SRB Retro-rockets for cars as an emergency braking system?

[szputnyik]

Let's say you're driving on the highway and suddenly a traffic-stopping accident happens in front of you, or you notice a developing traffic jam too late and you're about to run into stopped or slowly moving cars. You floor the brake pedal, the tires squeal, but it's just not strong enough to stop you in time, you either underestimated your speed, the magnitude of the situation, or the accident just happened too quickly. Suddenly, two SRB rockets ignite on the front bumper of the car, quickly decelerating it and stopping the collision.

Would a system like this work good for cars as an added safety measure?

[iamaphazael]

Using an SRB would be a bad choice. Consider that once it's lit, it will burn all of its fuel until there's none left, generating thrust the whole time. As the speeds and conditions (eg the weight of occupants/cargo in the car at the time) of emergency situations are highly variable, an SRB charge might either not provide enough impulse, or provide too much and cause the car to end up moving in reverse, which could be bad. If you want a rocket-propelled safety device, a throttleable liquid fuel setup is the way to go.

[Kilmeister]

What good is stopping if you incinerate your victims...err other drivers?

[John Nowak]

Drag chutes would seem a much safer option.

[DonLorenzo]

You don't stop. You make it go up. Plop a LES on top of your car and be done with it, I'm sure it'd be doable to make the seats detach from the rest of the vehicle, probably even in time if it activates at the same time an airbag would. Yeah you'd be screwed in a tunnel, but if you're really worried about that, put a sensor or something that checks it

[Jordanpumah]

i've always thought the best way to stop a car dead in it's track would be like some sort of heavy duty metal pole that shoots out of the car at a certain angle so that it shoots into the ground, although going from 80 to 0 in 3 milliseconds might be a bit jarring.

[DonLorenzo]

Yeah, that's basically the same as the crash you're trying to prevent Except no protection from crumple zones (yours and the other guy's!). As always, making a steel cage that survives highway-impacts is relatively easy, it's the squishy humans inside that don't deal with that so well. Cars get all mangled and crushed up not because we can't make them stronger, but because the crumpling and folding is a good way to dissipate energy, much better than instantly stopping dead. Incidentally the bit around the human doesn't crumple nearly as readily, for obvious reasons.

If you are deadset on forward firing SRBs, I would calibrate them to take off say 25m/s and not trigger above speeds of 20m/s. In the case of going faster, the impact isn't averted but significantly reduced. Good enough.

[DonLorenzo]

Any opinions on whether designing a SRB that gives a car a 10g acceleration for 250ms? Would that short of a firing duration present a design problem?

[SuperFastJellyfish]

There are too many failures of this setup that could easily hurt innocent bystanders or yourself for this to ever be implemented. First one off the top of my head would be an SRB explodes and throws shrapnel and burning chemicals in all directions.

[DonLorenzo]

Clearly

Then again, much the same might be said of the automobile itself

[Kilmeister]

I did some quick googling, but couldn't find an answer. What would the shelf life a booster like this be? I recall solid rockets have kind of an expiration date, but I don't know what it is with current technology.

[Ralathon]

Putting explosives and detonators in the bumper to burn during crashes... Well, at the very least it'll justify Michael Bay movies.

It could probably work, but I think the main reason it'll never be implemented is politics. Giving people easy access to explosives will probably make a few branches of the government very nervous.

[K^2]

There are a range of problems.

First, if you go with a retro rocket, you don't go with SRB. You go with a hybrid. Much safer, and you can control burn duration. That said, rockets are just bad for this. A typical car is about 1.5T. At highway speeds you'd want to kill at least 55mph of forward speed. (That would bring you to safe collision speeds even if brakes completely fail. Otherwise, with brakes you might be able to stop fully.) Your hybrid's I_SP is going to be around 300s. That means your rocket is going to have more than 12kg of fuel. This is a) Expensive, Presents huge risk in case you can't avoid collision.

This part can be solved. You don't actually need high I_SP here, so you can bring a bunch of inert reaction mass with you. Say, water. Water can be sprayed from the front in a way that makes it relatively safe at moderate distance. You'd still need a rocket to push the water out, but it'd be a much smaller rocket. Say, you have 50L of water in your retro jets. Now you can get away with less than 1kg of propellant to push it out. This can be contained even if it decides to explode. The whole system would probably add about 100kg to your car's total mass, but if it would make it significantly safer, that would be acceptable.

On to the next problem. Your tires can decelerate you at about 1g. If you are flying at 70mph, that gives you braking time of 3.2 seconds. Basically, by the time you realize that you need to brake and you can't make it, it's really too late. "Lets just fire these automatically!" Alright, say I'm driving on a freeway. There is a slow moving vehicle in front of me, so I'm going to shift lanes. Just as I start, retro rockets fire, spin me out of control, and put me in the middle of the lane at zero speed. That's a great situation to be in on a freeway. Now, you could probably come up with some way to make this work, like use the radar, make sure closing speed is too fast to stop, and the driver is applying full brakes. But even that might create new hazards. So it's a question of whether this can even help you in more cases than it would hurt you. Which brings me to my next point.

You are in traffic. You need to stop. You do, thanks to the retrorockets. But what about the guy behind you? Unless he has a similar system installed, he's still plowing ahead at 70mph, and probably just starting to apply brakes. And suddenly, he has a stationary car in front of him. Now he slams into you at probably higher speed than you would have slammed into somebody, drags you forward, and quite possibly pushes you into something as well as getting rear-ended himself. You've just turned what could have been a two car collision into a pileup.

Is that always going to happen? No. But the odds of such outcome increase with speed, and therefore, it's more likely than not in all of the situations where it would have a chance of saving your life, rather than just your car. And don't forget. The main purpose of all safety features is to save lives of people, not reduce damage to cars. If we worried about cars, we could just make their frames tougher. They'd survive minor collisions much better. They'd kill driver at higher speed collision, but even then a lot of the spare parts would be recoverable. That seems much easier than retro rockets. But again, that's just not the point.

[DonLorenzo]

All excellent points. A world in which cars have rockets would still be good though

One thing I don't quite understand, why do you invoke the tires and their braking capacity? The deceration due to rockets doesn't have to act through the tires, so why is their performance important?

[Kerbin Dallas Multipass]

You are in traffic. You need to stop. You do, thanks to the retrorockets. But what about the guy behind you? Unless he has a similar system installed, he's still plowing ahead at 70mph, and probably just starting to apply brakes. And suddenly, he has a stationary car in front of him. Now he slams into you at probably higher speed than you would have slammed into somebody, drags you forward, and quite possibly pushes you into something as well as getting rear-ended himself. You've just turned what could have been a two car collision into a pileup.

I regard this argument as invalid. You could say that about any form of braking improvement or even improved tyres with better grip. "The guy behind me" is never an excuse for plowing into the car/pedestrian/obstacle in front of you.

Secondly: All automatic brake assist system are designed to trigger in quite exactly the moment when a collision is unavoidable. So, without the system you would come to a sudden standstill as well, the potential pileup problem is exactly the same.

[panzerknoef]

All I can think of with this is:

(In KSP style)

*Is that a traffic jam, Bob?

- I don't know, it's not very clear...

* Oh nooooo it is! Stop car stop!

-Jeb, pull the brakes we're gonna crash!

-pulls brakes-

*It's not enough Bob, we wont make it!

- activate the braking SRB's quickly!

*Yeah!

-activates SRB's car stops...-

-Phew, That was close!

* It sure was Bob, but... Is that smoke I see coming from the car in front of us?

- Well, it looks like it...

* Oh my gosh! That car is on fire! The SRB's must've torched it!

- Darn, run Jeb, Ruuuuunnn!!!

...

The News: Hundreds die, many more wounded due to devastating traffic jam fire. Cause unknown.

So far the cutting edge braking technology.

[DerekL1963]

* Oh my gosh! That car is on fire! The SRB's must've torched it!

Yep, the plume from the SRB's is going to be a significant problem. And the exhaust is generally... neither pleasant or healthy.

[K^2]

I regard this argument as invalid. You could say that about any form of braking improvement or even improved tyres with better grip. "The guy behind me" is never an excuse for plowing into the car/pedestrian/obstacle in front of you.

Secondly: All automatic brake assist system are designed to trigger in quite exactly the moment when a collision is unavoidable. So, without the system you would come to a sudden standstill as well, the potential pileup problem is exactly the same.

The only improvements on the market currently that allow you to brake faster than the car behind you are spikes and chains. Everything else just removes the driver error. As well as both of these are useful only in limited situations, ones where people tend to drive very differently to begin with, and come with limitations and/or requirements to carry a clear sign identifying such a feature on your car in many jurisdictions. And this is for the precise reason that such cars can stop better than somebody behind them.

Furthermore, even if we consider systems like ABS actual assists, despite the fact that a skilled driver can stop just as well or better, they are, generally, incremental improvements. You might say that going from .3g to .9g is a lot, but it's nothing like going from .9g to 5g. Not only is the increment higher, the reaction time is also reduced dramatically. Which means that it is plain impossible for a driver behind you to stop unless they are driving half a football field behind you.

Finally, there is the question of velocity difference. Say I'm driving without ABS with someone who is driving with one. I drive only 2s behind at highway speeds. Say, 70mph. Person in front of me slams the brakes. He decelerates at .9g. I slam my brakes .5s later and decelerate at .3g. Car in front of me has time to stop, and I slam into the back going 43mph. Given that it's an impact into a car of similar mass, my car is trashed, but I'm completely unharmed, save for a bruise from the airbag.

Or, lets say, I'm completely careless, and I ride the guy's bumper just .5s behind. In that scenario, assuming I'm a careless driver, collision happens just 2s after he began braking. But he doesn't have time to slow down all the way, and I still slam into him at only 31mph of relative velocity. No matter how things go from there on, I'm fine. And so are people further behind.

Now, lets suppose that instead of braking at .9g, the car in front suddenly pulls a 5G maneuver. In the .5s scenario above, even braking at .9g, I slam into car in front still going 63mph. Ever seen a collision at 63mph? Or the aftermath. I'd be lucky to survive that, and even that, only if car in front of me is of similar mass. Passengers in that car would also be lucky to live through this collision. (If I'm driving a semi, they're all dead.) At 2s, lead, provided nothing goes wrong, I have time to stop. But given a 2s lead, the front car would also be able to stop safely, and everyone would be fine.

To put it in simplest terms, it is generally going to be safer for the front car to crash then slow that abruptly if other cars on the road don't have similar capability.

Finally, it is almost impossible to say what actually is an inevitable collision. If a car 1s in front of me slams into a concrete block and stops instantly, I can still swerve around it. In many cases, it's going to be way safer than stopping. At .5s I can still swerve around if I already began the maneuver. To come to a stop from 70mph in less than .5s, the car would have to undergo acceleration of over 6g. That's assuming that there is zero lag in firing the engines, which is not going to be true. So realistically, we'd be looking at what, 7-8g? That gives a stopping distance of about 12m. That's about the length of the impact absorbing barriers on the freeway. Ever seen a car ram into these? It's survivable. That's kind of the point. But the car's trashed. And that's with impact distributed over the entire front of the car. Now you want that load to be focused on just the rockets. So just what sort of a frame are you planning to attach these to?

So we have to consider lower deceleration rates. Something like 4-5g should be doable. But this requires predicting inevitable collision more than a second ahead. How does the system know if I'm planning to swerve? If it's safe to swerve? If swerving or braking is more likely to get me killed? Am I even alert enough to react this fast? Guess wrong, and the system kills the driver who would have otherwise escaped the collision. Now, being useless is one thing. When the system designed to protect actually causes deaths, and fairly frequently, that's a very bad system.

The only way to use retro-rockets as an actual safety feature is that all cars are operated by AI linked into a swarm that can coordinate traffic. Then, in the worst case scenario, such a system would be in position to decide if it's prudent to fire retro-rockets. But I suspect, with such a system in place, need for this would be very rare.

[UnthinkableThinker]

Sure, you can strap on SRBs to stop a car, but what about the people inside the car itself? Inertia. Airbags, bumpers, all are made to increase the time of the contact, that's what keeps the passengers alive (or at least tries to) inside the car during a collision. I believe the best way to avoid crashes, especially high speed crashes such as in highways, are what I call "Intelligent highways", and they must work along cars, to detect a recent collision and once the car is located in that radius (let's say a 1km radius), the system fires and reduces the speed of all of the cars in that area, very similar to the Yellow Flag we see on races, except this will all be done through computer systems and automatically, so basically the car will slow itself down, whether the driver wants it or not.

[Drunken Hobo]

Imagine Alfa Romeo made one...

[Justy]

I read a novel in the early 1990's about a supertanker collision, oil spill, and firestorm off the coast of British Columbia. It was, shall we say, not the greatest piece of literature ever penned by man. But as a last-ditch measure to try to avoid the collision, the novel's brand new top of the line bigger than ever supertanker did feature emergency braking rockets to try to bring its stopping distance down to something reasonable. (of course, it wasn't enough. It'd be a short novel if it was.)

Imagine Alfa Romeo made one...

Safest rocket ever... the ignition will never work!

[Kerbin Dallas Multipass]

Let's make one thing clear: A computer would have to detect imminent impact and this detection mechanism would have to be foolproof and 99.9% reliable.

Sentences quoted in quotes are quoted from K^2

"Which means that it is plain impossible for a driver behind you to stop unless they are driving half a football field behind you."

Half a (american) football field is 50 yards, is that correct? That's 45m

"Finally, there is the question of velocity difference. Say I'm driving without ABS with someone who is driving with one. I drive only 2s behind at highway speeds. Say, 70mph."

2s at 70mph is 62 meters. Assuming 1 shock second until your foot is on the brake pedal you have less than half a football field to come to a stop or you crash. Fact of life, sorry.

I do not see the problem keeping a 45m distance to the car in front of me. Half a football field is not that much in a car.

"I ride the guy's bumper just .5s behind."

Are we still at 70mph? Thats a distance of 15m. One would describe it as one car length. At this speed this is roadrage.

"Finally, it is almost impossible to say what actually is an inevitable collision. If a car 1s in front of me slams into a concrete block and stops instantly, I can still swerve around it."

The point where you can still swerve around things can be defined by physics. We are debating a hypothetical rocket decelerator which, unlike automatic wheel braking, does not rely on wheel to ground friction but has a predictable deceleration force in any condition.

Edited February 15, 2014 by Kerbin Dallas Multipass
-since

[FanaticalFighter]

That idea can only come from KSP players xD.

No, it'll be a bad way to stop a car. Not just SRBs, but any type of rocket. For one, rocket exhaust velocities have to be enormous, and I mean REALLY enormous. You'd kill the guys in front of you a lot faster than an accident would.

[Kerbin Dallas Multipass]

That idea can only come from KSP players xD.

Walk up to a stranger and ask that person what he/she thinks about an explosive charge stored in their steering wheel which will explode directly in their face in case of an accident. It's safe because there is a 0.08 mm membrane separating you from the explosion.

And.. yes, if it blows up spontaneously it can kill you or will break your arms. It will definitely break your thumbs.

Most of us do have cars with airbags.

[FanaticalFighter]

(Actually, in India, Airbags are still not that common)