Rocket Equation Homework: Explaining Mass Infinity

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Homework Help Overview

The discussion revolves around the Rocket Equation, which relates the mass of a spacecraft to its propellant and exhaust velocity. Participants are exploring the implications of the equation, particularly the claim that the mass of the spacecraft approaches infinity under certain conditions related to the velocity change ratio.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants are questioning whether the mass ratio should equal one if the mass is very large and discussing the relationship between dry mass and propellant mass. There is uncertainty about the specific reference to mass going to infinity and whether the question pertains to total mass or just dry mass.

Discussion Status

Multiple interpretations of the problem are being explored, particularly regarding the definitions of M and M+P. Some participants have raised concerns about potential typos in the values provided and whether the entire question has been presented.

Contextual Notes

There is mention of the problem being sourced from a study guide, which may imply constraints on the information available to participants. The discussion also notes that some values may be incorrect, which could affect understanding.

Biosyn
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Homework Statement



The following equation is known as the "Rocket Equation":

\frac{M+P}{M}= e^{ΔV/C} = mass ratio

M = dry mass
P = mass of propellant
C = exhaust velocity
ΔV = velocity changee^1 = 2.72
e^2 = 2.74
e^3 = 20.4

As ΔV/C goes up, the mass of the spacecraft goes up faster than the exponential, so much so that depending on the lightness of the structural materials and the density of the propellants employed, somewhere between ΔV/C = 2 and ΔV/C = 3 the mass of a single spacecraft will go to infinity! Please explain how and why the mass of the spacecraft will reach infinity?

Homework Equations



\frac{M+P}{M}= e^{ΔV/C}

The Attempt at a Solution

Shouldn't the mass ratio be equal to 1 if the mass is a really huge number?
Or, does the propellant mass have something to do with it. I know that the propellant mass needs to increase along with the dry mass of the rocket.
 
Last edited:
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It's unclear whether the question posed refers to M or M+P. If M is fixed, then M+P goes up exponentially with ΔV/C. But I don't get the bit about going to infinity between 2 and 3. I've no idea where that's coming from. Is this the whole question, or is something left out?
Btw, the quoted value for e^2 is wrong. Looks like a typo.
 
haruspex said:
It's unclear whether the question posed refers to M or M+P. If M is fixed, then M+P goes up exponentially with ΔV/C. But I don't get the bit about going to infinity between 2 and 3. I've no idea where that's coming from. Is this the whole question, or is something left out?
Btw, the quoted value for e^2 is wrong. Looks like a typo.


I think it's the total mass of the spacecraft (M+P). I'm not entirely sure. This is from a USAD Science section study guide.
 
Biosyn said:
I think it's the total mass of the spacecraft (M+P). I'm not entirely sure. This is from a USAD Science section study guide.
Can you provide a link or is it behind a paywall?
 
haruspex said:
Can you provide a link or is it behind a paywall?

It's behind a paywall. :/
 

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