The rockets are stored in a rack at the rear of the roof that folds up when activated by a combination of intense braking and hitting the shiry red button on the dashboard. The rockets are fired at a slight upward angle so as not to melt the roof and also provide extra downforce to the rear of the car adding stability and increasing the effectiveness of the rear brakes.

The foldy panel acts as an air brake and has a gap at the bottom to let the deflected air push the car roof down, cancelling out the lift created as air hits the panel. This small braking and downforce effect is amplified by the air curtain created by the rocket jets, as oncoming air cannot penetrate the faster moving jet of air and is deflected down towards the panel and roof.

I have no idea how to work out the effectiveness of the air brake or the additional rear brake capability, but here are some approximate numbers for the effectiveness of the rockets alone:
Model rocket -> weight = 100g, thrust = 100N in 1sec, diameter = 30mm, length = 10cm
Car -> Weight = 1500kg, speed = 30m/s (around 110kph or 65mph), momentum = 45kN, normal braking time = 4secs + reaction time, braking distance = 60m (about 200ft) + reaction time.

150 rockets would give 15kN thrust in 1 second, slowing the car by 10m/s. During that second, normal braking would have slowed the car by 7.5m/s. Normal braking will then take a further 1.6 seconds to completely stop the car.

Total braking time = 2.6s, total stopping distance = 33.75m

I made the mistake of spending quite a while working out the details before searching the bakery for similar ideas, so there are 2 very similar ideas, but I spent too much time on the idea to not post it. I'll leave it up to my fellow bakers to judge the originality.
-- marklar, Apr 10 2008

Similar idea 1 Vertical_20thrust_20braking
Fire rockets up to increase downforce [marklar, Apr 10 2008]

Similar idea 2 Rocket_20emergency_20brake
Rocket under car on castor wheel [marklar, Apr 10 2008]

C-130 rocket braking http://uk.youtube.com/watch?v=8YOtm9UCQEc
Too much of a good thing ..... [8th of 7, Apr 10 2008]

Smash Lab episode: Rocket Science http://blogs.discov...rocket-science.html
The idiots on Smash Lab ruin their own attempt to try this idea. Disclaimer: The entire Smash Lab show is only one-quarter baked. [land, Apr 16 2008]

...and then the ejection charge fires and all the hot model rockets shoot out onto the road. [-] for littering [+] for making big fireworks a safety feature [+] for doing the math = [+]
-- elhigh, Apr 10 2008


[+] for an idea involving the reckless use of pyrotechnics in a public place.
-- 8th of 7, Apr 10 2008


/Weight = 1500kg, speed = 30m/s (around 110kph or 65mph), momentum = 45kN/

No, that would be 45000kgm/s. The maths is a bit fishy thereafter.
-- Texticle, Apr 10 2008


I was gonna say "I'm with [8th]", but he'd say "dig your own foxhole!" [+]
-- lurch, Apr 10 2008


No, we'd say, "Come .... join us .... don't be afraid...."
-- 8th of 7, Apr 10 2008


"...Resistance is futile."
-- BJS, Apr 12 2008


How far would the car travel while the rockets are lighting up?
-- RayfordSteele, Apr 12 2008


[Texticle] 1 Newton is the force required to accelerate 1kg by 1m/s.

[RayfordSteele] I dunno, I think model rockets light pretty quickly with an electric starter. I'm not thinking of firework rockets that have a taper.
-- marklar, Apr 12 2008


They initiate very fast, some will be giving full output in under 300 mS.
-- 8th of 7, Apr 13 2008


A rocket is using the opposite-but-equal reaction of throwing something at high velocity in the opposite direction. In the rocket's case, that's hot exhaust gases. Since you're trying for rapid deceleration, it might be more effective to use roof-mounted canon. Firing a shot gives much more instantaneous deceleration. The cannonball could harmlessly explode into beautiful fireworks, or could gently parachute back to earth.
-- gregor-e, Apr 13 2008


Good point, [gregor-e]. The Davis recoiless cannon, carried by combat aircraft during WW1, used a paired barrel with a mass of fabric or grease being ejected as the reaction compensator.

In this case, lots of balloons full of water are hevay enough and safe enough to be a suitable reaction mass, but you then have the problem of hauling all that extra equipment (therefore poorer acceleration and fuel economy) round on the vehicle just in case you need to stop suddenly.
-- 8th of 7, Apr 13 2008


[marklar], momentum is not measured in Newtons, because it is not the same as force. They are related, yes, but I stand by my fishiness qualm.
-- Texticle, Apr 13 2008


"In classical mechanics, momentum (pl. momenta; SI unit kg·m/s, or, equivalently, N·s) is the product of the mass and velocity of an object (p=mv)." from Wikipedia (yeah yeah, wiki is unreliable blah blah).

It appears that I missed the time component out of it, ie, force required to remove the momentum is 45kN applied for 1 second, so the maths is correct as the rockets fire for 1 second.
-- marklar, Apr 14 2008


Wikipedia is on the money in this case.

Ah, so the rockets give 15kN thrust _for_ 1 second?

I can follow your maths now, and it seems correct enough for the situation at hand. A force balance with respect to time would be a better option though, in my opinion, if you were really interested in accurate stopping distance (to anything like 2 decimal places, for example).
-- Texticle, Apr 14 2008


Surely it's simpler to fire the engine out of the front of the car?

A) the engine can be a third of the weight of your car and will slow you down "lots", and

B) will also slow down anything you're just about to hit.

Don't try this with a Porsche, folks.
-- james_what, Apr 15 2008


And for the passenger, a chest mounted rocket.
-- ldischler, Apr 15 2008


A bumper-mounted rocket would not be suitable when you hit the brakes to avoid running over an old granny.

Also, this idea is about more than just the primary effects of the rocket, there's the additional stability and rear traction. You would get less stability if the front of the car was pushing backwards, it would try to spin.

There's also the air curtain brake, which would be less effective. At the front of the car, without the roof to stop the airflow, most of the deflected air would go under the car.
-- marklar, Apr 16 2008


If you pointed the rockets downwards, you could hop over whatever you were trying to avoid ...
-- 8th of 7, Apr 16 2008


or mount the rockets on the base of the drivers seat.
-- WcW, Apr 17 2008


I have seen advance shoulder-launched anti-armor weapon system that is recoilless. It uses a jet of salt spray in the rear to disperse the resulting reaction of the projectile. As the shock wave of the normal hot gas of ordinary weapon of that kind is absent, the human launcher can stay in an enclosed space with only salt mist to contend with.

So, instead of hot gas as a projectile by rockets, my proposal in support to your brilliant idea is to use powdered salt to the cannon propelled by gun powder, and disperse it elastically so not to damage the mounting struts and cushioned the braking impact. The salt spray is not hazardous to humans and the environment. If it is effectively dispersed forward evenly and slightly angled upward, it would not pose a threat to anything in front of the braking car. The added benefit is the resulting denser air in the front of the car to make any aerodynamic brake system much more effective.

Good luck to your idea, friend. I hope this would greatly help and encourage you to follow this up into realization. We really need your system to avert disasters looming in the streets.
-- rotary, Apr 17 2008


I thought salt was often used a sub-lethal load in shotguns.
That's gotta sting.

I once saw a live demo of a recoil-less door-breaching "weapon" also ballasted with water, for use by police assault teams. I still wouldn't have wanted to stand behind it.
-- AbsintheWithoutLeave, Apr 17 2008


Hell, yeah!
-- wagster, Apr 17 2008


Salt powder is much safer than sand and much denser than water. Let me explain the mechanics:

I said it is powdered salt, not a big chunk of salt crystal. It may be partly colloid, with dissolved gas perhaps. When released through the arrayed flat-ellipse-shaped roof-contoured nozzles/slits, the pockets of pressurized fluids would expand, effectively dispersing the salt powder evenly into a wider transparent plume. The hot combustion gas ensuing from the ignited gun powder will act as the propellant of the colloidal charge as well as the dispersant of the plume. The plume would descend considerably assuring a soft direct hit to any aerodynamic braking system, preferably made by the launching platform in the car roof.
-- rotary, Apr 17 2008


[rotary] not all ideas on ths site are intended to be seriously marketable products. Car companies have enough trouble with people sueing them for airbag injuries.

I assumed that rockets would have a better thrust to weight ratio as you don't see many salt engine space shuttles. Still, thanks for explaining it a second time, I'm sure lots of people would have been confused if you just said "fire salt forwards".

I don't know how far the rocket jet would go, but I don't think it would even protrude beyond the front of the car.

[AbsintheWithoutLeave] Asalt rifle, hehe.
-- marklar, Apr 17 2008



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