Science Buddies: "Ask an Expert"

Hello,

I would just like to state that this is the first time I am posting and in all honesty I found this website on 6/12/2016… Nevertheless I digress let's get to the question I have to ask.

When launching a rocket does having something behind it change its energy loss when launching or even if there was a way ( I know that you cannot put a fan on a sailboat but bear with me) to allow something to fly behind it very near the exhaust of the rocket increase the amount of velocity it gains.

I designed an experiment that could test this but have not gotten around to doing it… Have a rocket with a force sensor on it to determine if there is a change in the amount of force the rocket is pushing forward with while the key to the experiment being breakaway door behind the rocket that will “break away” after the rocket gets to a steady force. Would the force sensors readout change when the door is removed and the rocket stops pushing off a solid and starts pushing on gasses?

SgtLlama,

Thank you for your question. This forum is normally for school science fair and project questions. However, it is an interesting question.

First let me start by stating that a rocket doesn't work by pushing on anything behind it. They work by virtue of Newton's 3rd law of physics which states that "for every action, there is an equal and opposite reaction." So the action of throwing tiny particles of mass (rocket propellant) out the back end of the rocket causes a force to be applied to the rocket in the opposite direction. This is described by the Tsiolkovsky rocket equation, or ideal rocket equation. The equation says that the change of velocity of the rocket is related to the velocity of the exhaust and the mass expelled during the maneuver by the following equation:

ΔV = Ve * ln(m0/mf)

where:
ΔV = the maximum change of velocity of the rocket during any particular maneuver.
Ve = the velocity of the exhaust stream.
m0 = the initial total mass, including propellant and structure.
mf = the final total mass after propellant has been expended and any staging has occurred.
ln = the natural logarithm function.

Additionally, the thrust produced by a rocket engine is given by the following:

F = m_dot * Ve + (pe - p0) * Ae

Where,
F = force of thrust.
m_dot = mass flow rate (i.e. how much propellant is being thrown out the back of the rocket per second).
Ve = exist velocity of the exhaust plume.
pe = the pressure of the exhaust plume.
p0 = pressure of the atmosphere.
Ae = area of the nozzle exit.

Neither equation relates the velocity or thrust of a rocket to a push off of something behind it. They work because of the conservation of momentum and is the reason rockets work in the vacuum of space with no atmosphere to "push" against.

You have an interesting experiment to show this concept in action. I would expect your trap door, having propellant mass particles hitting it and bouncing back, would transfer momentum back to the rocket structure, thereby preventing forward motion significantly, if not entirely.