Designing a hard-shell rooftop tent

[aerohead]

Originally Posted by Frank Lee

That's what autonomous mode is for.

I'm holding out for the autonomous,pre-masticated,5-star meals,air-freighted in from Paris!

[freebeard]

Thanks for the interesting questions! I'll go with these in return:

How much computing power (in petaflops) behind Autodesk Flow Design?
Is the posted design the one you tested?
You have a Tesla and a stack of 1" R-40 vacuum panels? what dimensions?

So... The shape — start with an ME-262. Overhang the windshield but not the nose. If you can't picture a trolley-top on 4-bars, just ask.

Here's a low-resolution-Templated armadillo shell:



Without more of a design brief, I'd only recommend retractable spider-leg camper jacks so you can drive out from under it. And consider basalt as an alternative to the carbon fiber. Lower cost and it bends instead of shattering. You can already get a Basalt Tesla body kit:


Larte Design use basalt fiber in Tesla Model S front bumper - Basalt Guru

[BigChief]

Originally Posted by freebeard

Thanks for the interesting questions! I'll go with these in return:

How much computing power (in petaflops) behind Autodesk Flow Design?
Is the posted design the one you tested?
You have a Tesla and a stack of 1" R-40 vacuum panels? what dimensions?

So... The shape — start with an ME-262. Overhang the windshield but not the nose. If you can't picture a trolley-top on 4-bars, just ask.

Here's a low-resolution-Templated armadillo shell:

Without more of a design brief, I'd only recommend retractable spider-leg camper jacks so you can drive out from under it. And consider basalt as an alternative to the carbon fiber. Lower cost and it bends instead of shattering. You can already get a Basalt Tesla body kit:

Yes, I ran the design I posted and similar through Flow Design. No idea how many petaflops it takes; it is a realtime sim and the initial calculation takes a minute or two to resolve on my skylake i7.

My vacuum panels are 8x8x1-inch square. I plan to just tile the inside walls and ceiling, and the exterior bottom. They're aluminum skinned with an additional shrink-wrap plastic shell so they're pretty tough as long as they don't get a hole poked in them.

I think I actually just had a eureka moment. That shell design you posted; what if I simply made a flat door on the front and back of the shell that could be "kicked down" to fold flush against the floor. Then when I'm putting down serious highway miles it's just a big tube that the air can flow through; hardly any frontal area (assuming 8-foot radius with 1-inch thick sandwich panel plus 1-inch of insulation, that's 0.347222 of square feet frontal area by my math). If the furniture is folding carbon fiber tables and chairs, they can all be either stored in the Tesla or folded flush against the floor/walls.

I had researched basalt before; it seems like both the strength-to-weight and cost is supposed to be an intermediate between fiberglass and carbon. But I want to keep the strength-to-weight as high as possible since we're talking roof-mounted, plus I've already got a great deal for surplus carbon lined up; it's easier to get good deals on it because there's so much more in production. I've heard that adding a final layer of kevlar cloth to carbon is a good way to bolster the impact/abrasion resistance so I might try that for insurance against the odd tree branch; either that or I might try UHMWPE tape on the likely impact spots.

[deejaaa]

here's some info on the '16 model:
Model: 2016 Tesla Model S
MSRP: From $88,000
Horsepower: 417 hp
Range: 253 mi battery-only
Curb weight: 4,824 lbs
sounds like a lot of coin to just put a space camper on top of. thought about converting it to a station type back end? it wouldn't add much more weight, height, length. might be able to reverse the mod for resale.
http://speednik.com/files/2013/02/hurst_hauler.jpg
https://lh3.googleusercontent.com/-E...3%252520PM.jpg
make one of these:
http://www.stationwagonforums.com/fo...18f895ec46a016
these are pretty cool:
http://98ron.superunleaded.com/wp-co...carcamp-sm.jpg
http://images.blog.autoshopper.com/8...o_camper_2.jpg
what yours will probably look like:
http://whitetrashrepairs.com/wp-cont...13/08/8839.jpg

[seifrob]

Originally Posted by BigChief

... could you give me some insight into how you arrived at that?...

As I said, it was back-of the envelope calculation without any tools. So I had to estimate height, width, drag coefficient and weight. That envelope in question (alright, it is back of IKEA leaflet in fact) is attached. I hope someone wiser (aerohead, freebeard, kach22i??) will check it.

What I don´t know is drag coefficient of large diameter tube. Regardless of that, general solution (formula) is there, use your own numbers (in metric).

EDIT: I should explain: If a car flips or not is question of ?moment? - i don´t know correct term in english. When flipping, you have two levers. One is weight of your car acting on half of your wheelspan, pinning you to ground, the other is aerodynamic force acting on lever as long as half of height of your car (approximately). When aerodynamic force multiplied by the lever length is greater than weight multiplied by half of the width, you are in trouble.

It would be much easier to explain it i my native language. (but much harder for you to understand :-) )

[slime]

okay all ask....why?

[BigChief]

Originally Posted by deejaaa

here's some info on the '16 model:
Model: 2016 Tesla Model S
MSRP: From $88,000
Horsepower: 417 hp
Range: 253 mi battery-only
Curb weight: 4,824 lbs
sounds like a lot of coin to just put a space camper on top of. thought about converting it to a station type back end? it wouldn't add much more weight, height, length. might be able to reverse the mod for resale.

Well, roof-mounted seems more reversible than what you're suggesting. I could always detach the camper and tow it behind the car on an ordinary utility trailer if my rooftop plan doesn't work out, but like I said there's advantages to parking/agility/registration by having it on top.

I don't want to do one of those station wagon / hightop conversions because it would feel like a car I extende and ripped the roof off of, as opposed to feeling like an actual tent/cabin. But the key advantage is that my vacuum panels can insulate heat and sound very well; it'd be sloppy to try to put them all over the car interior.

The Tesla is a lot of coin, but it also has 90 kilowatts of battery. That can handle climate control and run a laptop and other devices for weeks between charges. It's really well-suited for off-grid escapades.

[BigChief]

Originally Posted by slime

okay all ask....why?

I saved up enough to either buy a really nice RV, or buy a really nice car and put a camper on top of it. I live in the city with only street parking and drive out to campsites on the weekends, so this makes more sense logistically.

I've been driving an EV for years now (Smart Fortwo with some of my own ecomods like pizza-plate hubcaps) and once you have an EV you can't go back.

So I want to make the switch to the Tesla because of all the synergy with my camper idea. My Fortwo is super fun but is essentially a highway-legal go kart, I certainly can't tow anything with it. But a Tesla is heavy, powerful, very stable, auto-leveling suspension, battery 5 times more capacity/range than mine, and all current Teslas are being produced with autonomous driving cameras installed.

Picture this: my camper like I described is 20 feet long above the Tesla, which gives it enough space for around 2400 watts of solar cells (these only add about 20 pounds of weight if you encapsulate them in the same method like they make solar race cars). At 5 hours of sunlight in the day, that's 12kwh, after the inefficiencies of charging I might gain 10kwh per day back to the 90kwh battery. That means every day I'd get back 40 miles of range, or 20 miles of range and surplus energy to heat/cool the camper and run accessories. Now imagine in 5-10 years, the Teslas log enough miles for the government to feel comfortable legalizing autonomous driving. I could hop into the drivers seat, take a nap and take up in another city.

A self-driving road-trip without ever having to fuel or charge anywhere. I could park anywhere I want or drive to the middle of nowhere and make it home for a night or a week. Now that you've asked why, I'll ask "why not"?

[BigChief]

Originally Posted by seifrob

As I said, it was back-of the envelope calculation without any tools. So I had to estimate height, width, drag coefficient and weight. That envelope in question (alright, it is back of IKEA leaflet in fact) is attached. I hope someone wiser (aerohead, kach22i??) will check it.

What I don´t know is drag coefficient of large diameter tube. Regardless of that, general solution (formula) is there, use your own numbers (in metric).

EDIT: I should explain: If a car flips or not is question of ?moment? - i don´t know correct term in english. When flipping, you have two levers. One is weight of your car acting on half of your wheelspan, pinning you to ground, the other is aerodynamic force acting on lever as long as half of height of your car (approximately). When aerodynamic force multiplied by the lever length is greater than weight multiplied by half of the width, you are in trouble.

It would be much easier to explain it i my native language. (but much harder for you to understand :-) )

Thank you for the explanation. I found this from https://en.wikipedia.org/wiki/Tesla_Model_S :

"Most notably, since the battery pack is the single heaviest component of the vehicle, the Model S has a center of gravity height of only 18 inches (46 cm)".

So center of gravity is 18 inches (46cm) on the car which weighs 4842 pounds (2196kg). The Model S is 56.5 inches (143cm) height. The entire camper structure is 300 pounds (136kg) at 8-foot diameter by 20-feet long. I calculated that for 3 layers of carbon fiber, then 1-inch layer of nomex honeycomb, and another 3 layers of carbon fiber.

My roommate is an engineer so I'll ask him for help too.

[freebeard]

My roommate is an engineer so I'll ask him for help too.

I support that choice.

I think I actually just had a eureka moment. That shell design you posted; what if I simply made a flat door on the front and back of the shell that could be "kicked down" to fold flush against the floor. Then when I'm putting down serious highway miles it's just a big tube that the air can flow through; hardly any frontal area (assuming 8-foot radius with 1-inch thick sandwich panel plus 1-inch of insulation, that's 0.347222 of square feet frontal area by my math).

Two words: Interference drag. That's sort of the opposite to what I'd hoped to convey. Have you thought about putting a propeller on the front to recharge the battery?

Let's try again. Are you familiar with the Template? Here it is implemented at a higher resolution.



This expression of the aerodynamic ideal combines a geodesic primitive with a Cartesian coordinate layout. Every edge length is a known quantity to 6 decimal places, and the layout can be based on triangles, diamonds or hexagons.

If you build that I'll owe you a six-pack of beer.

Now, as to the lever arm from the Tesla's natural roll center to the new center of pressure. Here is a post from 2009 that introduced Englar's work at Georgia Tech

http://ecomodder.com/forum/showthrea...ots-10972.html*

It's all gone behind a paywall but if you follow up you will see that he demonstrated using pumped air to increase/decrease drag, lift and ruddering. So if you truncate the rooftop 'tent' and it has kilowatts of power available you could use it to steer the top of the car relative to the bottom.

OTOH, I worry about the 8x20ft floor plan. Could you get by with 7x16ft?



I had one of these once. It was 23ft with the hitch, 7 ft wide with a 6' 2" ceiling. You're talking about perching the equivalent on top of you car. I hope you have a plan for off-street parking (a store-and-lock or something) so it doesn't have to be on top of the car all the time.

Edit: *Not the best link, it degenerates into a discussion of injecting steam into the wake. Two papers you want are

A New Aerodynamic Approach to Advanced Automobile Basic Shapes
Alberto Morelli

and

Alternative approaches to rear end drag reduction Technical Report
Torbjörn Gustavsson