Mach Jetpack Humans

1. Jan 26, 2012

Albertgauss

If a human were to wear a jetpack and could fly at the speed of sound, what would he feel? Would he feel the sound waves piling up on his head? What would he hear? Inside supersonic aircraft he would hear nothing, and anyone outside would hear the pressure wave, but, what if his head is the point where the sound waves pile up? What if he had a really strong helmet so that he would not be killed?

What would his body feel? Would he feel the sound waves build up on his body? Assume he has a suit that he can't be killed.

2. Jan 27, 2012

meldraft

If we count out the intense heat from air friction, we would feel nothing unless he was accelerating. Simply Newton's second law :)

Edit: Of course the air pressure would still crush him to death, but I assume we also disregard that. Btw, inside a supersonic aircraft sound travels as it does everywhere else, so people can talk like they can on the ground.

3. Jan 27, 2012

This isn't true. If you are flying through the air, the only way to be moving at a constant speed is to exert a force on the fluid to move it out of the way. Short of not moving, there will always be a force that you feel because drag doesn't go away. This isn't flying through a vacuum.

4. Jan 27, 2012

meldraft

Unless we suppose, however, that the person can handle these forces, it doesn't really make sense to talk about the hypothetical scenario

5. Jan 27, 2012

Albertgauss

Meldraft,

Half the stuff on this website is hypothetical, and its where the hypothetical ends that the creativity for the real world begins.

Yes, let's suppose he could handle these forces. I did get to thinking about a human torpedo. They would certainly feel the water piling up in front of them, so at mach speed in air, by analogy with water, he would at least feel the air piling in front of him. I think I agree with boneh3ad--the human jetpacker would be pushing air out of the way, so he would certaihnly feel that. He would also be engulfed in a fireball, due to all the air friction. That's as much progress as I've made for now.

6. Jan 27, 2012

Drakkith

Staff Emeritus
Air would still be rushing by you, so you would hear the sound of...air rushing by you. What would you feel? Stick your hand out the window when your going down the road and multiply that by about 100.

7. Jan 27, 2012

Drakkith

Staff Emeritus
This is not true. You would not generate nearly enough friction to do that.

8. Jan 27, 2012

Staff: Mentor

Not true. Please re-read the Rules link at the top of the page. Posts must be based in science here at the PF.

Your question is allowed because it is an experiment that could be carried out. It is physical to ask about the effects. But as you can imagine, the person would need enough protection not to be killed, which means that they would be wearing a protective suit, which means that they would not be "feeling" much of anything...

9. Jan 27, 2012

Even the SR-71 only had temperatures in the region of 700 K near stagnation points, which is pretty ridiculously high, but not high enough to engulf anything in a fireball. For that to happen, you either need to have an ablative heat shield that is designed to catch on fire or you need to be moving so quickly (Mach 20-ish) that the molecules in the air dissociate and ionize, creating a plasma. Since I don't think the rocket man will be re-entering the atmosphere, I don't think this will be a problem since we can't currently accelerate anything to that speed in the atmosphere.

Come on, nothing wrong with a little bit of an academic exercise. It isn't like he was asking about a perpetual motion machine. No reason to take it too literally.

10. Jan 27, 2012

Drakkith

Staff Emeritus
He was responding to the statement that half the stuff on PF is hypothetical, which is untrue.

11. Jan 31, 2012

Airwalker

The feat talked about here was accomplished many years ago in the :Project Man-High" altitude tests. Although no Jetpack was used, the speed was achieved by a very brave, courageous or as some say, "crazy" man named Joseph Kittinger. Back in the early days of the space program testing he did a 13min. free-fall from an altitude of 103,000 ft. to test the possibility as well as the affects on a human of a return from outer-space if a capsule were to malfunction while orbiting. On an earlier test flight his glove seal malfunctioned and he almost lost his hand due to his internal body temperature and pressure and the low atmospheric pressure outside his gloved hand. His craft was a helium filled balloon. His record free-fall flight still stands to this day. Google his name for the complete story.

12. Jan 31, 2012

While a very cool experiment, Kittinger never broke the sound barrier so that doesn't really apply here.

13. Jan 31, 2012

Airwalker

Aerospaceweb.org | Ask Us - Fastest Skydiver Joseph Kittinger
www.aerospaceweb.org/question/aerodynamics/q0243.shtml
Sep 18, 2005 – Joe Kittinger was born in 1928 and became interested in aeronautics .... that Joseph Kittinger broke the sound barrier during his 1960 skydive.

I beg to differ with your statement. He did break the Speed of sound at altitude, not at sea level like the English THRUST II land-speed record holding car. Joe is now working with a Frenchman who is training to break Joe's altitude/speed record.

14. Jan 31, 2012

You do realize that the site you just linked to says he did not break the sound barrier, right? It does this several times actually.

He claims he went 614 mph, but even at 100,000 feet altitude, the speed of sound is roughly 675 mph. In other words, he didn't break the sound barrier.

15. Feb 1, 2012

Airwalker

Yes, I assumed you read the recent reports like that one and some others, but many years ago the calculated speed during Joe's nearly 6 mile free-fall was approx.690mph. He then deployed a small drogue chute to slow down and stabilize himself. Perhaps someones measurements we off back then but as years have gone by his speed has gotten slower and slower. I think a numbers game is now being played because of the upcoming record attempt by Red Bull guy. I have no doubt that when this new guy points himself head first into a dive he will break all the speed records. I wish him the best.

16. Feb 1, 2012

I have some doubt as to whether it is even possible to break the sound barrier simply in a free fall like that. All the quotes I have seen about Kittinger have said 614 mph, including his own account, so personally, I tend to believe he didn't, and therefore no one has.

The problem is once you get up against the sound barrier, you end up having to contend with wave drag, and that is a huge amount of aerodynamic drag that you don't have to contend with at subsonic speeds. Complicating the issue is the fact that a person is a blunt body and therefore would experience even greater wave drag due to the bow shock that would form.

17. Feb 1, 2012

Drakkith

Staff Emeritus
True, but free falling from a very high altitude where the density of air is extremly low might allow this.

18. Feb 1, 2012

I thought about that for a bit. Consider the following. A person jumps from a balloon in the stratosphere and enters free fall. Gravity is slightly weaker so they accelerate ever so slightly slower. However, the rarefied gas exerts very little drag, so they accelerate a bit faster. This is probably pretty close to a wash. Of course, at higher altitudes, the speed of sound is slower, so it would be easier to reach the speed of sound. However, it took Kittinger something like 13,000 feet of free fall to reach his speed, and the farther you fall, the drag increases quite a bit faster than the increase in gravitational acceleration (meaning you are more likely to reach terminal velocity or even decelerate) and the faster sound moves (meaning that Mach 1 target keeps moving farther out of reach). That means that the farther the fall, the less you are accelerating, the harder air is pushing back - especially when you start entering the transonic regime - and the faster the speed of sound gets.

It just raises some doubts in my own mind. Given Kittingers experiment and the fact that I don't know of any sort of CFD that has been done to actually determine the free fall speed of a skydiver from that high all the way down, I am inclined to believe it is likely not possible in Earth's atmosphere until someone shows that it is.

19. Feb 1, 2012

Drakkith

Staff Emeritus
Keep in mind that the density of air is about 0.0001846 kg/m^3 at 80,000 ft. Terminal velocity at this height is MUCH higher than it is near sea level. Rough calculations using an online calculator gave me about 10,000 mph for a person at this altitude, but obviously you could not free fall to this speed as you would be moving into higher density air well before you ever hit it. However I think the point stands, that terminal velocity can be higher than the speed of sound.

20. Feb 1, 2012