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Oberth Effect?

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ka9q:

--- Quote from: Northern Lurker on May 20, 2019, 05:06:45 PM ---The effect of the Effect is simple enough; the faster the rocket is going, more delta-V it gets from a kg propellant.o any constructive

--- End quote ---
This is actually incorrect; you get the same delta-v regardless of the rocket's velocity relative to the observer (ignoring relativistic effects). What Jay said is correct; you get more kinetic energy from each kg of propellant when you're going faster.

Some years ago I worked out the powers and energies for each stage of the Saturn V. It turns out that during much of the third stage burn more mechanical power (force times velocity) is being given to the Apollo spacecraft than the heat power released in the J-2's combustion chamber. This is not "free energy"; that extra energy is coming from the kinetic energy that had been stored in the J-2's propellants before they were burned. That energy was put there by the lower stages of the rocket. So rockets aren't quite as inefficient as they seem early in flight when they're furiously burning propellant and still moving fairly slowly. Only a small fraction of the released energy may be going into the payload but much more is being "invested" into the kinetic energy of the as-yet-unburned propellants, some of which will be recovered later.

The most energy-efficient rocket is one that increases its Isp (effective exhaust velocity) linearly with speed so that its exhaust is always stationary in the inertial frame of the launch site. Of course this would be a much more massive rocket because energy efficiency is usually not the primary consideration in rocket design. Size and weight are more important, which is why a highly energetic propellant like liquid hydrogen is so attractive even though it costs a lot of energy to make.

bknight:

--- Quote from: ka9q on May 21, 2019, 11:29:21 AM ---
--- Quote from: Northern Lurker on May 20, 2019, 05:06:45 PM ---The effect of the Effect is simple enough; the faster the rocket is going, more delta-V it gets from a kg propellant.o any constructive

--- End quote ---
This is actually incorrect; you get the same delta-v regardless of the rocket's velocity relative to the observer (ignoring relativistic effects). What Jay said is correct; you get more kinetic energy from each kg of propellant when you're going faster.

Some years ago I worked out the powers and energies for each stage of the Saturn V. It turns out that during much of the third stage burn more mechanical power (force times velocity) is being given to the Apollo spacecraft than the heat power released in the J-2's combustion chamber. This is not "free energy"; that extra energy is coming from the kinetic energy that had been stored in the J-2's propellants before they were burned. That energy was put there by the lower stages of the rocket. So rockets aren't quite as inefficient as they seem early in flight when they're furiously burning propellant and still moving fairly slowly. Only a small fraction of the released energy may be going into the payload but much more is being "invested" into the kinetic energy of the as-yet-unburned propellants, some of which will be recovered later.

The most energy-efficient rocket is one that increases its Isp (effective exhaust velocity) linearly with speed so that its exhaust is always stationary in the inertial frame of the launch site. Of course this would be a much more massive rocket because energy efficiency is usually not the primary consideration in rocket design. Size and weight are more important, which is why a highly energetic propellant like liquid hydrogen is so attractive even though it costs a lot of energy to make.

--- End quote ---

With Blue Origin and mow SpaceX opting for methane is because it is easier to handle than liquid H2?

ka9q:
The reasons are all practical tradeoffs.

Methane/LOX has roughly the same performance as RP-1/LOX, so that's a wash.

Methane is less dense than RP-1, requiring a larger tank, and liquid methane is a cryogenic liquid, unlike RP-1 (but not nearly as cold as LH2). Those are strikes against methane.

Methane doesn't coke (release elemental carbon) as easily at high temperatures. That's an advantage, as it makes it easier to reuse a regeneratively cooled engine. More complex (and efficient) engine cycles (like preburning) may be possible. That's another advantage.

Methane can supposedly be made from raw materials on Mars.

So while methane has had no particular advantages (only disadvantages) until now, you can see why BlueOrigin and SpaceX are interested in it.

JayUtah:
Plumbing fittings for liquid hydrogen are hard to engineer.  It's a very tiny molecule, and it likes to sneak past seals.

jfb:
IINM, methane and oxygen have similar boiling points, so the tanks can share a common bulkhead, which saves some mass.  But I think the real attraction is that methane can be synthesized on Mars relatively easily. 

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