Hypothetical engineering problem - the 'handbag car'

1. Oct 5, 2008

qbit

There is an advertisement that has a large 'pick-up' or SUV come to a stop, fold itself up around the driver, lower her to the ground and continue to fold up until it's about the size of a matchbox. Humorously, it gives the 'secure' bleep noise before leaping into the drivers' handbag.
Hypothetically, if an engineer were to go about designing such a vehicle, would she begin with the 'decompressed' configuration or the 'compressed' configuration? I guess this is similar to the engineering problem of a 'solar sail' that must first be packed into a payload before being deployed.

2. Oct 5, 2008

Staff: Mentor

Is this a serious question? If any engineer went to her boss and said she wanted to design such a vehicle, when her boss finished laughing, s/he would fire her.

3. Oct 5, 2008

qbit

Well, in a way it's a very serious question. I understand that some (many? most?) physicists believe that all the laws of the universe existed at the moment of the 'big bang'. Our universe has 'unfolded' to give rise to the complexity we observe because of those laws.
I was more interested in any general principles engineers could give to solve such problems. Obviously, such a car would be technically laughable, but instead of choosing a common problem such as a prefabricated house that needs to be transported to its location, I thought an example that can't yet be done would be more useful to the question I was trying to ask.
It did occur to me that such 'devices' generally need have only one criteria in the 'compressed' configuration and that is to be small. The 'uncompressed' configuration usually has many criteria and functions to fulfill. However, being small is a very limiting factor. The question I guess I wanted to ask is: Is there a relationship between the upper limit of 'functionality' in the 'uncompressed' configuration and the 'smallness' of the 'compressed' configuration known to engineers? Bit of an ugly question. I was hoping to get a few responses so I could figure out how to word this question in terminology that engineers are more familiar with.

Last edited: Oct 5, 2008
4. Oct 5, 2008

qbit

On reflection, the term 'functionality' is vague and, as far as I know, unquantifiable. Would it be more meaningful if the upper limit for the uncompressed configuration be restricted to measurable quantities such as tensile, compressional & shear strength? This would eliminate other tricky stuff like resilience to high energy particles/photons; optical transparency/opacity and chemical reactivity. No doubt I missed some large categories that would fit into 'functionality'. But this could be a good place to start?

5. Oct 7, 2008

qbit

It has occurred to me that my question I was trying to ask is probably meaningless in the face of 'shape memory alloys'. I had assumed that things like connections, joints, and telescoping components would become predominant if a given device were to be shrunk too small - overwhelming the original purpose of an uncompressed device.
If by applying heat or a current a device simply conforms to a new shape, then that enters a world where, even if I knew how to ask the question, I doubt I'd understand the answer. Maybe if I do ten years of study; researchers in the field publish freely in the meantime; and this forum still exists, I'll ask then.

6. Oct 7, 2008

Mech_Engineer

The question is ridiculous for many reasons, not the least of which is the fact that conservation of mass (and conservation of volume in the case of metals) will apply. If the car weighs 4000 pounds while driving around, it will weigh 4000 pounds in any "configuration" you fold it into.

7. Oct 7, 2008

Danger

A mere drop in the bucket compared to the other crap in W's handbag.
The only way that I can think of to get maximum compressibility in a structure is if it's inflatable. I don't mind driving around in a car with airbags, but I don't want to drive around in an airbag with no car around it.

8. Oct 7, 2008

qbit

I'll try to articulate my question more clearly. An SUV is maybe not the best example but I'll try.
If you take the volume of each of the thousands of components that make up the SUV, one could simply sum these volumes and say that that's as compressed as the SUV will ever get, assuming we're not trying compress it to something that resembles a portion of a neutron star. This is a trivial exercise if given the data and one doesn't actually have to do the measurements.
But I'm thinking a bit more pragmatically. If one were to actually pack all the parts, say, for transport, and one wanted to fit all these parts into the smallest container possible, there are bound to be larger components, for example, the roof, that will stick out and force one to use an otherwise larger volume container. A way around this is to make the roof in smaller pieces that can be assembled when the shipment gets to its destination. But now the roof needs to have components that join it together increasing the total volume. Even if that means weld seams. On the other hand if it were possible to 'extrude' and 'inflate' a car in one piece (impossible, I realise), you would have far less joining components for all the pieces reducing material volume but the overall volume would be larger because a car has some empty space within its extremities and it can't be packed.
A (normal) car is not really a good example because it is made up of so many pieces and most of them are relatively not too big compared with others. What I'm trying to get at is this: is there a general method for optimizing a device for packing which takes into consideration its final form and function.
People actually buy all sorts of things based partly on the attribute that they 'pack flat'. But as far as I can determine that's about as quantitative as this property gets. I don't even know the name of this attribute except to call it 'compressibility'. I'm confident I'm not inventing a new science here. There must be lots of people who have thought about, named, and even found mathematical relationships for this.
Note: I'm note including any self-assembly attributes here.

9. Oct 8, 2008

Staff: Mentor

Neutron star? That's an odd example to use. Just so we're on the same page, why don't you do for us the calculation you suggest. Let me frame it a little more specifically for you, though: Assume you have a SUV that weighs 4,000 lb and has an average density equal to the density of steel. What is the minimum volume of the material that makes up the car.

Regarding your question, though: the designers of cars don't consider the need for them to be dis-assembled and packaged into a small container. It simply isn't on the list of necessary attributes for the vehicle to have.

10. Oct 8, 2008

qbit

Mass of car: 4000 lb = 1814.4 kg (I hear Americans are inching their way to metric)

Density of low grade steel: 7850 kg/m^3

Volume of SUV: 0.231 m^3

Or rather, the minimum volume. I agree, no one would transport a SUV compressed to this volume.

I also agree that a SUV is NOT a good example. Maybe in the future where parking is really at a premium, some form of compression of whatever vehicles are used at that time would be desirable.

Do you ever go camping? The next time you do, consider the parts needed to construct your tent, how many there are, and what space it packs into.

I thought the advertisement that I originally mentioned highlighted this kind of problem, albeit in a dramatic and unrealistic way.

Are engineers not the right people to ask concerning this kind of problem? Who would you recommend?

11. Oct 9, 2008

Danger

This is the right place to ask, but you have to keep in mind that the answers that you receive will be based upon known laws of physics. Outlandish speculation is sometimes accepted in the GD section, but not in the serious science forums. This is not meant to demean you or your ideas. Please just accept that what you suggested initially is impossible, and continue with your thinking. We don't want to discourage people here; merely point them in the direction of something attainable. There are lots of things that could benefit, in a practical manner, from being compressed. A car, however, is not one of those.
Keep thinking, dude—that's what makes the world go around.

12. Oct 9, 2008

Staff: Mentor

Cartoonists, maybe? Seriously, it sounds like you are trying to invent a Transformer. What you describe is nowhere close to being realistically possible and engineers are constrained by reality.

13. Oct 9, 2008

redargon

There's your answer. That is the smallest volume you could fit everything into if you kept the density constant.

In terms of your original question in your original post, and without being as hyper critical as everyone else, I would propose you try to design the "item" (suv or whatever) in expanded form first, then see where things can be adjusted to get it to fit into a smaller and smaller space. If you start your design from the compressed phase, then you may inadvertently sacrifice functionality aspects so that you can fit everything in, instead of being able to sacrifice the lower functional requirements at a later stage (after you have the working model with full funcitonality) to fit everything in.

hope that helps