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What does one to one ratio mean?

What does it mean when the SpaceX Grasshopper's thrust to weight ratio is greater than one at touchdown?

  • According to SpaceX (http://www.spacex.com/press.php?page=20130310), this would "prove a key landing algorithm for Falcon 9". Can someone explain this to a non-Rocket Scientist?

  • Answer:

    It means that the rocket engine was still producing enough thrust (i.e force) to lift the remaining weight of the vehicle. This allows it to make it a soft, controlled landing instead of a hard landing (crash landing), so that it will not rely on shock absorbers. As the fuel burns, the weight of the rocket decreases, but, when the fuel/oxidizer tanks are nearly empty the thrust decreases too, so the http://en.wikipedia.org/wiki/Thrust-to-weight_ratio will decrease. Each rocket has a characteristic thrust curve which determines its behaviour as burn time elapses. Usually rockets do not have throttle, so the form of the curve cannot be controlled. The Grasshopper's engines have throttle and, to be usable for landing, they must be throttled down controllably, to a value asymptotically approaching 1 from above, at a time limited by the rocket's height off the ground. If that can't be achieved, the rocket will bounce back or, if the ratio drops below 1, it will plunge.

Achilleas Vortselas at Quora Visit the source

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What they really mean is that even at the lowest throttle, the thrust is still larger than the weight. This means that the engine produces more force than the weight of the rocket, so it continually accelerates upwards. That in turn means that it will lose speed all the time while coming down, and if it ever comes to a stops, then it will start moving up instead. Thus it will have to have the stop right as it touches the ground, then the engine can be switched of and it has landed. So there is no second try: If it stops to soon, it starts going up, and there is no way to get it down again without switching the engine off, and then it would crash. The safer way would be to be able to throttle the engine down to less thrust than the weight of the rocket. Then you can stop at a safe altitude, hover, and slowly edge down to the ground. Moving fast downwards, though continuously decelerating, and then coming to a stop at exactly the right time is what they call "a hoverslam". The reason it is important to be able to do is that there are fundamental limits to how low you can throttle a rocket engine, and adding a smaller engine would be expensive and add weight.

Kasper Emil Feld

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