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# Rocket Assisted Emergency Brake

Momentum in a crash has to go somewhere. Cancel it before the crash.
 (+7, -1) [vote for, against]

Certain flying vehicles use rocket assisted braking. It works. Other vehicles should benefit the same way.

- Jet assisted emergency braking for trains

- Jet assisted emergency braking for cars

- Jet assisted emergency braking for ships

- Jet assisted emergency braking for gliders

- Jet assisted emergency braking for commercial airliners.

- Jet assisted emergency braking for helicopters

- Anything that moves and carries passengers could potentially benefit

The simplest example is a train. When the conductor presses a big red "emergency stop button" a massive rocket mounted to the roof of the train fires in the opposite direction of the train's motion. It will safely stop the train before slamming into a line of gravel filled wagons that accidentally detached from a previous freight train.

The toughest example (but most impressive) will be something like a helicopter. Imagine that the helicopter's blade breaks off as a result of manufacturing defect. The helicopter loses all control and comes tumbling down as hard as a sack of bricks. The helicopter is equipped with 6 rockets (up,down,left,right,forward,back). The onboard computer looks at velocity, distance to ground, and orientation to determine which combination of these rockets should be fired seconds before impact. Instead of impacting the ground at 300 KM/h it will impact at a more reasonable 30 KM/h. With more than 6 rockets (or steerable rocket), there is a potential for gentle 1 KM/h landing. (Think moon landing)

 — ixnaum, Apr 15 2014

How to stop a train by firing AK-47 rounds at it. http://what-if.xkcd.com/18/
Not quite the same as the problem expressed in this idea, but similar. [hippo, Apr 16 2014]

C130 JATO braking http://youtu.be/fSFjhWw4DNo?t=1m25s
[DIYMatt, Apr 16 2014]

similar version....except where it is different. Vertical_20thrust_20braking
[Ling, Apr 17 2014]

Here's what happens when you try this... http://www.military...sters/912500478001/
The fun begins at 2:00 and gets really good at 2:20 [normzone, Apr 17 2014]

 How big would that train rocket have to be?

 Passenger train with 10 cars will weight roughtly 800,000 kg. Let's say it's only got 50 km/h momentum left before coming to a stop. That's 12 m/s. 800,000 kg * 12 m/s = 9,600,000 Ns. That is a class X motor. Same as they use on the space shuttle. Ok so that won't be very practical, but it would sure be cool to see in action.

Car might work bit better. Mass is 2000 kg. Let's say it's speed is 70 km/h. That is roughly 38,000 Ns. Which is a Class O rocket motor. These weight only ~150 kg and can be had by amateurs without FAA license.
 — ixnaum, Apr 15 2014

 There's also the problem that in order to not make the stop worse than the crash, the rocket has to be mounted onto the last car in the train, which will result in the rocket wash hitting the entire train.

I seem to remember covering it here, but you're actually better off mounting the rockets vertically (driving down), increasing the normal force on the wheels and thus the stopping power of the existing brakes.
 — MechE, Apr 15 2014

Yes, a space shuttle sized rocket slamming into another object would make a pretty big boom. Why not mount more emergency rockets on it and eject it into a nearby field if it doesn't do it's job in time? Sure, it's a Rube Goldberg approach but with enough engineering we'll get the kinks out (eventually)
 — ixnaum, Apr 15 2014

//rockets should be fired seconds before impact//
No no no. You want the rockets to be firing as soon as an imminent impact is detected - longer deceleration means smaller forces. If you use a short, powerful, "last second" rocket blast, it's just the same as the impact, only earlier, and it roasts the "target" too.

 Ah, the theory of unintended consequences. I personally think it would be pretty funny to obliterate the object you were about to crash into with your superheated plume of retro-rocket exhaust gasses. You would likely ablate away most of the offending object during the firing of your retro-rocket.

 A couple hundred tonnes of APCP exhaust at 5000 or so metres a second is about as close to a death ray as I could imagine.

Next up - advanced ceramic armour and carbon-carbon composites for cars and trucks to protect against "safety" devices. I wonder what's worse? Dying from being run into by a train, or dying a fiery death as you and your car are picked up and hurled maybe a mile or so into the distance in a spectacular fiery plume?
 — Custardguts, Apr 15 2014

 I read this as rocket assisted bra, which has yet to be done on halfbakery...probably

I'm thinking along the lines of custardguts above, wouldn't it be easier to use the rocket to destroy whatever it is you're about to crash into?
 — not_morrison_rm, Apr 16 2014

 It would be fun too. I'm sure [8th] will be along in a minute to bun this for irresponsible and gatuitous use of dangerous explosives.

 Imagine being the driver of a large truck, fitted with the latest whizz bang retrocket kit, in heavy traffic. Some toolbag in a new bright green VW beetle pulls out in front of you, waving their middle finger and laughing, causing you to have to jump on the brakes. Do you a) apply appropriate pressure with your right foot to the brake pedal to prevent tragedy, or b) press the big red button and wipe them off the face of the earth?

Sooner or later, people would start choosing option b. Maybe, (or maybe not,) I want to live in that world.
 — Custardguts, Apr 16 2014

This kind of reminds me of the launch escape systems used on some spacecraft, except that those worked and this wouldn't.
 — Spacecoyote, Apr 16 2014

I think there should be a pedestrian version as well. You know when you are hurrying through a crowded city street and sometimes someone steps in front of you and you dont have time to stop or dodge aside.
 — pocmloc, Apr 16 2014

Well carrying around a double barrel 4-bore rifle would have a similar effect, both halting your forward movement, and similar downrange effects too I imagine.
 — Custardguts, Apr 16 2014

A couple of jatos aiming forward would have the added bonus of blowing away whatever it was that caused you to slam on the breaks if it was light enough.
 — doctorremulac3, Apr 16 2014

 // helicopter //

 Anyone mad enough to climb into one of those Heath-Robinson deathtraps deserves everything they get.

The idea of using a mass of rockets to brake a falling object was pioneered in the 1940's by D.M.W.D.'s "Hajile" device, which was only partly successful and was never deployed operationally.
 — 8th of 7, Apr 16 2014

 //The idea of using a mass of rockets to brake a falling object was pioneered in the 1940's by D.M.W.D.'s "Hajile" device, which was only partly successful and was never deployed operationally.//

Also by the lunar lander and Soyuz reentry module.
 — DIYMatt, Apr 16 2014

Braking and landing aren't quite the same thing.
 — Spacecoyote, Apr 16 2014

Breaking and landing sometimes are.
 — Custardguts, Apr 17 2014

 — Ling, Apr 17 2014

I seem to recall a recent successful Mars NASA event in which braking rockets were used to slow down entry.
 — RayfordSteele, Apr 17 2014

What [Ling] said, only again...
 — normzone, Apr 17 2014

Just to extend the idea, put a pair of small rockets on every engine and every car. The rocket would be sized based on the fully loaded mass of the car. An emergency signal would ignite all rockets by RF signal. Bring the train to a 1G slow and stop in about 1/10 the current stopping distance. Since trains usually have lots of time to see an object in their way before needing to stop. 1/10 of the stopping distance would save a lot of lives.
 — LloydofDSS, Feb 25 2015

 If you aimed the train's rockets down where the wheels meet the rails, you could melt away the low-friction shiny bits and really get some braking traction.

 For automobiles, I would build a retractable arm that extended out behind the car, with the rocket pointed up and forward. When it fired, the flame would go up over the car, roasting fewer pedestrians and melting enough of the roof to discourage gratuitous use. Under the end of the arm would be a friction pad that the rocket would force onto the road surface (or, in the deluxe model, a plow blade).

 (Or you could just be boring and mount the rocket in the car's trunk, pointing up and forward, so the blast would both slow the car and increase the downforce for better braking. (Or, in the deluxe model, smash the back end of the car into the pavement so hard that it digs a hole.))

(By the way, a free-flight rocket cannot be stabilized by locating the motor to pull, instead of push, so my idea is not going to help steering.)
 — baconbrain, Feb 26 2015

Weld the wheels to the rails with high current...
 — Ling, Feb 26 2015

If a train engine weighs 100 tons and is going 56 mph, a rocket with the thrust of a Titan I (40 tons) would slow it to a stop in 6.4 seconds. In the recent train wreck in Oxnard California the engineer applied the brakes 8 seconds before impact. The Titan weighs 50 tons, but most of that weight is to produce a 140 second burn. This would only need a 6 second burn, so it might only weigh around 4 tons. Each burn of a Titan costs 1.3 million. So each burn of this rocket would cost about 55 thousand dollars. Every train car would need an appropriate sized pair of rockets of its own.
 — LloydofDSS, Feb 26 2015

Every car needs one, and a train might be a hundred cars in length. That's \$ 5,500,000 per stop. Spendy. Consider also that you are lighting a mile long oxyacetylene jet around often flammable cargo that might well be passing through a highly populated area. Also, to equip the entire fleet, including the cars that are shuttling back and forth lightly loaded or empty will require an initial investment of trillions of dollars, framed against the current situation of a rail fleet that includes thousands of cars grandfathered into use that cannot be brought to safety code but will not be retired until they wear out due to toothless legislation.
 — WcW, Feb 27 2015

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