How did the Juno space probe reach 165,000 MPH?
I'm not sure what Juno's orbital speed is at closest approach.
Quote from: Nomadd on 08/07/2016 11:11 pm I'm not sure what Juno's orbital speed is at closest approach.At perijove, approximately 130K mph Jupiter-relative and approximately 160K mph Earth-relative. The difference is due to Earth-Jupiter orbital velocity.
For long duration missions cryogenic propellants are problematic because they tend to boil off to a gaseous state. To circumvent this we hear of designs that go to the trouble of keeping it cold enough that it stays liquid, or just switch to another propellant. But is there a straightforward reason that it couldn't be allowed to turn to gas, and then recooled and liquified only when needed? Perhaps the energy and mass of the equipment needed to reliquify is always going to be demonstrably greater than insulation?
This surplus gas can be used in gas thrusters or burnt in H/O gas thrusters for station keeping.
A quick question. Somewhere I got the impression that the oxygen-rich staged combustion cycle is inherently more efficient than a fuel rich staged combustion cycle.Perhaps that is incorrect - I can't think of a good reason why it would be.But if it is more efficient, why?Something is niggling in the back of my brain now about smaller molecules of oxygen vs kerosene, but my last chemistry lesson was in 1984 when I was 18...
To a very rough first approximation, the various gases involved (hydrogen, oxygen, methane) have similar (within a factor of 2) specific heats per molecule. In other words, it takes a similar amount of energy to raise the temperature of a mole of oxygen molecules from 699 K to 700 K as it does to raise the temperature of a mole of hydrogen molecules from 699 K to 700 K. That means that the propellant present in the larger quantity (by number, not mass) will tend to provide the most pumping power at a given temperature. Lox-hydrogen engines run at O/F mass ratios of 5-6, corresponding to number ratios of about 0.35. Hence, fuel-rich staged combustion offers about three times the power of the oxygen-rich alternative.When it comes to lox-methane, O/F is usually just slightly fuel-rich, meaning that O/F is a bit over 3 by number. Hence, there is more pumping power available in the oxygen-rich cycle (though the polyatomic nature of methane raises its molar heat capacity, reducing oxygen's advantage somewhat).
One problem with over-expansion is the turbulent flow as it detaches from the nozzle wall; this can tear up an engine and cause guidance problems--the flow will be asymmetrical.
Aren't most rockets designed with engines that are slightly over-expanded at sea level
Why is an overexpanded nozzle less efficient than an ambient nozzle? According to the attached diagram, the nozzle is "full" all the way to the end with both an ambient and overexpanded nozzle (not grossly overexpanded, the bottom one, I know that has other issues). How is the rocket affected by the exhaust "collapsing" after it has left the engine?I understand that an underexpanded nozzle is less efficient because it fails to capture the maximum possible energy from the expanding exhaust, but just to be clear, it is still producing the same amount of thrust that it does at ambient, right? Because both nozzles are "full".