Apollo 10 contingency plans

[Zakalwe]

The descent stage was safed and deactivated by venting the helium using to pressurize the tanks. Once that is done, the engine is dead, and can't be used for anything. And the amount of fuel in it was only a few hundred liters at maximum.

On average, there was 5% of the total average fuel loading left that was usuable.

[Allan F]

So around 400 kg? How much dV is that with the descent stage still attached? High enough to stage successfully?

[ka9q]

May I ask, why is the g-value being used related to the earth and not the moon? 

It's an artifact of the utterly obsolete English system of units, and shows why using them ought to be made a capital crime.

Although rocket specific impulse is typically quoted in seconds, this involves implicitly canceling pounds-force (lbf) with pounds-mass (lbm).

An I_sp of 300 seconds means that the engine can produce 1 pound-force of thrust from 1 pound-mass of propellant for 300 seconds. But a pound of  force and a pound of mass are two completely different things. Only in standard earth gravity does a mass of 1 pound-mass exert a downward force of 1 pound-force, and these units remain the same regardless of the local gravitational acceleration and the actual weight in pounds-force of 1 pound-mass.

The correct measure of rocket engine performance is effective exhaust velocity, which is what you get when you don't incorrectly cancel units. In SI, the units of I_sp are simply m/sec, which makes far more intuitive sense anyway.

[Luke Pemberton]

It's an artifact of the utterly obsolete English system of units, and shows why using them ought to be made a capital crime.

Yes, given we would hang people for accidentally standing on a ladybird with 4 spots, it seems surprising that we invented a system that when used was not punishable by death. I think it is possible that most people collapsed of an aneurysm when using the system, so no one was ever brought to trial for its use. 

[VQ]

It's an artifact of the utterly obsolete English system of units, and shows why using them ought to be made a capital crime.

Although rocket specific impulse is typically quoted in seconds, this involves implicitly canceling pounds-force (lbf) with pounds-mass (lbm).

An I_sp of 300 seconds means that the engine can produce 1 pound-force of thrust from 1 pound-mass of propellant for 300 seconds. But a pound of  force and a pound of mass are two completely different things. Only in standard earth gravity does a mass of 1 pound-mass exert a downward force of 1 pound-force, and these units remain the same regardless of the local gravitational acceleration and the actual weight in pounds-force of 1 pound-mass.

The correct measure of rocket engine performance is effective exhaust velocity, which is what you get when you don't incorrectly cancel units. In SI, the units of I_sp are simply m/sec, which makes far more intuitive sense anyway.

Technically (see http://xkcd.com/1475/), the "I_sp in seconds" issue can be created using both pseudo-SI and pseudo-FPS systems of measurement. You need the same "G" correction factor if kgf are used instead of N and the I_sp is expressed in seconds by an SI user. This is just a less common error than using the non-consistent unit of lbm instead of the consistent unit of slug (the amount of mass accelerated at 1 ft/s² by one lbf, which weighs ~32 lbf on earth).

Even less commonly, you can use lbm as your unit of mass and the poundal as your consistent unit of force. One poundal is equal to about 1/32 lbf.

[Bob B.]

It's an artifact of the utterly obsolete English system of units, and shows why using them ought to be made a capital crime.

I don't have a problem with it.  It's just another system.  If you understand it and are practiced in using it, it's no more difficult than SI.

[Bob B.]

The correct measure of rocket engine performance is effective exhaust velocity, which is what you get when you don't incorrectly cancel units.

It a way, I wouldn't say it's the correct way.  There's nothing wrong with the English system way of calculating specific impulse, it's just different.

In SI, the units of I_sp are simply m/sec, which makes far more intuitive sense anyway.

That's the same thing as effective exhaust velocity.  I wouldn't call that specific impulse.

[ka9q]

It a way, I wouldn't say it's the correct way.  There's nothing wrong with the English system way of calculating specific impulse, it's just different.

It's both different and wrong because force and mass simply don't cancel (equivalently, gravitational acceleration is not dimensionless). Specific impulse has units of velocity, not time, so to be correct the English way would give results in units of feet per second, not seconds.

Using the correct units can additionally provide insight into a fundamental tradeoff of rocket engine design by equivalently representing effective exhaust velocity as power per unit thrust with a dimensionless scale factor of 0.5. E.g., an effective exhaust velocity of 1 m/s implies an exhaust kinetic power of 1/2 W per N of thrust, and an effective exhaust velocity of 3 km/s corresponds to a power of 1500 W/N.

One might claim an intuitive meaning for I_sp measured in seconds: the time that a rocket engine could hover in 1 g gravity. But even this doesn't work because the mass of the rocket doesn't remain constant as it depletes its propellant. Nor does it include the mass of the engine and payload.

[Bob B.]

It's both different and wrong because force and mass simply don't cancel (equivalently, gravitational acceleration is not dimensionless).

You're thinking about it the wrong way; we're not canceling force and mass, we're canceling force and force.  In the English system, specific impulse is defined as the impulse per unit weight ejected (standardized to Earth gravity).  The weight of 1 pound-mass at standard gravity is 1 pound-force, thus we have force•time/force = time.

Specific impulse has units of velocity, not time, so to be correct the English way would give results in units of feet per second, not seconds.

The English system does use feet/second, it's called effective exhaust gas velocity.
 
 

[Peter B]

The descent stage was safed and deactivated by venting the helium using to pressurize the tanks. Once that is done, the engine is dead, and can't be used for anything. And the amount of fuel in it was only a few hundred liters at maximum.

So essentially during, the course of a descent to the surface of the Moon, there is not likely to be any abort situation where the preferred choice would be to land as opposed to pushing the throttle to max and starting an ascent back to orbit?

[Dalhousie]

It's an artifact of the utterly obsolete English system of units, and shows why using them ought to be made a capital crime.

I don't have a problem with it.  It's just another system.  If you understand it and are practiced in using it, it's no more difficult than SI.

SI is always easier, more logical, less error prone and for efficient.

[Bob B.]

SI is always easier, more logical, less error prone and for efficient.

The only thing I find easier about SI is that standard measurements are in multiples of ten or a thousand.  Otherwise I find it effectively the same.
 
 

[ka9q]

The only thing I find easier about SI is that standard measurements are in multiples of ten or a thousand.  Otherwise I find it effectively the same.

The recurring confusion about which acceleration of gravity to use in specific impulse calculations is a pretty good example of a basic problem with English units. That's why I brought it up.

[Echnaton]

The descent stage was safed and deactivated by venting the helium using to pressurize the tanks. Once that is done, the engine is dead, and can't be used for anything. And the amount of fuel in it was only a few hundred liters at maximum.

So essentially during, the course of a descent to the surface of the Moon, there is not likely to be any abort situation where the preferred choice would be to land as opposed to pushing the throttle to max and starting an ascent back to orbit?

We are of course discussing a hypothetical landing that would not be by the book.  So there is no reason to assume that the engine would be safed after landing, by the book.  Anything physically possible, even though well beyond normal procedures remains viable.    That said, the method by which they could have landed without the guidance of the computer during the final stage is unclear to me.  Particularity if Stafford and Cernan had not trained for it.   Perhaps if the goal was just to land, rather than to land is a specific spot, it was possible. 

[Bob B.]

The only thing I find easier about SI is that standard measurements are in multiples of ten or a thousand.  Otherwise I find it effectively the same.

The recurring confusion about which acceleration of gravity to use in specific impulse calculations is a pretty good example of a basic problem with English units. That's why I brought it up.

People who are properly trained and practiced in the use of English units aren't confused.  Most confusion arises from the fact that there are two systems of units, not that there is something inherently wrong with English units.  I don't doubt for an instant that there are non-scientists and non-engineers who have been exposed to metric units their whole life who would get confused by some uses of SI units.  For instance, a layman probably doesn't know what a Newton is or how it's used.  People who need to know do know because it is part of their training.  Likewise, people who have learned and are practiced in the use of English units know what they need to know and aren't confused by it.