4 ms·

Does anyone know why it is 5x the effect of gravity for reentry? I would think it would just fall back into the Earth's gravitational pull after its thrusters a

Does anyone know why it is 5x the effect of gravity for reentry? I would think it would just fall back into the Earth's gravitational pull after its thrusters are turned off. Or is it because it has an arcing path, and thrusters are deliberately not turned off during reentry so that the rocket spends less time burning up in the atmosphere? Anyone know? It's not clear to me why it would be 5x the effect of gravity on reentry.

aparent 11y ago
This comes from deceleration of the capsule due to air resistance as it enters the atmosphere.
david-given 11y ago
It's not attaining anywhere near orbital velocity, but it's still going pretty fast. All that speed has to be shed somehow, and they do that through atmospheric drag. The deceleration is high because the atmosphere gets thick pretty quickly, so you end up losing all that speed quickly, which means a high acceleration. They could use the rocket to slow down --- but to do that, they'd have to carry fuel up, which makes the whole thing much larger. It's vastly more efficient to let atmospheric drag do it for you for free.
VLM 11y ago
Someone else can provide the math, but if due to the tanks being mostly empty you can burn anything from practically 0 to 5 G thrust, intuitively you burn the least total fuel by flooring it to 5G at the last minute. Consider the alternative of burning less than 1G of acceleration, you'll fail to slow down although you'll descent acceleration will be somewhat lower. At exactly 1 G it'll keep falling at a constant speed till it runs out of fuel, then impact. Or if you're asking why performance is so much more spritely at the end of mission, unlike boats or cars the fuel in the tanks is like 95% of the mass of the rocket, so something that barely waddles off the launch pad with full tanks weighs practically nothing at landing when the tanks are nearly empty.

aparent 11y ago

This comes from deceleration of the capsule due to air resistance as it enters the atmosphere.

david-given 11y ago

It's not attaining anywhere near orbital velocity, but it's still going pretty fast. All that speed has to be shed somehow, and they do that through atmospheric drag. The deceleration is high because the atmosphere gets thick pretty quickly, so you end up losing all that speed quickly, which means a high acceleration. They could use the rocket to slow down --- but to do that, they'd have to carry fuel up, which makes the whole thing much larger. It's vastly more efficient to let atmospheric drag do it for you for free.

VLM 11y ago

Someone else can provide the math, but if due to the tanks being mostly empty you can burn anything from practically 0 to 5 G thrust, intuitively you burn the least total fuel by flooring it to 5G at the last minute. Consider the alternative of burning less than 1G of acceleration, you'll fail to slow down although you'll descent acceleration will be somewhat lower. At exactly 1 G it'll keep falling at a constant speed till it runs out of fuel, then impact. Or if you're asking why performance is so much more spritely at the end of mission, unlike boats or cars the fuel in the tanks is like 95% of the mass of the rocket, so something that barely waddles off the launch pad with full tanks weighs practically nothing at landing when the tanks are nearly empty.