How does burn time affect the parameters in the rocket equation?

  • Thread starter Thread starter zenterix
  • Start date Start date
  • Tags Tags
    Rocket equation
AI Thread Summary
Burn time in rocket propulsion primarily influences the acceleration profile rather than the final speed change, as long as the ejection speed and mass parameters remain constant. In the context of the rocket equation, varying burn time does not affect the overall velocity change if the initial and final masses are the same. However, in practical scenarios like launching payloads to low Earth orbit, burn time becomes crucial due to atmospheric and gravitational losses. The first stage of a rocket typically burns fuel quickly to overcome atmospheric resistance, which can impact performance. Ultimately, while burn time is not a factor in theoretical calculations, it is significant in real-world applications.
zenterix
Messages
774
Reaction score
84
Homework Statement
a) Before a rocket begins to burn fuel, the rocket has a mass of ##m_{r,i}=2.81\times 10^7\mathrm{kg}##, of which the mass of fuel is ##m_{f,i}=2.46\times 10^7\mathrm{kg}##. The fuel is burned at a constant rate with total burn time ##\mathrm{510s}## and ejected at a speed of ##u=3000\mathrm{m/s}## relative to the rocket. If the rocket starts from rest in empty space, what is the final speed of the rocket after all the fuel has been burned?

b) Now suppose the same rocket burns the fuel in two stages ejecting the fuel in each stage with the same relative speed. In stage one, the available fuel to burn is ##m_{f,1,i}=2.03\times 10^7\mathrm{kg}## with burn time ##\mathrm{150s}##. Then the empty fuel tank and accessories from stage one are disconnected from the rest of the rocket. These disconnected parts have a mass of ##1.4\times 10^6\mathrm{kg}##. All the remaining fuel is burned during the second stage with a burn time of ##\mathrm{360s}##. What is the final speed of the rocket after all the fuel has been burned?
Relevant Equations
##\vec{F}_{ext}=m_r(t)\vec{v}_r'(t)-um_r'(t)##
The items (a) and (b) are provided for context. I am not concerned with solving the problem. That is relatively easy.

My question is about the burn time. It doesn't seem to matter for solving the problem as it has been posed. All we care about is the states at the beginning an end of each stage.

I noticed that in (b) the first stage burns most of the fuel in a way shorter time compared to the second stage. I can see how this is realistic in some cases: a rocket has to leave the atmosphere first and this requires the most power and fuel. But the problem above takes place in empty space.

But doesn't this first stage scenario affect the ejection speed of the fuel in any way?

In general, how does the burn speed affect the problem?
 
Physics news on Phys.org
It determines how long the ejection lasts.
 
zenterix said:
In general, how does the burn speed affect the problem?
As you suspect, varying the burn time does not change the final speed change you get from the rocket equation as long as the ejection speed is kept the same (i.e. same rocket technology) and the initial and final masses are the same, as is explicitly stated in this problem.

In some practical applications however, like for a launcher that has to burn to lift a payload mass from ground to low Earth orbit, the exact acceleration profile matters a lot mostly due a combination atmospheric and gravitational losses, so for those kinds of problems the burn time (i.e. the acceleration) do indeed factor into how much effective speed change the rocket can provide.
 
Thread 'Collision of a bullet on a rod-string system: query'
In this question, I have a question. I am NOT trying to solve it, but it is just a conceptual question. Consider the point on the rod, which connects the string and the rod. My question: just before and after the collision, is ANGULAR momentum CONSERVED about this point? Lets call the point which connects the string and rod as P. Why am I asking this? : it is clear from the scenario that the point of concern, which connects the string and the rod, moves in a circular path due to the string...
Thread 'A cylinder connected to a hanged mass'
Let's declare that for the cylinder, mass = M = 10 kg Radius = R = 4 m For the wall and the floor, Friction coeff = ##\mu## = 0.5 For the hanging mass, mass = m = 11 kg First, we divide the force according to their respective plane (x and y thing, correct me if I'm wrong) and according to which, cylinder or the hanging mass, they're working on. Force on the hanging mass $$mg - T = ma$$ Force(Cylinder) on y $$N_f + f_w - Mg = 0$$ Force(Cylinder) on x $$T + f_f - N_w = Ma$$ There's also...
Back
Top