Tesla Model X CES 2015 walkaround by TechVideo screencap
Aluminum is best suited when loads are spread over a wide area--but it is vulnerable to shearing or stretching forces exerted on a small area.
The mounts for the large torsion springs of the falcon doors must thus spread their loads over a wide area, which may be a challenge given the narrowness of the roof spine shown in the concept versions of the Model X.
The roof structure cannot be vulnerable to warping over time, which is why Tesla has reportedly tested a number of high-strength alloys.
One of our sources suggests that the company could end up having to use an expensive titanium alloy to get the proper durability for the door-hinge mountings.
2014 Tesla Model X all-electric crossover with 'Falcon Doors' open
(3) TOWING CAPABILITY
The final challenge for the Tesla Model X is in providing the towing capability not currently offered by any plug-in electric vehicle.
Large SUVs are often used to tow anything from boats to horse trailers.
While the Model X will have nothing like the towing capacity of a heavy-duty pickup truck, its buyers might reasonably expect it to be able to tow a trailer with a couple of motorcycles or personal watercraft.
But sustaining the maximum output of electric motors in continuous use puts huge thermal loads on them--just as racing does.
The power of electric motors is often quoted at two levels: sustained and peak outputs. Towing requires high output to be sustained for miles or hours, which presents a huge cooling challenge.
Tesla Model X prototype in Culver City, California [photo by Instagram user jmtibs]
Spreading the load over two motors helps--one source suggested that was why Tesla eliminated the single-motor Model X a couple of years ago.
But even the glycol-based coolant system in the Model S P85D may not suffice to protect the Model X motors while towing at capacity, especially if going from, say, San Francisco to Reno, a 5,000-foot increase in elevation.
Tesla may now be testing a thermal-conditioning system that uses refrigerant, like an air-conditioning system that actively removes heat from the coolant rather than simply shedding it via radiators.
In the end, Tesla has a track record of solving engineering challenges that conventional wisdom suggests are close to impossible.
Tesla Model X - Official Debut, Los Angeles, February 2012
And automotive engineers as a class are expert at identifying problems, analyzing their root causes, and then designing and testing solutions that solve them.
The big worry for more than one of our sources is this: Has the Model X team managed to solve all of these problems to the point that the electric SUV can go into mass production six month hence?
"I personally think we are 18-24 months away from a general Model X release right now," said the most pessimistic of our sources.
"I am just not seeing a company ramping up tooling and equipment to deliver a new vehicle in less than four or five months from now."
Tesla affirmed just last week, however, that in fact it is "on track for a Q3 launch."
The company is scheduled to release its fourth-quarter and full-year earnings this Wednesday after the stock market closes.
It may offer further details on the Model X schedule at that time.