All Questions
24
questions
20
votes
4
answers
8k
views
What is the "pendulum rocket fallacy" as it relates to analogizing a pencil balanced on a finger to maintaining attitude of a hovering rocket?
I've received comments that explain to me that the analogy between rocket attitude control during a hovering maneuver and the act of balancing pencil on the end of a finger is a helpful and good one; ...
- 149k
13
votes
6
answers
3k
views
Why does this formula say rocket efficiency depends on velocity?
Wikipedia gives the following equation for the efficiency $\eta_p$ of an engine here:
$$\eta_p= \frac {2\, (\frac {v} {v_e})} {1 + ( \frac {v} {v_e} )^2 }$$
where $v$ is the rocket speed and $v_e$ ...
- 2,035
9
votes
5
answers
5k
views
Why do space rockets not start from an angle? Why go straight up? [duplicate]
I know that they used to have this idea, back in the day, of a "space gun" to fire somebody into space, and I understand why that was not safe or practical/feasible.
However, once they had come up ...
- 107
9
votes
3
answers
4k
views
Oberth effect for Earth vehicles
I don't understand this and must ask a probably very stoopid question here:
The Oberth effect says that a rocket is much more efficient when (and in the orbital direction of) a payload when it ...
- 27k
8
votes
3
answers
14k
views
What does the exhaust plume of a rocket look like in vacuum?
There are plenty of photos of rocket tests and launches in atmosphere, and in these the exhaust plume tends to be a long thin flame.
Is this true for when rocket engines operate in a vacuum? My guess ...
- 83
8
votes
2
answers
878
views
Is engine thrust noticeably increased near the ground?
I know next to nothing about rocketry and space flight.
I assume that the majority of the force generated by a rocket's engine is simply the result of throwing gasses out of the bottom very fast, and ...
- 181
6
votes
2
answers
793
views
Modeling a rocket using Tsiolkovsky's equation and ordinary differential equations
So I have this uni assignment to make a model out of ODEs, and my idea was to use rockets. After some research I found about Tsiolkovsky's equation and tried using it on a Falcon 9 (disconsidering ...
- 61
5
votes
1
answer
802
views
Contracting rocket engine nozzles
The efficiency of rocket engine nozzles depends greatly on their expansion ratio an how well the ambient pressure matches the rocket nozzles exit pressure. An optimal expansion ratio means that there ...
- 1,222
4
votes
3
answers
3k
views
What (if anything) limits the efficiency of a rocket engine?
Humans have developed lots of rockets. I observe that in most cases to increase the payload capacity we just increase the amount of fuel.
Is it not possible to exponentially increase the efficiency ...
- 79
4
votes
2
answers
758
views
Design of starship fins
How much can the falling speed of starship approximately be reduced by the bellyflop fall with fins? Are we talking about numbers around 5% or 50% here?
Wouldn't it make sense to put holes where the ...
- 143
4
votes
2
answers
457
views
Could a fat rocket be used on Mars?
Since Mars' atmosphere is around 2% the density of Earth's then could a much wider rocket achieve orbit?
Could the rocket be in the shape of a capsule or an optimal shape for reentry on Earth?
- 1
3
votes
5
answers
991
views
Why is hydrogen better than helium as remass?
I just watched a very good YouTube video on why nuclear engines might be useful, and it also goes into why Hall Effect thrusters are super good at squeezing obscene ISP out of things.
During the video,...
3
votes
1
answer
154
views
What is missing in the conversion of specific impulse from units of seconds to thrust/mass flow rate? [duplicate]
I was trying to get the Specific Impulse of the Saturn V engines, hoping for a value in N/kg/s, as I need to know the mass consumption rate per thrust value. Any and all sources give me the value in ...
3
votes
1
answer
481
views
Pendulum rocket fallacy - so why *do* skyrockets have sticks?
This question is inspired by two, linked questions.
Skyrockets (of the pyrotechnic/firework kind) have guide sticks, which are (according to internet sources) to enhance stability: "The guide ...
- 131
3
votes
2
answers
2k
views
At a height of 100 km, what speed do you need to be going to escape Earth's gravity?
I understand that the escape velocity of Earth is 11 km/s. However, Earth's gravitational sphere of influence is not infinite, so it is possible to go slower than that and still escape the sphere of ...
- 33