I wish to investigate an active vehicle braking method for use in extreme situations. Very high accelerations are acceptable in situations where this would prevent the much worse accelerations present in a collision.

I was thinking of a system that will use ejected gas/liquid to reposition the position of a car and/or assist in braking. We know that ejected mass (gas or other) create a propulsive force. I was thinking to utilize this mechanism to create a force that will decelerate the car. Or maybe change its position/trajectory (eg. when it moves out of the road in a steep turn).

Would such system be viable?
What parameters would such system have?
eg to decelerate a car travelling at 30m/s within 3 meters (about 15g deceleration!).
How much mass should we eject and at which speed?
Have such systems being proposed/researched/built?

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    $\begingroup$ What do you mean by reposition? The title and first sentence seem to be disconnected from the rest of the question. $\endgroup$ Jul 28, 2015 at 11:45
  • $\begingroup$ Could you please add what research you have already done? Purely hypothetical questions are hard to answer because there are a lot of variables that effect the answer. $\endgroup$
    – hazzey
    Jul 28, 2015 at 11:54
  • $\begingroup$ We know that ejected mass (gas or other) create a propulsive force. I was thinking to utilize this mechanism to create a force that will decelerate the car. Or maybe change its position/trajectory (eg. when it moves out of the road in a steep turn). I haven't done any research, I am just exploring it. Any input would be helpful. $\endgroup$
    – sloupioc
    Jul 28, 2015 at 12:18
  • $\begingroup$ There is a relevant discussion here on Meta Engineering about questions that ask us to broadly evaluate "novel ideas." The consensus opinion so far is that we want people to do their homework first and bring us focused, well-researched problems. $\endgroup$
    – Air
    Jul 28, 2015 at 17:45
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    $\begingroup$ I'm not an automotive engineer but it seems impractical for use in passenger cars. $\endgroup$
    – Air
    Jul 28, 2015 at 20:48

4 Answers 4


I gave this idea some thought some years ago -= catalysed by the events depicted in the video linked at the end.
What you are describing is essentially a rocket. The concept is obviously possible as it's been done long ago. Whether it is practical in any real world situations is the main point to be considered.

It is hereby proclaimed that for the balance of this answer.

  • g = 10 m/s/s
    (Multiply this by 0.98 if you wish)

    1 kgf is the "weight" of 1 kgm of material in a 1g gravitational field.

    So 1 kgm "weighs" 1 kgf = 10 Newton

    Units of kgf.s/kgm = s (seconds) !!! :-) (Only valid for "rocket scientists)

For a rocket to be useful and practical in your scenarios requires it to be "safe", reliably able to be operated instantaneously at any time, suitably powerful and suitably compact and low mass.

There are many potential solutions but one which seems most likely to be a practical contender is a HP (Hydrogen Peroxide) catalytic decomposition rocket. In such a system pressurised HP is expelled via a catalytic decomposition promoter (often a silver wire screen) and hence into a classic convergent divergent rocket motor.

A catalytic HP system has the advantages of being RELATIVELY low temperature, nothing but water and Oyxygen are expelled, it can be stored ready for use without need for cyrogenic cooling and is relatively non corrosive =(although cleanliness is utterly essential). The rocket is able to be "throttled" over most of its range and the thrust can be easily controlled in level and started and stopped at will.

Monopropellant Hydrogen Peroxide is not an especially high energy per volume or mass propellant. The standard "figure of merit" is Isp = specific impile which can be thought of as the amount of time in seconds that a unit mass of propellant can exert a force equal to its own weight in seconds. This can be expressed as eg lbf.s/lbm (pounds force x seconds per pound mass) and a bit of sloppy laziness allows people to cancel the lbf/lbm and call the resulting units "seconds". In fact it's really (lbm. x m/s/s x s / (lbm) = m/s = velocity!

Anon - an Isp of 100 is not too hard and 200 is getting rather hard. Lets goo with 120 to start. 1 kg of HP will provide 120 kg force = say 1200N for 1 second. A little plugging in to formulae shows that you are going to need a significant mass of HP compared with vehicle mass to get very large accelerations. eg your suggested 30 m/s in 3m would (as Olin says) be bone crushing - but in fact is entirely achievable using ABOUT the same amount of propellant as doing it at a more sensible rate - see below. BUT lets look at what it takes to apply 1g of acceleration (10 m/s/s) for 1s to a say 1000 kg car.
1g acceleration to a 1 tone car takes 1000 kg x g = 10,000 N. Working in kgf, with an Isp of 120 to get 1000 kgf thrust for one second requires 1000/120 = 8.5 kg of HP.
HP density is usefully greater than 1 but call this 8 litres.
Taking the original 30 m/s vehicle - to stop it at 1g requires 30 m/s / 10 m/s/s = 3 seconds of thrust so about 25 kg and 25 litres of HP.
That's about 25/1000 = 2.5% of the vehicle mass - which is not insignificant BUT also 'useful' for something which can either accelerate the vehicle over about 0 - 100 kph in about 3 seconds OR bring it to a standstill in the same distance - even on mud, snow, ice or while in the air.

Above I said that stopping the vehicle in 3 metres was impractical. That's essentially true, but it's not impossible. And a little consideration shows that you notionally need no more propellant to do do As long as you can get the same Isp you just need to consume the same amount of propellant at a higher rate. That requires a higher maximum capacity rocket motor, greater flow rates so bigger "pluming" bigger forces etc- but it's doable.
To stop from 30 m/s (= 108 kph) in 3 metres requires
a = V^2/(2 d) = 900/6 = 150 m/s/s = 15g (as Olin said)
Time to stop is t = V/A = 30/150 = 0.2 seconds.
Doing a 0-100kph ~= 0.60 mph in 0.2 s makes by long long log ago neighbours best toys look like toys indeed - and would make any occupant very sore indeed. BUT not necessarily dead - accelerations of that order over such short periods are survivable given proper containment - and extending it out to say 7.5 metres and 0.5 seconds and 6g has you well inside fighter aircraft and below peak moon-rocket levels.

More useful than applying gross front/back velocity changes might be applying forces to allow steering authority in skids - or for a race car which is sliding across a ploughed surface towards a tyre wall - or a concrete wall. Quite modest amounts of propellant could achieve quite useful results. Under computer control such a system has the ability to extricate a vehicle for otherwise irrecoverable situations. Also, without due care, to insert them into them.

Properly done such a system could even allow a vehicle to "fly" for short periods - like this

(Armadillo Aerospace hydrogen peroxide mono-prop 1st manned flight)

enter image description here

  • $\begingroup$ you are saying that such systems have been built before? I can't find anything online. I was thinking to propose it to a competition as an active braking method for extreme situations, so isn't it unique? $\endgroup$
    – sloupioc
    Jul 28, 2015 at 13:58
  • $\begingroup$ @sloupioc I am NOT aware of it having been used as a braking system. But rocket cars have existed since 1930's or before. Rockets were the sole propulsion. Fatally dangerous vehicles in a number of early cases. . Also Ky and Kitty - 1970s - standing 1/4 in 3.22s [VVVBG} and 422 mph on peroxide.That's 12.7g average across the 1/4 mile by -= and the Great Admiral Truax designed Evel Nievel's Snake River Canyon peroxide powered rocket car. $\endgroup$ Jul 28, 2015 at 14:17
  • $\begingroup$ Peroxidedragsters $\endgroup$ Jul 28, 2015 at 14:17
  • $\begingroup$ Original restored 1970's record breakinnnnnnnnnnng dragster here $\endgroup$ Jul 28, 2015 at 14:18

First, what you are asking about is basically what a rocket engine does. The physics is a momentum balance. The total momentum the car looses is equal and opposite of the momentum of whatever you eject out the front. You can answer the last question yourself using just high school physics.

Second, decelerating a car from 30 m/s to 0 in 3 meters is absurd. At best that would happen within 200 ms, which means 150 m/s², which is over 15 g. Neither you nor the car are going to survive that.

  • $\begingroup$ Olin, are you saying that we are able to stop cars at the minimum possible distance and anything shorter than that is absurd? If not, what is the minimum possible distance we can decelerate a 30m/s moving car? $\endgroup$
    – sloupioc
    Jul 28, 2015 at 13:04
  • $\begingroup$ @sloup: The minimum stopping distance for 30 m/s is a function of how many g you can tolerate, both you and the car. I don't know what exactly the threshold is, but 15 g sounds too high. $\endgroup$ Jul 28, 2015 at 15:12

Yes, this has been done for other vehicles. The Soyuz spacecraft uses parachutes to slow, but then fires solid rocket motors just before landing to slow down. According to this site the engines fire 1 sec before touchdown at 24 ft/s. And This site says that landing is at 3 m/s (9.8 ft/s). If these are both correct, the Soyuz decelerates with a delta-v of 14 ft/s in 1 sec just before touchdown.


15 g might seem like a lot, and it is, if the 15g is sustained as it would be in something like an aircraft's aerobatic maneuver. 15g isn't unreasonable as a limit to what humans can take in a short duration event like a crash, Indy car drivers have sustained horizontal impact forces many times that without significant injury. However, consider how those drivers are restrained to avoid injury, 5 pt harness, helmet, arm restraints, head restraints and so on. The level of G-force that humans can tolerate depends on the individual, their training and also the direction in which the G-force is applied.

Here's an interesting read on human tolerance of impact forces



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