# Homework Help: Relativistic rocket equation (intuitive derivation)

1. Jan 7, 2017

### TubbaBlubba

1. The problem statement, all variables and given/known data
In Newtonian mechanics, the rocket equation is derived by solving the simple differential equation -dm U = m dV, where U is the velocity of the expelled material relative to the rocket; a matter of conservation of momentum. In order to get the correct relativistic equation, one wishes to arrive at the differential equation $-dm U = \gamma^2(V) m dV$. I know of lengthy derivations using velocity addition and so on, but I want to try to understand it in a more intuitve and managable way. My textbook (Rindler) is not exactly helpful by providing the hint of writing M for the rest mass of the rocket and dM for the "relativistic mass" of the expelled material...

2. Relevant equations
We wish to arrive at $-dm U = \gamma^2 m dV$ using relativistic mechanics.
3. The attempt at a solution
My line of reasoning is this: The final velocity V is the velocity relative to Earth (or wherever the rockets starts from). At any given moment, we will observe the rocket of mass m changing its velocity by dV to increase its momentum by $\gamma(V) m dV$. On the other hand, the expelled material is expelled at a velocity U, constant relative to the instantaneous rest frame of the rocket. So from Earth, the expelled material will be observed to have momentum $-\frac{U}{\gamma(V)}dm$, giving us the desired differential equation $-dm U = \gamma^2(V) m dV$..

Is this line of reasoning acceptable, intuitively speaking?

2. Jan 7, 2017

### Orodruin

Staff Emeritus
No. Mainly because the expressions you have for the change in the momentum of the rocket and the momentum of the ejected material are not correct. Note that the change in the rocket's momentum will have the contribution you quote from the change in its velocity - but it will also have contributions from the changes in the mass and in the gamma factor. Likewise, the momentum of the ejected material needs to be considered more carefully. Note that the velocity of the ejected material is not necessarily in the opposite direction of the space ship velocity in the Earth frame. If $V > U$, then the ejected material will still be going in the forward direction.

A priori, you should be able to do the analysis in the Earth's rest frame, but you need to take more care when deriving the expressions for the momenta.

3. Jan 7, 2017

### TubbaBlubba

Thank you, I will think a bit more about it.

4. Jan 8, 2017

### TubbaBlubba

OK, I worked it out a priori using differential calculus and conservation of momentum/energy. Yeah, it's not that hard as long as you keep various Lorentz-factor identities in mind.

5. Jan 8, 2017

### Orodruin

Staff Emeritus
By the way, Rindler has written several textbooks on relativity. Not that it mattered here, but it will generally be helpful if you quote the title of the book you are referring to.

6. Jan 8, 2017

### TubbaBlubba

Yes; I'll keep that in mind!

7. Jan 8, 2017

### Ray Vickson

OK, so which one ARE you referring to?

8. Jan 8, 2017

### TubbaBlubba

"Relativity: Special, General and Cosmological, 2nd ed".

Rindler suggests as a hint the equation dM U = M du', where du' is the speed of the rocket relative to its instantaneously inertial frame and dM the "relativistic mass" of the jet.

What I eventually did was to set up conservation equations (g for gamma):
MVg(V) = (M + dM)(V + dV)(g(V + dV)) + ((V-U)/(1-UV))g((V-U)/(1-UV)))dM'
and similarly for energy, some manipulations, and you get there eventually.

9. Jan 8, 2017

### Orodruin

Staff Emeritus
Yes, it is rather straight forward really once you keep track of everything. I still prefer the derivation where you start by looking at the instantaneous rest frame because there are simply less things to keep track of. The addition of velocities is simply
$$V + dV = \frac{V+dV'}{1+V\, dV'} \simeq (V+dV')(1-V\, dV') \simeq V + dV' (1 - V^2) = V + \frac{dV'}{\gamma^2},$$
directly identifying $dV' = \gamma^2 dV$.

10. Jan 8, 2017

### TubbaBlubba

Ah, that's a nice trick - using velocity addition with V + dV. Will have to remember that one.

But a differential equation with M as referring to a conserved quantity and dM to an unconserved, yeah, that irks me.

11. Jan 8, 2017

### Orodruin

Staff Emeritus
Where is this differential equation?

12. Jan 8, 2017

### TubbaBlubba

It's just me being confused. -dM U = M dV' relates the rate of change in the rocket's rest mass to the velocities. But dM is not the rest mass of the exhaust jet at that moment. Or is it? Ugh...

13. Jan 8, 2017

### Orodruin

Staff Emeritus
No it is not, it is the change in the rest* mass of the rocket. Note that the rest* mass of the exhaust will be zero if $U = 1$. The equation is the conservation of momentum equation after taking conservation of energy into account. By conservation of energy, you know that
$$M = (M+dM)(1 + d\gamma) + \mathcal E = M + dM + \mathcal E \quad \Longrightarrow \quad \mathcal E = - dM$$
where $\mathcal E$ is the energy of the ejected material in the instantaneous rest frame of the rocket (note that $d\gamma = \mathcal O(dV'^2)$). The momentum $p$ of the ejected material is related to the energy by $p = - U \mathcal E$. From there you can go to the conservation of momentum to deduce that $-U\, dM = M\, dV'$.

Edit: * Physicists normally only talk about mass. Relativistic mass is an antiquated concept - it is just another name for total energy.

14. Jan 9, 2017

### TubbaBlubba

Ahhh, thank you. That makes it a lot more clear to me.

Yeah, I know. Thankfully I have lecture and course notes that do away with it, but it can still be very confusing to work with textbook material that uses it.

15. Jan 9, 2017

### Orodruin

Staff Emeritus
Just out of curiosity, do you know why you are using that book and not the other book by Rindler (Introduction to Special Relativity) if your course only covers SR and no GR?

16. Jan 9, 2017

### TubbaBlubba

Hm. It's a pretty brief course (3/5ths of one, the rest being Lagrangian mechanics) with extensive lecture notes, some homework and a relatively short exam (IIRC, it used to be oral), based on selections from a given set of problems from the book. Maybe the instructor uses it in several courses and is simply familiar with it, or they want students to be able to use it further on.

Last edited by a moderator: Jan 9, 2017