Designing a hard-shell rooftop tent
 

Hey All, I'm attempting to design a rigid rooftop camper tent that will be permanently mounted to the roof of a Tesla Model S. This is going to be an around 7-foot diameter tube (I want it to be standing height inside) and 20 feet long so there's no overhang on the sides and about a foot and a half of overhang at the front and rear of the vehicle.

Here's a concept picture here, I used a plane fuselage model since that gets the idea across: imgur.com/BhKIclp

I'm making it out of carbon fiber and nomex honeycomb, I did the math and it'll come in around 300 pounds which is very light for such a huge structure. The Tesla has about the lowest center of gravity of any production car and I don't drive over 45mph so I'm not worried about crosswinds.

The two big aerodynamic concerns I have are:

1) how best to shape it (if it were 40-feet long, it would be easy to shape it like a killer whale with standing room in the middle and a narrower sleeping area in back, but 20 feet with a standing-height requirement makes it really hard to keep the drag coefficient down and still be practical

2) how much of a gap I should have between the floor of the camper and the roof of the car. If there's little gap, I'm worried the frontal area will act like a scoop and totally mess up the aerodynamics of both. The Tesla is 4.7 feet high and I would need to keep everything under 13.5 feet to be street legal.

I tried modeling everything in "Autodesk Flow Design" which is a 3D wind tunnel simulator but I'm not convinced it is giving me Cd values that are anywhere near accurate.

Uh....um....Is there any particular reason you wouldn't figure out a way to make a trailer out of this and pull it behind the car? I have no idea about the rooftop venture you propose, I don't believe it has been attempted.

Here's your pic/illustration.
http://i.imgur.com/BhKIclp.png

Thanks for posting the pic. The reason I want it on top is that it makes finding parking a thousand times easier, keeps it away from dirt and insects, and the Tesla's air suspension will "auto-level" the car when parked, therefore also leveling the camper. It also doesn't have to be licensed like a trailer. So most of the same reasons that rooftop campers are gaining popularity in general.

I know that is terrible for aerodynamics versus just towing it, but I live in an apartment with street parking, so if I want to go camping on the weekends and still have a place to park during my weekday commute, roof-mounted seems like the only way to go.

Might be some adverse behavior in windy conditions.

Consider bridge clearances can be as low as 13.5 feet. I would make a trailer and look for a rental storage bay. A canoe and a good tent would be easier.

Definitely paint it yellow and brown and put "Oscar Meyer" on the side. Also note the wiener is the perfect piercing shape. Much like a nuclear sub full of seamen in an emergency blow, or the famed No-Dong missile of North Korea entering a black hole. Forget these modern aerodynamic software programs, the wiener has been getting it done for a million years.

Welcome to the forums! :)

Have you considered a collapsible design something like roof top tents but with hard sides? No need to make it completely rigid all the time while traveling. ;)

Do you really have to have standing room while on top of the car? Consider the limitations you would have when driving through low clearance areas. ;)

Even parked, the Tesla will likely not have enough weight to keep that contraption stable in windy conditions.

I saw a guy lose a large toolbox he was using to weigh down a large cardboard box. A gust of wind lifted them out of the pickup bed.

Why not an actual tent that can be thrown in the back seat or trunk? Even a big cabin tent packs up small enough to fit easily in most trunks.

Where do you go camping at 45 mph in an electric car that doesn't have a cheap motel nearby?

A guppy airplane comes to mind...
https://i.ytimg.com/vi/dgZxOKNX1h8/maxresdefault.jpg

Have you considered a single wheel trailer and a Wacky Shack? ;)
WACKY SHACK

While the structure may only weigh 300lbs, will it remain an empty tube for you to stand in while camping? It'll get a lot heavier pretty quick when you add you and whatever camping supplies.

Hi, i dont want to be a spoilfun, but my back-of-the-envelope calculation indicates that this structure has potential to flip over the car in relative wind speeds of around 100 km/h. So i would not recommend it.

https://sussexcampers.files.wordpres...aab-camper.png.

For others: it's Toppola what jakobnev posted. It offers standing place with small footprint. Visit http://www.toppola.com for more info.

To jakobnev: is it yours? How does it handle in crosswinds? Was your post meant as suggestion what to do instead rooftop whale, or as contradiction to my claim?

I don't have one myself, I was just trying to suggest a design that was a bit more sane.

I don't necessarily doubt you, but could you give me some insight into how you arrived at that? Keep in mind the Tesla Model S 90D is 4842 pounds curb weight and it is very bottom-heavy.

Aha, them i rise whole hand for toppola style. I only fear that one has to have some balls to remove hatch in - how much is for Tesla car??

If I remember right, some lady even made trip around the world in Saab and Toppola, so it definitely is viable proposal.

I'm making the seats/tables from the same carbon fiber sandwich panel I'll be using for the structure, so that weight would be negligible. Luckily a lot of camping supplies are designed for the new "ultralight backpacking" fad so there's many things like sleeping bags/pillows that would hardly add any weight at all. I'd be piping the heat/AC in from the Tesla via some flexible ducting when needed, and any occasional-use appliances would probably be stored in the Tesla's frunk.

One of the main reasons I don't want a collapsible tent is that they all suck for insulation. I actually have a stash of vacuum insulated panels which are an inch thick and R-40 but they can only be mounted on a rigid structure.

overhang
 

I don't think you'll have proper forwards/upper vision to see traffic lights while stopped at intersections.The sight line will be occulted by the overhang.

That's what autonomous mode is for.

auto
 

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

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? :cool: 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:

http://ecomodder.com/forum/member-fr...17-trdrp1a.jpg

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:

http://basalt.guru/wp-content/upload...ont-bumper.jpg
Larte Design use basalt fiber in Tesla Model S front bumper - Basalt Guru

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.

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

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 :-) )

okay all ask....why?

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.

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"?

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.

I support that choice.

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.

http://ecomodder.com/forum/member-fr...07-7-35-02.png

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?

http://ecomodder.com/forum/member-fr...iler-show0.png

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. :eek: 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

I did check out the template, but it seems like it would need to be very wide across (eight or more feet) in order to have standing headroom (six or more feet). It seemed like messing with the aspect ratio severely compromises the Cd? I don't suppose you'd know where I can find a Sketchup or Autocad model of what you just linked so I can scale it to my model and see how it measures out? I can indeed fabricate something like that very precisely, because I can have the whole thing carved from styrofoam and the carbon wrapped around that; it'll conform to curves perfectly. It's certainly a lot easier than building from triangles of sheet metal, at least.

I don't know if you were joking about the propeller; only way it could work is only when the car is parked, and only if I mounted it on the hood or trunk of the car low enough that it wasn't casting a shadow on the roof (shadows drastically reduce the output of the photovoltaics). I understand it creates more drag than it outputs energy when moving, of course.

When you mention interference drag, are you saying that a wide-open pipe on the top of the car would cause amplified drag by interfering with the stream of air coming up from the windshield? I guess that makes sense. I was checking out a model of the space shuttle before and I assume that is why they use struts to give it distance from the booster rocket, so that it wouldn't mess with slipstreams: http://i.imgur.com/ueqoTqt.jpg

That's a conundrum though; if I can minimize interference drag by raising the camper farther up from the car, that makes the whole thing easier to tip and of course is less practical.

Still, I don't see why on-street parking would be an issue in any case. Where I live the legal height maximum is 13 feet 6 inches, and the Tesla is 4 feet 8.5 inches. So I have almost 9 feet to play around with above it.

But yeah, two questions for you; how do I find or generate a 3D model of the Template, like you have there, and what is the make/model of that trailer? I'm assuming it's an airstream of some sort?

Thought I'd post the Flow Design screenshots of my most recent mockup, a pipe with two teardrop shaped beams connecting the camper to the roof:

http://i.imgur.com/gUkpBkj.png

http://i.imgur.com/RqDxCQq.png

You can see the wake in the rear of the Tesla, the lines with a blue hue. The drag coefficient the program gives always seems to be wildly inaccurate though, even against shapes and vehicles where the real life Cd is known.

Just out of curiosity: can you rotate the model 30 degrees and test it again?
With this software on hand, you should have pretty good idea whats happening in crosswinds.

Thee Template is a cruel mistress. It's a half-body of revolution; one simple curve swept around an axis. One member here generated a lot of discussion about what that curve is exactly: Equation and Spreadsheet for Template.

The half-body only has headroom in the top center. The full body only has floorspace in a strip down the middle. This is why I suggested the ME-262 over the airliner nose. In fact, I've questioned the need for it to be a body of revolution.

http://i.imgur.com/6EYZT.png

From left to right — round, squircular and square. Later I considered lowering the equator and ditched the cartesian coordinate layout the spreadsheet necessarily imposes with the geodesic geometry. Here's a compounded curved rendition:

http://ecomodder.com/forum/member-fr...-w-caption.jpg

At that point I was outright mocking you.

That makes no sense. An open-ended pipe will self-interfer. Consider seifrob's comment and then put 'vena contracta' into the search box in the upper left. An empty culvert is a long way from an inhabitable space, no?

Sure thing:

http://i.imgur.com/3nDhNH4.png

Well, again, if you have any idea where I can download a 3D model I'll be happy to try it out. I'm just an amateur modeler so I don't think I'd have much luck modeling it myself.

I figured; just making sure. I don't know why I warrant the derision. I never said I was a super genius; I have no engineering/math background, just blessed with some time/money/insanity and really want to make this work.

Well, have a look at these then:

Here's a plain Tesla, the drag area is at the grille:

http://i.imgur.com/UuJRTfM.png

Now, if I put the culvert on top, it seems like it squished the air going over the windshield as it passes in between the car's roof and the culvert's floor, creating an additional drag area:

http://i.imgur.com/YfMkgh2.png

But, if I raise the culvert up even higher in the simulator, the drag area seems to go away:

http://i.imgur.com/ek8j1zw.png

Again, I mostly don't trust this program's accuracy as the Cd is all over the place, but that at least makes sense to me, it's why I posted the space shuttle picture earlier, I think they spaced the two structures out in the same way to reduce interference drag.

But wouldn't it still be less overall drag than one of those streamlined bodies you posted on account of how little frontal area it has? That's what I'm trying to figure out. If the enclosed streamliner still is more efficient then of course I'd head back in that direction.

Yes, an empty culvert is not a home, but with some fold-down doors and folding furniture that could be stowed in the Tesla during highway travel, it might be a nice compromise between habitable and transportable.

Since you're considering folding aspects of it you could just as well do a collapsible design.

Have you noticed that biplanes have fallen out of favor? That's because the wings in proximity interfere with each other. Their advantage was the shorter spans. A barrel shape has the problem whichever way you look at it. Back to basics: In addition to form drag you have skin drag. The barrel/culvert has twice the wetted area.

But wait, there's more.... Go back and look at the Average Drag Coeffiecient in the pictures in Permalink #32 and #35. 0.52 goes to 0.72 at 30°. In the environment an automobile operates in it's almost never 0°.

OTOH if you look at the 1.14Cd returned in #36 for the slick-top Tesla we're back to whether your Skylake i7 is up to it.

https://www.pugetsystems.com/labs/ar...-i7-6700K-641/

So, 100 GigaFLOPs is 0.1 TeraFLOP. But notice the software must be matched to the processor architecture. It's an up-to-date platform though, Keep an eye open for CFD done in neural networks.

There's other issues, how to generate the Template in four easy steps, the longer pylons would work just as well under a half-body, etc. What is the actual design case? It sure is nice outside...

Because it still doesn't make sense. You have to leave the camper up there all the time or would need somewhere to park it, it might as well be a trailer all of the time. A small single axle hi-lo would be towable by the car, is insulated, has full stand up height, and not much added frontal area in the down position. It even has a bathroom. You could find a really nice one for under $10,000. Look for a 17T model.
You are way underestimating what any structure up there is going to weigh especially once you add gear. The beauty of having it all in a trailer is it is ready to go all the time, and then doesn't need unloading after the trip. Weekend trips all packed out of the trunk take forever to load, forever to set up, forever to take down, and then forever to unload back home. Got a trailer? Feel like bugging out? Gone with a stop maybe at the grocery store on the way.

if the time and money are not an issue then I would just drive the Tesla and enjoy it for what it is. Then, use something designed to sleep in for the rest of the equation.

Doing the exercise for the heck of it is good, but I think that reality and sanity needs to be considered too. I would not be comfortable driving within a country mile of your proposed vehicle :eek:

Simon