Skip to main content
Space Exploration

Return to Answer

clarification based on comments
Source Link
GremlinWranger
GremlinWranger
  • 25k
  • 1
  • 67
  • 97

There would be thrust but it would be similar to that of an open pot of boiling water which is not noticeably heaviergoing to be very slight. This is basically a rocket where there is no nozzle, so looking at specific impulse the exhaust velocity would be defined by velocity of the evaporating molecules (10s100s of meters per second)*sin45 due to the random departure direction time the mass per second of ablation.

If you wanted to improve this, you would have an internal pipe network in the vehicle skin that heated the working medium to vapor and exhausted it through proper nozzles, at least getting rid of the sine losses by constraining the thrust to the needed direction but the exhaust velocity of this system is still pretty low, quite possibly above Mach one (100s of meters a second) but nowhere near the kilometers a second a chemical rocket has.

In general doing a powered re-entry will need similar performance to reaching orbit since you are doing the reverse so you need either similar efficiency per unit weight of fuel or a massive amount of 'fuel'. If your objective is to land bulk material, say a water from a comet there are some options here, but all will involve turning >90% of your payload to vapor. For things like manned vehicles a conventional heat shield does a better job for far less mass.

There would be thrust but it would be similar to that of an open pot of boiling water which is not noticeably heavier. This is basically a rocket where there is no nozzle, so looking at specific impulse the exhaust velocity would be defined by velocity of the evaporating molecules (10s of meters per second)*sin45 due to the random departure direction.

If you wanted to improve this, you would have an internal pipe network in the vehicle skin that heated the working medium to vapor and exhausted it through proper nozzles, at least getting rid of the sine losses by constraining the thrust to the needed direction but the exhaust velocity of this system is still pretty low, possibly above Mach one (100s of meters a second) but nowhere near the kilometers a second a chemical rocket has.

In general doing a powered re-entry will need similar performance to reaching orbit since you are doing the reverse so you need either similar efficiency per unit weight of fuel or a massive amount of 'fuel'. If your objective is to land bulk material, say a water from a comet there are some options here, but all will involve turning >90% of your payload to vapor. For things like manned vehicles a conventional heat shield does a better job for far less mass.

There would be thrust but it is going to be very slight. This is basically a rocket where there is no nozzle, so looking at specific impulse the exhaust velocity would be defined by velocity of the evaporating molecules (100s of meters per second)*sin45 due to the random departure direction time the mass per second of ablation.

If you wanted to improve this, you would have an internal pipe network in the vehicle skin that heated the working medium to vapor and exhausted it through proper nozzles, at least getting rid of the sine losses by constraining the thrust to the needed direction but the exhaust velocity of this system is still pretty low, quite possibly above Mach one but nowhere near the kilometers a second a chemical rocket has.

In general doing a powered re-entry will need similar performance to reaching orbit since you are doing the reverse so you need either similar efficiency per unit weight of fuel or a massive amount of 'fuel'. If your objective is to land bulk material, say a water from a comet there are some options here, but all will involve turning >90% of your payload to vapor. For things like manned vehicles a conventional heat shield does a better job for far less mass.

Source Link
GremlinWranger
GremlinWranger
  • 25k
  • 1
  • 67
  • 97

There would be thrust but it would be similar to that of an open pot of boiling water which is not noticeably heavier. This is basically a rocket where there is no nozzle, so looking at specific impulse the exhaust velocity would be defined by velocity of the evaporating molecules (10s of meters per second)*sin45 due to the random departure direction.

If you wanted to improve this, you would have an internal pipe network in the vehicle skin that heated the working medium to vapor and exhausted it through proper nozzles, at least getting rid of the sine losses by constraining the thrust to the needed direction but the exhaust velocity of this system is still pretty low, possibly above Mach one (100s of meters a second) but nowhere near the kilometers a second a chemical rocket has.

In general doing a powered re-entry will need similar performance to reaching orbit since you are doing the reverse so you need either similar efficiency per unit weight of fuel or a massive amount of 'fuel'. If your objective is to land bulk material, say a water from a comet there are some options here, but all will involve turning >90% of your payload to vapor. For things like manned vehicles a conventional heat shield does a better job for far less mass.