So.. many of you guys here are pilots.. and I am guessing many know energy management principles from fighter training. From Wikipedia..

Quote

the law of conservation of energy states that the total energy of an isolated system remains constant, it is said to be conserved over time.[1] This law means that energy can neither be created nor destroyed; rather, it can only be transformed or transferred from one form to another

And you know that kinetic energy of a moving body is 1/2 x m x(v squared), where m is mass of the body and v is velocity.

Imagine you are in a spacecraft in outer space, beyond the galaxy.You are observing another spacecraft that is quite close to you and has no relative motion with respect to you. These two spacecraft are the "isolated system" and are free from any outside gravitational influences. Now let's say the other spacecraft fires its engines (Burn 1) and accelerates to velocity v with respect to your spacecraft. Then the engines pause. After it travels a distance d away from your spacecraft, it fires its engines in the opposite direction (Burn 2) and decelerates to having zero velocity with respect to your spacecraft.

So let's say the chemical energy in rocket fuel is converted to kinetic energy after Burn 1. But after Burn 2, more fuel and thereby chemical energy is expended but is used up to decelerate it back to zero velocity. After Burn 2, the spacecraft has zero kinetic energy and also a net loss of chemical energy. Where did this chemical energy go?

Energy is said to be a scalar quantity that has no direction, so how was the kinetic energy in one direction cancelled by kinetic energy imparted in other direction? Where did this kinetic energy go?

So energy can be destroyed?? Appreciate any takers.. Thanks in advance!

It depends.

3 hours ago, katdude said:

But after Burn 2, more fuel and thereby chemical energy is expended but is used up to decelerate it back to zero velocity. After Burn 2, the spacecraft has zero kinetic energy and also a net loss of chemical energy. Where did this chemical energy go?

Why do you think more energy would be used to decelerate the spacecraft? In other words, where are you getting the premise that there would be a net loss?

Edited August 31, 20187 yr by Klepto

@Klepto It is well known that more energy is needed to decelerate a spacecraft. Space shuttle needed to do a de-orbit burn in order to reenter earth's atmosphere. There is a net energy loss because the fuel has been used up and the final kinetic energy is still zero in our scenario.

@PilotCandidate Well some chemical energy of the fuel is converted to light energy and heat energy but majority of it is used to thrust the spacecraft and impart kinetic energy to it. Similar to how jet engines impart thrust.

I honestly think this question is what is preventing us from creating spacecraft that can dart from planet to planet at incredible speeds. In other words, we will truly have the SPACE FORCE that Mr. Trump wants.. haha.. Star Wars for real! and American will be the first human to set foot on Mars!!

19 hours ago, katdude said:

@Klepto It is well known that more energy is needed to decelerate a spacecraft. Space shuttle needed to do a de-orbit burn in order to reenter earth's atmosphere. There is a net energy loss because the fuel has been used up and the final kinetic energy is still zero in our scenario.

...

I honestly think this question is what is preventing us from creating spacecraft that can dart from planet to planet at incredible speeds.

It is actually well-known that it takes exactly the same amount of energy to decelerate a spacecraft in a closed system as it does to accelerate it... even less if you consider the friction of space and the reduced weight of the spacecraft after fuel burn.

It IS well known that a de-orbit burn occurs in... orbit. Not a closed system by any stretch of the imagination.

Even if your question had a valid premise, and it wasn’t your first post ever here, I doubt we would solve interstellar travel on this forum.

Sorry.

Edited September 1, 20187 yr by Klepto

You are making a couple false assumptions.."it is well known"..and premises. I suggest you research the laws of thermodynamics as well. There is nothing to be discovered

@Klepto Agree with the points you made. However the question still stands even on earth’s surface.

Lets say you’ve built a huge room in a lab and managed to suck out all the air in it. There’s a very long absolutely frictionless horizontal table in this room. Now apply the above question to this scenario. A model spacecraft is stationary at one end of the table. Now it does Burn 1, accelerates toward other end of table till it reaches velocity v. After traveling distance d, it does a Burn 2 in opposite direction and comes to a halt. 

If we think the chemical energy in fuel used in Burn 1 is converted to say 90% thrust derived kinetic energy, 5% light and 5% heat energy, and same thing happens with Burn 2. Accounting for the light and heat energy loss in both burns, it seems the 90% thrust derived kinetic energy from Burn 2 cancelled the kinetic energy from Burn 1 because it was in opposite direction.

Agree that due to the reduced weight of model spacecraft after Burn 1, you will need to expend less fuel energy in Burn 2. So Burn 1 energy is greater than Burn 2 energy.

But 90% of Burn 1 energy and 90% of Burn 2 energy created thrust derived kinetic energy which ultimately cancelled each other. How can this happen? Can energy be cancelled because of opposite direction? So energy is not a scalar?

There was a net loss of chemical energy stored in the fuel as it was used up, and I don’t see it converted into anything else apart from the small heat and light energy, so energy can be destroyed.?

Any aerospace/ aeronautical engineers on this forum? Any takers?

Edited August 31, 20187 yr by katdude

You've kicked out particles in each burn (equal and opposite reaction). While the aircraft velocity is back to zero those particles still have mass and velocity. 

[mention=78602]Klepto[/mention] Agree with the points you made. However the question still stands even on earth’s surface.
Lets say you’ve built a huge room in a lab and managed to suck out all the air in it. There’s a very long absolutely frictionless horizontal table in this room. Now apply the above question to this scenario. A model spacecraft is stationary at one end of the table. Now it does Burn 1, accelerates toward other end of table till it reaches velocity v. After traveling distance d, it does a Burn 2 in opposite direction and comes to a halt. 
If we think the chemical energy in fuel used in Burn 1 is converted to say 90% thrust derived kinetic energy, 5% light and 5% heat energy, and same thing happens with Burn 2. Accounting for the light and heat energy loss in both burns, it seems the 90% thrust derived kinetic energy from Burn 2 cancelled the kinetic energy from Burn 1 because it was in opposite direction.
Agree that due to the reduced weight of model spacecraft after Burn 1, you will need to expend less fuel energy in Burn 2. So Burn 1 energy is greater than Burn 2 energy.
But 90% of Burn 1 energy and 90% of Burn 2 energy created thrust derived kinetic energy which ultimately cancelled each other. How can this happen? Can energy be cancelled because of opposite direction? So energy is not a scalar?
There was a net loss of chemical energy stored in the fuel as it was used up, and I don’t see it converted into anything else apart from the small heat and light energy, so energy can be destroyed.?
Any aerospace/ aeronautical engineers on this forum? Any takers?

Does this have anything to do with the Earth being flat?


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On 8/31/2018 at 2:31 PM, KWings06j said:

You've kicked out particles in each burn (equal and opposite reaction). While the aircraft velocity is back to zero those particles still have mass and velocity. 

This just adds another layer to the question. Chemical energy in each burn is converted to light, heat, kinetic energy of the particles you mentioned, and kinetic energy of the model spacecraft. The kinetic energies imparted to the spacecraft each time in opposite directions cancel out as final kinetic energy of it is zero. So the question is still valid.

Other way to put it: If energy is non-directional, how can kinetic energies imparted by thrust in opposite directions cancel each other?

On 8/31/2018 at 7:47 PM, PilotPitts said:

Does this have anything to do with the Earth being flat?

Earth is supposedly a sphere. And men supposedly walked on the moon.  lol

Earth is supposedly a sphere. And men supposedly walked on the moon.  lol

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The issue is that, while the question is valid, you're ignoring half the system. Let us assume that there are 100 units of energy in your system. Here are the various types of energy and their starting values:

Potential Energy (rocket fuel) - 100

Kinetic Energy 1 (spaceship motion) - 0

Kinetic Energy 2 (any other object motion) - 0

Thermal Energy (heat) - 0

Light Energy - 0

Taking the first letter from each energy type (simplified I know) we come to the equation P+K1+K2+T+L=N (net energy) = 100

After your first burn you've converted some of that potential energy to the other forms. Thus you end up with P=80, K1=9, K2=9, T=1, L=1. Net energy remains at 100. What is important to note is that the kinetic energy is evenly split between the spaceship and the exhausted particles that created the thrust. You do a second burn, dropping K1 to zero. However to do that you increase K2. Note that the new particles coming out are moving faster because they have the added velocity of the spaceship. Since the spaceship was lighter due to less fuel you consumed less potential energy in the process. Thus your new numbers are P=62, K1=0, K2=34, T=2, L=2. Your K2 increase is due to its original energy, the energy from the new burn, and the energy imparted on it by the ship movement.

 

This is all, of course, drastically overly simplified but I don't feel like dusting off my old textbooks. 

On 9/7/2018 at 2:17 PM, KWings06j said:

Note that the new particles coming out are moving faster because they have the added velocity of the spaceship. Since the spaceship was lighter due to less fuel you consumed less potential energy in the process. Thus your new numbers are P=62, K1=0, K2=34, T=2, L=2. Your K2 increase is due to its original energy, the energy from the new burn, and the energy imparted on it by the ship movement.

I love your answer! Indeed, the new particles have more speed due to the added velocity of the spaceship. Thanks!

@ihtfp06 I am completely fine being stupid in order to find an answer to something that I don’t know.