Could a completely frictionless projectile "fireball" if it moved fast enough?

[FungusForge]

Lets say somehow I managed to make a completely frictionless projectile, and fired at some speed north of Mach 100, could it still "fireball" like our brave kerbonauts returning from the Mun?

[SargeRho]

Ram heating would still occur. Reentry heat isn't from friction, but primarily due to ram heating.

[The Yellow Dart]

Correct, the heating is due to extremely high pressure that the air reaches before it can get out of the spacecrafts way. If you made a very slender spike, or something like a pointed spear, that pointed perfectly prograde as it descended, it would sort of achieve what you are looking for. You would lessen the heating by making it easier for the air to move out of the way and could dip further into the atmosphere before experiencing heating, but by that same token, that object would not slow down nearly as much as an object of the same mass but is, say, spherical. Since you almost invariably are looking to slow down when you slam something into an atmosphere at orbital speeds, this doesn't usually end up being useful.

[FungusForge]

Alrighty.

I figured ram heating would still make it fireball. I just asked for "I told you so" rights. Thanks for the responses.

[RainDreamer]

At mach 100, you are at 0.0001c . I imagine you would trigger some sort of fusion reaction from atoms of your object hitting the atoms of air molecules at that speed, and a fire ball would occur regardless.

[cantab]

On 08/04/2016 at 7:03 AM, The Yellow Dart said:

Since you almost invariably are looking to slow down when you slam something into an atmosphere at orbital speeds, this doesn't usually end up being useful.

Unless you don't need to stop. The real-world example would be ICBM re-entry vehicles. Hypothetically, the "rods of god" concept would have even more need to maintain its speed.

Edited April 9, 2016 by cantab

[Steel]

Why do these threads exist? There is no point discussing how something outside the laws of physics interacts with the laws of physics, because it doesn't make any sense. If you have a "completely frictionless projectile" how is it interacting with the air molecules to cause ram heating?

[sevenperforce]

1 hour ago, Steel said:

Why do these threads exist? There is no point discussing how something outside the laws of physics interacts with the laws of physics, because it doesn't make any sense. If you have a "completely frictionless projectile" how is it interacting with the air molecules to cause ram heating?

Frictionless doesn't imply a lack of force interaction; it just specifies that all force interaction between the projectile and the surrounding medium is via conservative forces. Nothing physics-breaking about that, technically. You would still have compression and internal heating in the medium per the ideal gas law. 

[Steel]

4 minutes ago, sevenperforce said:

Frictionless doesn't imply a lack of force interaction; it just specifies that all force interaction between the projectile and the surrounding medium is via conservative forces. Nothing physics-breaking about that, technically. You would still have compression and internal heating in the medium per the ideal gas law. 

I'm fairly sure something interacting with the atmosphere via only conservative forces would break a fair bit of physics. I.e if you move an object to the ground and back up to where it started there would be no net work done on it by the atmosphere.

Edited April 9, 2016 by Steel

[sevenperforce]

7 minutes ago, Steel said:

I'm fairly sure something interacting with the atmosphere via only conservative forces would break a fair bit of physics. I.e if you move an object to the ground and back up to where it started there would be no net work done on it by the atmosphere.

In practice, yes, but not in principle. For example, if you used a space elevator to lower something from orbit to the surface and back again, the total work done to it by the atmosphere would be effectively zero. 

[PB666]

 

The answer to that question is that there is no surface that could withstand striking the atmosphere at that speed even at oblique angle.
The hardest substance we know carbon which could create the best surface, the diamond lattice, is a gas about 2000'C which means it could tolerate speeds up to maybe 3000 m/s
The surface would pit after reaching the degassing temperature at which point the surface would no longer be oblique at all points

It would pit, and there would be a huge explosion and the ship would dissappear in a nuclear blast type of cloud. Every nose piece has a point that is flat, that point will explode the second it hits 14 PSI gas traveling at 30,000 m/s.

SR71 blackbird is one of the sleekest jet ever created, the underwing and leading edges get hot enough to boil water at mach 3. There is no space craft without a leading edge, to say that the craft has no coefficient of drag is to say that it is volumeless, because every volume will create drag when pushed against another volume.

 

Edited April 9, 2016 by PB666

[Angeltxilon]

If you move extremely fast a object without friction, this will continue having friction and pressure losses (heat losses).
This is due to the object stay having a "x" surface of interaction with the air, and the air has friction losses.

By the second law of newton, the forces of friction are distributed between the object itself and the air. Also the different pressure zones can cause more friction and the ram effect.

[Camacha]

7 hours ago, Steel said:

Why do these threads exist? There is no point discussing how something outside the laws of physics interacts with the laws of physics, because it doesn't make any sense. If you have a "completely frictionless projectile" how is it interacting with the air molecules to cause ram heating?

It is called a thought experiment, which is a cornerstone of the inquisitive mind and therefore science and physics throughout the centuries. Not to mention physics often simplifies variables to make life a bit easier. You know the joke about the spherical cow, right? Looking at something from a hypothetical perspective with some variable eliminated or inflated helps you understand what you are looking at.

[Steel]

7 hours ago, sevenperforce said:

In practice, yes, but not in principle. For example, if you used a space elevator to lower something from orbit to the surface and back again, the total work done to it by the atmosphere would be effectively zero. 

Not true, work would be done by the atmosphere on the way down and on the way back up again by the atmosphere, which would require input of power to overcome. The net work done by gravity, however, would be zero.

 

1 hour ago, Camacha said:

It is called a thought experiment, which is a cornerstone of the inquisitive mind and therefore science and physics throughout the centuries. Not to mention physics often simplifies variables to make life a bit easier. You know the joke about the spherical cow, right? Looking at something from a hypothetical perspective with some variable eliminated or inflated helps you understand what you are looking at.

I'm aware of thought experiments as well as the use and value of simplifying assumptions. However in this case we are looking at an effect caused by an object interacting with the atmosphere, with the catch that this object has been hypothetically modified to not interact with the atmosphere in any physical way, so what does this show?

[sgt_flyer]

Mandatory What Ifs from randall  - and it doesn't even need to be frictionless !

throw anything fast enough, after ram heating, you can even have air molecules ending up fusing with whatever you threw  though in this case, you'd end up with a plasma ball  

https://what-if.xkcd.com/1/

there's also the one on reentry steak if you want more infos on compressive heating calculations  

https://what-if.xkcd.com/28/

it's crazy to think how much randall's what-ifs can be adapted to other questions  

Edited April 10, 2016 by sgt_flyer

[A Fuzzy Velociraptor]

On 4/8/2016 at 2:03 AM, The Yellow Dart said:

Correct, the heating is due to extremely high pressure that the air reaches before it can get out of the spacecrafts way. If you made a very slender spike, or something like a pointed spear, that pointed perfectly prograde as it descended, it would sort of achieve what you are looking for. You would lessen the heating by making it easier for the air to move out of the way and could dip further into the atmosphere before experiencing heating, but by that same token, that object would not slow down nearly as much as an object of the same mass but is, say, spherical. Since you almost invariably are looking to slow down when you slam something into an atmosphere at orbital speeds, this doesn't usually end up being useful.

Even then you would generate an oblique shock which would have a significant temperature change. At Mach 100 even at a turn angle of only 5 degrees the temperature after the shock is 22 times the temperature before the shock. Even 2 degrees of turn angle and the temperature will still be almost 5 times the temperature before the shock.

[sevenperforce]

2 hours ago, Steel said:

Not true, work would be done by the atmosphere on the way down and on the way back up again by the atmosphere, which would require input of power to overcome. The net work done by gravity, however, would be zero.

Not quite. A climber on a space elevator can move at an arbitrarily low speed. Since the force required to displace air depends on velocity, the force can be made arbitrarily close to zero, meaning that the work done to move through the atmosphere can also be arbitrarily low. 

There actually is a fairly significant contribution to the energy budget of the system that might not be readily apparent: buoyancy. At any given point in the atmosphere, the air pressure on the bottom of an object is just slightly greater than the air pressure on the top of the object. This force differential is what allows sufficiently lightweight objects like helium balloons to rise. An object moving up through the atmosphere will have work done to it; an object falling down will do work to the atmosphere. But the total energy will be conserved. 

[PB666]

4 hours ago, Camacha said:

It is called a thought experiment, which is a cornerstone of the inquisitive mind and therefore science and physics throughout the centuries. Not to mention physics often simplifies variables to make life a bit easier. You know the joke about the spherical cow, right? Looking at something from a hypothetical perspective with some variable eliminated or inflated helps you understand what you are looking at.

Right but the framework of the argument is not set up properly, the idea is that you have a rigid space that either has momentum do to mass or thrust do to some force application push the object through a non-rigid volume

 

The are several ways to go, wecan ask the question what happens is

if we were to increase the pressure of the  gas, it  goes into the idea of titration, if you are traveling through a very low density gas or plasma, at high speed what happens to individual molecukes, for example does hydrogen reflect elastically or does is ablate even if only one molecule strikes a flawless structure, like a diamond lattice. The answer to this is probably both, some reflect, and some displace. What happens as you increase density? 

Then there is the math question, if every rigid volume must have a leading edge or surface, what happens with the surface of the leading edge decreases. IOW what if we make the leading edge a point, when we do that we might not realized that we have stepped into a messy domain of quantum dynamics, that in itself is a project. But we can address something, because the rigity has to have momentum, and that implies mass, to create a moment of rigity at least two molecules would need to be bonded or linked, and for it to be a volume implies at least a tetrahedral. So if the objects momentum is given by its mass, and the mass falls along a single dimension line that follows a point, the the gas ca interact with points along the line. . . . . side friction, the pressure of the gass and its temperature are pushing  the molecules (their electrons) in the gas to strick the line causing its moleculs to absorb, reflect and possibly displace a rigidities constituants. Quantum mechanics tells us that for a line, nomatter how perfect is composed of quanta whose position at the quantum scale cannot be rigidly defined. So that the electrons of a gas molecule moving in the direction of the side of something might entirely miss a collision or slam directly into and be knocked free of the charged nucleus that binds it. Ocassionally,mwith high enough energy two nuclei might come close enough to reflect..

Thus in trying to reduce drag to zero a critical problem is exposed within the implicit rigity. The de broglie equations point this out that everything has a frequency, they vibrate,the fields can interact if close enough and transfer momentum. Even the most recent studies indicate that quantum transfers can occur across several atoms distances. So there is no way to avoid the base question, in the real world how can you make a object so slippery it would not creat drag. That things that compse the rigity will not interact with the molecules of the gas, the only possible answer is that the object is smaller than a quanta volume and/or its not massive/energetic which is to say its not an object.

There are ways to trick the system

-rf frequency to displace gas in front of the objects motion,mproblem is rf creates thrust against the motion of travel. Heating up the gas to 10,000'K would get rid of the mach affect compression and reduce its density according to pV = n RT, but the problem is that the gas is now plasma, it is a fireball. 

-lasers, again to move the fireball away from the surface of the ship. 

Bsically what these are doing is saying that there was a dense volume of gas to be entered , but our machines reduced the density of the gas to the point its effects are hard to detect or emits the same as the evacuated controls. 

[Yourself]

2 hours ago, A Fuzzy Velociraptor said:

Even then you would generate an oblique shock which would have a significant temperature change. At Mach 100 even at a turn angle of only 5 degrees the temperature after the shock is 22 times the temperature before the shock. Even 2 degrees of turn angle and the temperature will still be almost 5 times the temperature before the shock.

Indeed, if I remember my undergrad classes well enough it turns out that pointy objects actually experience higher heating loads than blunt bodies because there's more heat transfer to the vehicle from the air.  Something about oblique shocks not doing as much to slow down the flow in the vicinity of the body (that and the shock is attached to the body, so the very hot fast moving air is right next to it).

[A Fuzzy Velociraptor]

3 minutes ago, Yourself said:

Indeed, if I remember my undergrad classes well enough it turns out that pointy objects actually experience higher heating loads than blunt bodies because there's more heat transfer to the vehicle from the air.  Something about oblique shocks not doing as much to slow down the flow in the vicinity of the body (that and the shock is attached to the body, so the very hot fast moving air is right next to it).

Attached shock is the main thing there vs the detached shock since air itself is an insulator. The oblique shock doesn't slow the air as much but the temperature effect is not as dramatic as a normal shock. Though the effect of the oblique shock with such a ridiculously large Mach value means the air itself should be glowing rather than the body (assuming no convective heat transfer to the body).

[Steel]

6 hours ago, sevenperforce said:

Not quite. A climber on a space elevator can move at an arbitrarily low speed. Since the force required to displace air depends on velocity, the force can be made arbitrarily close to zero, meaning that the work done to move through the atmosphere can also be arbitrarily low.

Yes you can make it arbitrarily low, but not zero unless the velocity is zero. The point is that any object that moves (i.e has a non-zero velocity) through the atmosphere will always have work done on it by the atmosphere to slow it down. Under no circumstances is this energy stored in the system with the capability to be put back into the object later.

 

7 hours ago, sevenperforce said:

There actually is a fairly significant contribution to the energy budget of the system that might not be readily apparent: buoyancy. At any given point in the atmosphere, the air pressure on the bottom of an object is just slightly greater than the air pressure on the top of the object. This force differential is what allows sufficiently lightweight objects like helium balloons to rise. An object moving up through the atmosphere will have work done to it; an object falling down will do work to the atmosphere. But the total energy will be conserved. 

The buoyancy of an object is unrelated to it's air resistance in this case. Also an object falling will have work done on it by the atmosphere, again you're confusing the issue by talking about rising and falling, which is a conservative system due to it being governed by gravity. If you take a space elevator and put it sideways (so that gravity is taken out of the equation) there is no way to move anything from one end to the other without losing energy from the system to drag. 

[sevenperforce]

14 hours ago, Steel said:

Yes you can make it arbitrarily low, but not zero unless the velocity is zero. The point is that any object that moves (i.e has a non-zero velocity) through the atmosphere will always have work done on it by the atmosphere to slow it down. Under no circumstances is this energy stored in the system with the capability to be put back into the object later.

Typically, forces which can be made arbitrarily close to zero are set aside for the purposes of analysis. 

Anyway, it isn't true that motion through the atmosphere must necessarily be non-conservative. Consider a frictionless cylinder which tapers internally like a scramjet engine. The air which is displaced is compressed as it enters and can then expand at the exit for no loss. 

[sgt_flyer]

1 hour ago, sevenperforce said:

Typically, forces which can be made arbitrarily close to zero are set aside for the purposes of analysis. 

Anyway, it isn't true that motion through the atmosphere must necessarily be non-conservative. Consider a frictionless cylinder which tapers internally like a scramjet engine. The air which is displaced is compressed as it enters and can then expand at the exit for no loss. 

if you compress gases, they will heat up, and will be higher temperature than the surrounding medium - you'll end up losing a slight part of this heat through convection and radiation before the gases expands again - but having lost a bit of heat in the process, the overall process won't be lossless. 

Edited April 11, 2016 by sgt_flyer

[PB666]

42 minutes ago, sevenperforce said:

Typically, forces which can be made arbitrarily close to zero are set aside for the purposes of analysis. 

Anyway, it isn't true that motion through the atmosphere must necessarily be non-conservative. Consider a frictionless cylinder which tapers internally like a scramjet engine. The air which is displaced is compressed as it enters and can then expand at the exit for no loss. 

Parasite drag consist of at least form drag and skin friction. Even if the form were to disappear the motion of a fluid over a surface causes skin drag.

 

E

Exceeding the speed of sound Cd increases, you can decrease CoD by decreasing the surface angle relative to the axis of motion, but if you hold volume constant this increases the surface area and eventually skin drag will supercede form drag.

A sears-haack body. Note that form drag exists on the leading edge (choose bottom left or top right), skin drag exists in the central and lagging region.

https://en.wikipedia.org/wiki/Reynolds_number

Quote

Skin friction:- is caused by viscous drag in the boundary layer around the object. The boundary layer at the front of the object is usually laminar and relatively thin, but becomes turbulent and thicker towards the rear. The position of the transition point depends on the shape of the object. There are two ways to decrease friction drag: the first is to shape the moving body so that laminar flow is possible, like an airfoil. The second method is to decrease the length and cross-section of the moving object as much as practicable. To do so, a designer can consider the fineness ratio, which is the length of the aircraft divided by its diameter at the widest point (L/D) - Wikipedia parasite drag

For any given volume there is a Sears-Haack body that confroms to a most efficicient shape that optimizes the flow, reducing the form drag without creating excessive skin drag.

Or to put it like this, if you shot a gas at 1ATM though a meter tube at 30,000 m/s and there was a pressure differential, then there is skin drag. Skin drag is not dependent on the surface, its dependent on the gas and the pressure of the gas, in this particular case since a bow shock is created there will be a pressure well between the skin of the aircraft and the shockwave, the only way to reduce this is to make length longer which increases skin drag.

The volume of such a body is V = LR_max3pi^2/16. Length(L) is going to be somewhat a function of velocity. Skin drag is going to be a trade off to reduce form drag created by a larger radius(R).

CoD = 24V/L³

 

 

[sevenperforce]

10 hours ago, sgt_flyer said:

if you compress gases, they will heat up, and will be higher temperature than the surrounding medium - you'll end up losing a slight part of this heat through convection and radiation before the gases expands again - but having lost a bit of heat in the process, the overall process won't be lossless. 

Indeed. But the compression/expansion cycle can be made arbitrarily close to adiabatic by choosing a particularly optimized form factor, so it is still valid to consider conservative forces only. 

9 hours ago, PB666 said:

Parasite drag consist of at least form drag and skin friction. Even if the form were to disappear the motion of a fluid over a surface causes skin drag.

Sure, but the OP specified a frictionless object.