make a difference on a ~2 tonne car should not be pointed at anything that wishes to continue living! I tried to illustrate this at 1:54 in the new video, and it's at the end of the original one as well. Let me know if this works for you.
I have no info on the regulators - I hadn't considered it. I'd be very interested in any info you could pass on to me?

 

[Brando]

Again, I see air pressure differences. atmosphere presses down the aluminium foil. I still can't see how a car could be pressed down as you can't "seal" (tape down) the car.

side note: Chaparral 2J used fans to create suction (lower the air pressure) under the car. Therefore the air pressure above the car is greater and that increased pressure pushes the car down. Google search will yield articles to read. famous car

Now I wonder, can you "skirt" the roadster and use air jets to replace the fans? I don't see it just yet, but... got me thinking...

 

[Joerg]

@MESM I think people here are still imaging single thrusters (maybe 8 units?) placed around the car kind of firing compressed air out to the sides in a very narrow jet. Couldn't it be more like in your model where a "single" thruster (your hair dryer pushes air out into a layer beneath the car with wide vents around? Or it could still be like many thrusters around but with a very flat but wide nozzle.
Now, Megan, won't you step up from building models to creating a simulated model with some software?

 

[MESM]

@Joerg Yes - I also think that the narrow nozzles are not the way that he went (we will find out at some point!). My reading of Elon's tweets is that his ten thrusters might be arranged to form one curtain around the car using three nozzles on each side with two front and back. (In my demo I only have enough steam for one part of the curtain)

A computer model... actually, I have two computer models (but they are not very 'forum friendly')!

Computer Model 1: The simple one was used for my original video and there is an attached engineering slide pack (http://www.mbtrusts.co.uk/DropBox/Roadster Thrust 2018 July 20/Post Video Details.ppsx). It has one COPV that fires for half a second and it is only limited by tyre friction.

Computer Model 2: I have a second model that is limited by tyre friction and battery pack power (I have assumed 1 MW - but only because I don't have better data)

If you know more about the constraints on the car then I would love to hear them.


By the way I am guessing that that COPV pressure drop led him to fit two (perhaps three) separate COPV spheres in the back seat and I think that the car uses them one after another.

Model 1

Model 2

 

[Asymmetry]

Such an interesting topic turned completely boring

 

[Joerg]

What topic turned boring why?

 

[Silent Ludicrosy]

Your basic assumptions are a little off. Sticky tires can pull over 1g. I'm guessing it would need to be around 1.2 - 1.3 g from the tires alone.

Also 30 kN of aerodynamic downforce at 60 mph is not realistic for the roadster. F-1 cars only get an extra 2 g's of downforce at 120 mph. Your model for the base roadster needs more tire grip and less aero downforce.

I agree with Largehamcollider, it's more likely they'll be converging diverging nozzles angled up away from pedestrians and other cars. If the forward thrusters were somewhere at the front of the car like on the hood, and angled up, they would keep the high velocity airflow away from other vehicles. Plus it would help hold the front wheels down when accelerating, which would help the rear tires from reaching their adhesion limits at 1.3g before the front tires.

 

[AMN]

Tweet at Elon and start polishing up your resume.

 

[MESM]

@Pmac727 Thanks for that.

Previously I had been looking for terms like ‘Coefficient of Friction’ and I got links like this http://theengineersjunction.blogspot.com/2011/02/tyre-adhesion.html which only gave µ up to 0.8, 0.9 or perhaps 1.0 – and a lot of these sources are unhappy with µ > 1.

Your note mentioned ‘adhesion limits’ and when I Google that then I get search results that are closer to your higher limits (if you have any links I would love to see them). I had not seen these when I searched for ‘Coefficient of Friction’.

So, in my model I use the ‘0-to-60 time’ for the Plain Roadster 2020 (less than 1.9 seconds) to measure the aerodynamic performance of the Roadster body (then I stick those constants into the SpaceX models). If I use your mean guess for the sticky tyres (1.25), then I ‘need’ the Plain Roadster’s aerodynamic downforce to peak at ~6 kN or I would not reach 60 mph in 1.9 seconds. (6 kN is ~ 0.3 g given the best car mass that I have).

So if I use no other refinements and just put your ‘sticktion’ numbers into the model then I get a number of really cool changes to my ‘suction model’ results for the SpaceX option (if I use my estimated 2% suction from the Thrusters):

1. The horizontal acceleration is an almost constant 2 g across the whole period (because the loss of pressure in the COPV balances the rising aerodynamic forces)

2. The aerodynamic forces needed to get to 60 mph drop to ~(2/3) g (~ 12 kN)

3. Peak power does not really need to go above 1 MW in order to get to 60 mph (the horizontal forces are so much bigger at the start so the engines do not have to work too hard at the end)

I am using my ‘Model 2’ as discussed with Joerg above with some details from a tweet between Elon Musk and he ‘Everyday Astronaut’ (YouTube).

It seems we have three models competing for the prize: rocket thrust up, rocket thrust sideways, and the suction-thrusters model. It will be good to see who wins.

(By the way your 'rocket thrust-up solutions' might work best if the exhaust was in the middle of the roof pointing straight up - they do not have to be at each corner).

(@AMN - thanks I will tweet him but he is a busy man)

So here is my prediction but with your (@Pmac727) numbers (it won't be exact - I can't know all the constants.)

 

[MESM]

I have been proposing a model where thrusters create suction (which is safe for 'strollers' because my gas velocities are quite low in comparison to 'thrusters as rockets')

BUT you may instead prefer your thrusters to act as rockets that point up, or sideways, (by projecting momentum). You might find these numbers useful/ interesting.

I took a still of the SuperDraco rocket and compared its pixel-size with Elon. Apparently Elon is 6'2'' so the COPV vessel is ~.6 metres across (That black sphere has been safety rated for the inside of the Dragon capsule and I guess that it is safe enough to put under an astronaut's seat. I guess that they will not change the design of anything that has been safety rated.)

Each COPV has a volume of ~0.1 m3. In this forum the pressure is quoted as 5500 psi which gives 54 kg air.

Three of these could line up to be narrower than a Model X so it may be that the total mass of compressed air is about 165 kg. Definitely enough to run a rocket!!!

Not stroller-safe though

 

[Joerg]

Oh man, this thread is far from boring. I try to summarize what ideas there are right now and think about pros and cons.

As @MESM said, there are 3 different models now:

1. rocket thrust sideways
2. rocket thrust up
3. suction-thrust (using lower thusting side vents)

Model 1 is popular because this is what we naturally think of if Elon talks about SpaceX-package and everybody visions the F9 thrusters in space. Also Elons mentioned it might be possible to do "hops". This model improves 0-60 (and handling thru corners) by directly generating a force in a specific direction.
Pros:

• Rocket like thrust ;-)

Cons:

• ultra loud
• dangerous air jet for nearby objects / people
• unknown if momentum is sufficient to push a 2 ton car

Model 2 is completely different because the thruster only generates a downforce to increase grip of the tires.
Pros:

• probably one thruster only on top of the car is sufficient -> easy design option as an option-package
• less thrust necessary then model 1 because it doesn't need to push the car to accelerate, just push down to increase downforce hence grip
• less thrust -> less noise
• less thrust -> longer thrusting capability
• thruster goes upwards -> much less risk of damaging something, hurting someone

Cons:

• different what Elon said (thrusters all around) and hops are not possible

Model 3 is like model 2, increasing downforce to increase grip, but in a completely different way. Here a relatively low pressured air jet coming underneath to all sides of the car kind of "ripping off" air from under the car and thus creating a zone of low pressure. This again creates a downforce that enhances the grip. Best for explanation is to watch the 2 videos from @MESM above.
Pros:

• even less thrust necessary than model 2 (just an assumption, maybe that's not true)
  
• less thrust -> less noise
• less thrust -> longer thrusting capability

Cons:

• air jets must come out all around the car -> complicated design
• still risk of pebbles flying around

I personally like model 3 because it is such an innovative idea and way off what everybody would think if you hear of a "thruster SpaceX package" on a car.
From an as simple as possible design standpoint model 2 seems more plausible.
Model 1? Sweet, but I can't think of that in reality because of thrust physics and safety.

Any more ideas?

 

[Joerg]

COPV wise I always thought of something like that:

Would also be a better fitting form factor. But certification is a good argument and therefore maybe reusing the round COPV.

 

[MESM]

@Joerg great video; I had not been able to get anything like it. I could be wrong about them reusing the exact design because they know a lot about this stuff - here is a video about the BFR that I liked

.

Interesting to see how he reuses the SpaceX tech.