Rocket engine operating in a vacuum

In summary, the conversation discusses the final speed of a chemical rocket's exhaust in a perfect vacuum, given the total enthalpy of the hot gas it generates. The energy equation and the specific heat capacity are mentioned as potentially useful equations. The attempt at a solution suggests using a simplified equation to calculate the exhaust speed, but raises concerns about the accuracy and feasibility of this approach. Further clarification is requested on the topic.
  • #1
oobgular
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Homework Statement


So suppose there is a chemical rocket operating in the vacuum of space (assume it's a perfect vacuum). It generates a hot gas with a total enthalpy of h. What's the final speed of the rocket exhaust, in terms of h?

Homework Equations


The energy equation seems to be useful here:
h0 = h_exhaust + 1/2v^2 (from my textbook-- it's stated in the middle of a derivation with no explanation, but I assume it's applicable in this case)
h = c_p*T

The Attempt at a Solution


I feel like there isn't enough information to solve the problem accurately, since we are only given h. My first thought was that because it is a perfect vacuum, we can assume the exhaust enthalpy is 0, and so v = sqrt(2*h0)

Not only does that seem too simple, but also I don't think it's reasonable that all enthalpy is lost-- wouldn't that assume T drops to absolute zero? And I thought all molecular movement stops at absolute zero, so how could there still be a flow of gas?

Can someone help me with clarifying this? Thanks!
 
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  • #2
A hypothetical ideal rocket nozzle would have indeed the gas leave at "0 K": all particles move in the same direction at the same speed. That would need an infinite nozzle and various other silly things, so real rockets are always just approximations, but you can assume the ideal case here.
 
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Related to Rocket engine operating in a vacuum

1. How does a rocket engine work in a vacuum?

A rocket engine operates in a vacuum by using the principle of Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. The engine expels hot gases at high speeds out of the back of the rocket, creating thrust in the opposite direction and propelling the rocket forward.

2. Can a rocket engine operate in a complete vacuum?

Yes, a rocket engine can operate in a complete vacuum. In fact, it is more efficient in a vacuum because there is no air resistance to slow down the rocket's acceleration. However, a vacuum can also pose challenges for a rocket engine, such as the lack of oxygen for combustion and the potential for overheating without the presence of air to dissipate heat.

3. How is a rocket engine different in a vacuum compared to in Earth's atmosphere?

In a vacuum, a rocket engine must rely on its own internal oxygen supply for combustion since there is no oxygen present in the surrounding environment. This usually means using a specialized propellant, such as liquid oxygen, which can be stored on board the rocket. Additionally, a vacuum allows for higher speeds and efficiencies due to the absence of air resistance.

4. What are the challenges of operating a rocket engine in a vacuum?

Operating a rocket engine in a vacuum presents several challenges. One major challenge is the lack of oxygen for combustion, which requires the use of specialized propellants. Another challenge is the potential for overheating, as there is no air to dissipate excess heat from the engine. Additionally, the lack of air resistance can lead to higher speeds and potential stability issues for the rocket.

5. How does a rocket engine in a vacuum affect the trajectory of a rocket?

A rocket engine in a vacuum has a significant impact on the trajectory of a rocket. The absence of air resistance allows the rocket to reach higher speeds and achieve greater distances. The lack of oxygen for combustion also affects the amount of thrust and control the engine can provide, which can impact the trajectory of the rocket. Additionally, a vacuum can cause stability issues for the rocket, making it more difficult to control its direction and trajectory.

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