Answering questions on vacuum, space, rockets, and space travel

On about September 21, 2017 I received via email some questions/comments related to vacuum and how rockets and spacecraft are able to operate in space.  I replied within about 1 day and my responses are presented below slightly edited.


How can anything be propagated through space if it is a vacuum, nothing?

If space is a vacuum then what does the rocket push off of?

How does the rocket maintain its heading and not spin out of control if there is no air to stabilize off of?

Space travel believers will say that they use special propellant that when ejected out of the thruster nozzle has an equal and opposite reaction.

If that’s the case then wouldn’t the thrusters on let’s say the space shuttle when maneuvering to the ISS have to eject the special propellant faster than 17,500MPH which is the speed they are traveling?

All tests show that rockets don’t work in a vacuum.


I am going to do the best I can to answer these questions as quickly as possible off the top of my head without consulting any sources of information beyond what I currently know.  These are all good questions and certainly more could be learned by studying these things more.


First regarding the question of vacuum, I think that even in space there is not a complete vacuum in the sense that you could find a cubic mile of space that does not contain at least one atom or atomic nuclei.

But even if you could imagine a cubic mile of space without even one atom in it, I believe that other physical phenomena could take place in this space, such as the transmission of light or other electromagnetic waves, and other things possibly (I would have to research this more).

Even if you could have completely empty space, it becomes kind of a philosophical question of what is “nothing.”  I think the view of physics is that even if you could have completely empty space, that is not nothing, but instead a part of reality, known as the space time continuum in physics.  I think this view that even empty space would not be nothing goes back to early philosophers even before Christ.  I would have to double check on this.

I know we have plenty of examples here on earth of physical phenomena taking place in a high vacuum, but we really cannot create a perfect and complete vacuum.

So before we had transistors, we had vacuum tubes.  These accomplished the same sort of electrical tasks that eventually became possible to do with transistors.  When multiple transistors integrated into an electrical circuit were eventually created on a silicon chip, we got computers and relatively shortly thereafter we got computers that ordinary people could buy and use.  But realize that there were digital computers even before transistors, and these computers used vacuum tubes.

But of course, the more common use of vacuum tubes initially was in radio and then eventually television.  So there is a lot going on within the vacuum of a vacuum tube, such as the flow of electrons in the vacuum, and I am remembering how vacuum tubes need a source of heat to operate, and that is done with the filament, which is basically like a separate electrical circuit within the vacuum tube that functions just like a conventional incandescent light bulb.  Also, the screens of the original televisions were cathode ray tubes which were themselves giant vacuum tubes where a fast moving electron beam is used to create the television picture.

Now moving on to the other space questions.  I will paste the questions/comments posed and intersperse my answers/comments.

Question: If space is a vacuum then what does the rocket push off of?

Answer: It pushes off from the mass of propellant reaction products that it expels at high velocity.  I answered this more completely in a separate blog post dated September 20, 2017.

Question: How does the rocket maintain its heading and not spin out of control if there is no air to stabilize off of?

Answer: A physical solid object in space would remain in motion unless acted upon by an external force.  If we were to imagine that it were far enough out in space such that there were no significant gravitational forces acting upon it, then it would continue moving in a straight line for example.  I am using moving in a straight line, because if I said it was at rest, it would raise the question of what is it at rest with respect to, so linear motion is easier to imagine for this thought experiment.  This would be conservation of linear momentum.  The center of mass of the object would continue to move in a straight line.  But if the object had a spin or rotation to it, that spin would also continue indefinitely, unless acted upon by a force to change the rotation.  Just to complicate the example a little for illustration, let’s say it was a spacecraft traveling with a couple of astronauts in it, and the spacecraft was of a long cylindrical shape with one astronaut at one end and the other at the other end, and the spacecraft was tumbling (spinning) end over end.  If the astronauts were to move from the ends to the middle of the spacecraft the rate of the spacecraft’s tumbling would increase.  This would be due to conservation of angular momentum.  By the way, an example of conservation of angular momentum that most people on earth are familiar with is when an ice skater gets to spinning with their arms extended outwards and perhaps even a leg extending outward, and then draws their arms inward as well as their leg, and then they end up spinning noticeably faster.

Question: Space travel believers will say that they use special propellant that when ejected out of the thruster nozzle has an equal and opposite reaction.

Answer: I would have to research this for details, but I can say this.  Once you are in space, to make changes or corrections in either linear motion or spinning motion you are going to need more than one thruster pointed in different directions in order to achieve any change that you might want.  Maybe it could be done with just one thruster if you had the ability to change the direction that the thruster points from the spacecraft.  But also, for in space use, the corrections you make are usually going to be relatively small, so you want your thruster to be easy to turn on and off.  So you wouldn’t want to be burning rocket fuel and oxygen for example.  This is where your special propellants come in.  For a propellant to work all it needs to be able to do is expel a mass at a certain velocity, the higher the better, because then you get more effect for the mass that you use.

Question: If that’s the case then wouldn’t the thrusters on let’s say the space shuttle when maneuvering to the ISS have to eject the special propellant faster than 17,500MPH which is the speed they are traveling?

Answer: The unreacted unejected propellant is already going at the same speed as the spacecraft before it is ejected.  To cause a change in the motion of the spacecraft it only has to be ejected at some velocity with respect to the velocity of the spacecraft.  A thorough discussion follows.

Presumably in this example the spacecraft is orbiting the earth in a stable orbit at whatever distance from the earth that would be necessary for orbiting at 17,500 mph.  This could easily be calculated but I am not going to do that now.  I just know from having heard these orbital speeds mentioned before that this would be at a distance from the earth where you would have a very high vacuum, meaning essentially no “air” drag on the vehicle.  This would mean that the spacecraft would keep moving in this orbit unless acted upon by an external force.

The reason it is not moving in a straight line is that gravity is continually exerting a force upon it in a direction perpendicular to its velocity.  This is known as centripetal force and it causes the spacecraft to be continually curving toward the earth.  But it is going fast enough that the curving is the right amount to keep the spacecraft circling or orbiting the earth.

If you wanted to cause the spacecraft to re-enter the atmosphere, all you would need to do is fire a thruster in the opposite direction to the velocity of the spacecraft.  Note that the propellant used would already be travelling at the speed of the spacecraft before it is reacted and released, so it is ejected at high velocity with respect to the spacecraft’s velocity.  In so doing, if it is fired long enough to decrease the velocity enough, and this could all be calculated ahead of time based on the performance characteristics of the thrusters, the slower speed will result in the force of gravity causing it to curve more than the curve required for the orbit, so the spacecraft will start to travel to lower altitudes and eventually start entering the atmosphere.  I have never felt the need to look into the numerical aspects of all of this, but it might be interesting to gather all of this information to see if it all makes sense.  But I suppose, if it is all fake, they could have just made it all calculate out correctly to deceive us all.


Just a little note about how physics works for the equation F=ma in order to help better understand what centripetal force is.  (I am reflecting upon when I first learned about this equation, taking physics for the first time in high school 44 years ago, where I learned how this equation explains why car accidents can kill you, but that’s another story.)  In reality this equation is a vector equation.  A vector has both magnitude and direction.  F is the force vector with both its magnitude and direction.  a is the acceleration vector also with both magnitude and direction.  Acceleration is also by definition the time derivation of the velocity vector.  So in words, F(magnitude&direction) = mass x a(magnitude & direction) or

F(magnitude&direction) = mass x (change in v/change in t)

Where v is the velocity vector also with magnitude and direction.

So if you were in a drag racer, and you got the green light, and you take off, you would feel yourself being pushed from behind by the back of the seat as the car accelerates in a straight line with you in it.  Your velocity would be changing in magnitude (increasing) while its direction stays the same, straight ahead.

Now if you were traveling in a car at a constant speed and in a straight line, but then you put the car into a turn of a certain radius but at constant speed, you would feel the side of the car pushing against you in the direction that the car is turning.  In this case you are experiencing a constant acceleration, that is, a change in the velocity vector with respect to time, but the velocity vector is changing in direction but not speed or magnitude.  So in this turn the centripetal force is the force being exerted by the ground on the tires of the car perpendicular to its forward motion in order to cause it to turn (with you inside), which is a constant acceleration maneuver.  To make it a little more personal for you in the car, the side of the car is pushing against you causing you to turn, a centripetal force causing the centripetal acceleration of you.

Most people are familiar with the term centrifugal force.  If you are sitting in the turning car, from your point of view as being in the car, known in physics as a rotating reference frame point of view, you feel a force throwing you to the outside of the car, that is indistinguishable from a gravity force from your point of view from inside the car.  This reminds me of the space station in 2001 A Space Odyssey, where it is spinning so as to create an artificial gravity, so the person is able to run around it as if they were running on a surface with a gravitational attraction.

Comment: All tests show that rockets don’t work in a vacuum.

Answer: I would like to review these tests so I can comment upon them.

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