Flying at the Speed of Sound: What Would a Human Feel?

[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.

 

[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.

 

[boneh3ad]

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

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.

 

[meldraft]

Of course the air pressure would still crush him to death

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

 

[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.

 

[Drakkith]

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.

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.

 

[Drakkith]

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.

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

 

[berkeman]

Meldraft,

Half the stuff on this website is hypothetical,

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...

 

[boneh3ad]

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.

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.

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...

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.

 

[Drakkith]

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.

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

 

[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.

 

[boneh3ad]

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.

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

 

[Airwalker]

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

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.

 

[boneh3ad]

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.

 

[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.

 

[boneh3ad]

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.

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.

 

[Drakkith]

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.

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

 

[boneh3ad]

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

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.

 

[Drakkith]

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.

 

[boneh3ad]

And what is your basis for that being the terminal velocity of a human being falling in an arbitrary orientation? Did you account for the form and viscous drag and their varying with density? Wave drag as the speed approached the speed of sound?

I ask because even the space shuttle, which passed through that speed, would continue to aerodynamically slow down, and that was both heavier and more streamlined.

 

[bbbeard]

And what is your basis for that being the terminal velocity of a human being falling in an arbitrary orientation? Did you account for the form and viscous drag and their varying with density? Wave drag as the speed approached the speed of sound?

I ask because even the space shuttle, which passed through that speed, would continue to aerodynamically slow down, and that was both heavier and more streamlined.

The shuttle flew most of the in-atmosphere reentry profile at a 40 degree angle-of-attack. This orientation intentionally generates a huge amount of drag, especially in the transonic regime. I don't think your observation has any bearing on the transonic human question.

 

[Drakkith]

And what is your basis for that being the terminal velocity of a human being falling in an arbitrary orientation? Did you account for the form and viscous drag and their varying with density? Wave drag as the speed approached the speed of sound?

I ask because even the space shuttle, which passed through that speed, would continue to aerodynamically slow down, and that was both heavier and more streamlined.

The shuttle isn't falling straight down. It is decelerating from orbit. The air is slowing down it's forward velocity, not its downward. And to answer you question, no, I don't need to, as at 80,000 feet the density is so low that it doesn't matter. As I stated, a person would never reach terminal velocity at that altitude from a free fall, as they have to use up time and distance to accelerate, thus moving into denser parts of the atmosphere.

 

[meldraft]

They could, however, attempt to glide for a portion of the fall. I think that this way they would take greater advantage of the low density region of the atmosphere. Whether this could contribute significantly or not, I cannot really say.

 

[boneh3ad]

The shuttle isn't falling straight down. It is decelerating from orbit. The air is slowing down it's forward velocity, not its downward. And to answer you question, no, I don't need to, as at 80,000 feet the density is so low that it doesn't matter. As I stated, a person would never reach terminal velocity at that altitude from a free fall, as they have to use up time and distance to accelerate, thus moving into denser parts of the atmosphere.

You still haven't answered my question. How did you calculate 10,000 mph? It doesn't matter how high you are, the drag on a complex shape like a person is not simple and will change drastically with velocity, nt only in magnitude but in its physical nature. I merely want to know how you came up with that figure. Even rarefied flows can exert significant drag, a fact you seem to dispute.

Additionally, yes the shuttle comes in at 40 degrees, but the drg still acts both horizontally and vertically. It comes in at such an angle so that it spends more time slowing down. Coming in too steep would mean it would have way too high a velocity to safely land by the time it hits the ground. My point with the shuttle is that there is plenty of drag even in the upper atmosphere when you are going that fast. It isn't a perfect analog and isn't meant to be.

 

[Drakkith]

You still haven't answered my question. How did you calculate 10,000 mph? It doesn't matter how high you are, the drag on a complex shape like a person is not simple and will change drastically with velocity, nt only in magnitude but in its physical nature. I merely want to know how you came up with that figure. Even rarefied flows can exert significant drag, a fact you seem to dispute.

I used a couple of calculators I found on google. And no, I'm not disputing that it can exert a lot of drag, I'm saying that terminal velocity CAN be higher than the speed of sound.

 

[boneh3ad]

I used a couple of calculators I found on google. And no, I'm not disputing that it can exert a lot of drag, I'm saying that terminal velocity CAN be higher than the speed of sound.

And I am saying that 10,000 mph is absolutely absurd. I don't know which calculators you used, but I would really look into how you pieced it all together because that is an incredible velocity. Consider that it would take roughly 8 minutes in free fall to reach that speed, traveling a vertical distance of some 700 miles under the influence of Earth's gravity, and that is just using the value at the surface.

Still, at Mach 14.7 (10,000 mph at 100,000 feet), the ratio of static pressures is 251.938333 across what effectively is a normal shock. That means that even though ambient air is only 1 kPa (roughly 0.01 atm), you will be feeling around 2 atm on your front and likely pretty close to nothing on your back. Now, you would go fastest head first, so let's assume your head is 6 in by 6 in, so 36 square inches. You now have 1080 lbf pushing on your head. You figure the average human being weighs maybe 180, and now you see you have 900 lbf to slow you down. Of course this is sort of a back of the envelope, poor approximation, but it still strongly implies that 10,000 mph isn't even in the right ballpark. Using that approximation, you MAY be able to make Mach 5 (3,400 mph at 100,000 ft), assuming you could fall at that altitude for long enough, which you can't.

I never said it couldn't be higher than the speed of sound, only that I didn't think that it would necessarily be supersonic given all the conditions going into it.

 

[Drakkith]

Of course this is sort of a back of the envelope, poor approximation, but it still strongly implies that 10,000 mph isn't even in the right ballpark. Using that approximation, you MAY be able to make Mach 5 (3,400 mph at 100,000 ft), assuming you could fall at that altitude for long enough, which you can't.

That's in the ballpark for me in my opinion.

I never said it couldn't be higher than the speed of sound, only that I didn't think that it would necessarily be supersonic given all the conditions going into it.

I have no idea if the fall would be supersonic or not.

 

[willem2]

Keep in mind that the density of air is about 0.0001846 kg/m^3 at 80,000 ft.

This number for the density is much too low. Did you use 80000m instead of 80000 feet?

I get 0.04064 kg/m^3 at 24400 m from the american standard atmosphere from 1976. (couldn't afford 210 swiss francs for the ISO standard atmosphere)
If the air is about 220 times as dense as what you think, the terminal speed would be about 15 times smaller than what you think. (because drag force is proportinal to the square of the velocity)

 

[boneh3ad]

This number for the density is much too low. Did you use 80000m instead of 80000 feet?

I get 0.04064 kg/m^3 at 24400 m from the american standard atmosphere from 1976. (couldn't afford 210 swiss francs for the ISO standard atmosphere)
If the air is about 220 times as dense as what you think, the terminal speed would be about 15 times smaller than what you think. (because drag force is proportinal to the square of the velocity)

Overall I don't disagree with your post, but be careful with saying drag is proportional to velocity squared. That is enter ally true at subsonic speeds but as you get faster it gets more complicated.

 

[Drakkith]

This number for the density is much too low. Did you use 80000m instead of 80000 feet?

Yikes! It looks like I did misread the graph. That was 80k meters.