# Rocket to slow freefall

Can survive 46g force so what type of rocket and how much fuel would be best to stop a human falling at terminal velocity without exceeding this g force limit.

I also read spacex use rockets not parashutes for the weight advantage so would a short (i think 1 second) burst rocket be better in anyway to a parashute (despite such deceleration causing injury(yet non permanent)).

Or would the rocket and fuel be far too big.

• SpaceX uses rockets to land their first stage because of the accuracy obtainable in landing and because they already have a rocket on board. – Eric S Mar 11 '17 at 22:25
• Where the heck did you get that 440g figure? The article talks of 46.2g. And while it's survivable, this doesn't mean there's no injury as result. – SF. Apr 11 '17 at 11:27
• Hahaha must have been a typo, I'll change it @SF – SRawes Apr 11 '17 at 21:56
• That figure for g-force isn't sustainable either. The euthanasia coaster, for example, sustains 10g's for 60 seconds, which is enough to kill you. – JMac May 12 '17 at 11:39

There are human portable rocket packs which do work, albeit to a fairly limited extent.

The minimum terminal velocity in a stable 'belly to earth' position is about 50 metres per second so even at a fairly modest 10 g that is only about 5 seconds to slow down to a stop. So it's certainly not impossible.

However parachutes work pretty well at what they do. They are very simple and reliable and light and compact enough that they don't encumber the wearer much and you can carry a second one as a reserve which further improves their reliability. They don't require much maintenance beyond proper storage and for a safety-critical item they aren't really that expensive.

Equally importantly they are pretty easy to use. If you jumped out of a plane wearing one you would have a pretty good chance of surviving, even with minimal instruction. Obviously a moderate amount of training is desirable for dealing with emergencies and reducing the risk of injury on landing but even so there are a significant number of competent amateur parachutists.

Also once a parachute has deployed successfully there is little left to go mechanically wrong

On the other hand rocket packs require significant skill to fly at all they usually launch from a standing position and the pilot stays upright throughout the flight whereas free-fall parachutes are usually deployed from a belly down spreadeagled position which is inherently stable and works nicely with a backpack type parachute harness.

With a limited fuel supply you need to be able to time the deployment of thrust pretty accurately so you have a fairly narrow window to get it right, as opposed to a parachute where any altitude above the minimum safe deployment is basically fine (obviously jumping from very high altitudes has a whole separate set of problems).

Equally a rocket pack is mechanically complex, heavy and bulky and most rockets use fuel which is hazardous and requires special handling.

So really you are talking about replacing a simple, inexpensive and proven system with something which has never really got beyond the prototype stage, doesn't offer any obvious advantages and has several inherent drawbacks.

• 0.5 seconds instead of 5 seconds, right? 10 g = 10 x 9.81 m/s2. – Gürkan Çetin Apr 19 '17 at 18:58

Your question does not provide a definition of better, so I'll try to answer as generally as possible.

Parachutes have a considerable advantage over a rocket in that they are passive once deployed and self-stabilize, whereas a rocket needs a fairly sophisticated control system to point it the right way (and remain so) during the burn. Additionally, pilots can only endure high accelerations like those involved with rocket engines thanks to flight suits, training and careful orientation to minimize the effects, which may not be feasible in an emergency scenario.

If the only criteria under which a rocket engine could outperform a parachute as a means of slowing a human from free-fall to a safe speed is the vertical space required for the deceleration to occur.

As for your SpaceX example, they already have a rocket engine, fuel tank and control system, thus if they can make those elements continue to perform after the launch to decelerate the rocket, it makes sense to drop the parachute system, especially since they want precise landings to reduce recovery costs (no fishing the boosters out of the ocean like with the shuttle missions). Hard to say if the added fuel weight and complexity would be worthwhile in any other scenario.

For your fuel question, you'd need to apply Tsiolkovsky's equation for a desired $\Delta V$ of 53m/s (if you mean atmospheric freefall), but more details would be needed to give an actual number, like what type of rocket engine you envision (to estimate an Isp) and how much can we assume your human+engine system to weight. Also, I'm pretty sure you've got a typo