4 ms·

Why is there so much excess fuel to begin with?

by adzm 4y ago

tuatoru 4y ago
Contingency planning.
Symmetry 4y ago
Physical systems aren't perfectly predictable, and so require feedback. You include a little more fuel than you think you need so that if you use more than expected, say because solar conditions make the atmospheric drag a bit higher, you won't run out with the payload just shy of the intended orbit. But during re-entry there's no reason to keep that with you and if by some chance your stage arrives intactish at the ground it might cause a fire. Hence the venting before re-entry.
pixl97 4y ago
In case something doesn't work as efficiently as possible. Since these rockets are not specialty designed for each launch you only save a very tiny amount of money/weight by not filling it all the way up.
LeonM 4y ago
To add to the other (good) answers: it is also because this makes (re)starting the engines in zero gravity easier. Liquids behave very weird in 0g, trying to suck fuel from a nearly empty tank in 0g can be challenging. Leaving more fuel in the tanks makes this less of a challenge.
Tuna-Fish 4y ago
That's not all that much fuel. There's always some margin in the tanks to make sure that the turbopump won't ingest a bubble, and probably a little more so that if the rocket underperforms slightly on ascent they won't lose the mission because they skimped on 50kg of fuel.
0xDEF 4y ago
There isn't that much excess fuel. The reason it looks so big and dramatic is because of water vapor and fumes reflecting sunlight.
BenoitP 4y ago
I wouldn't say there is a lot of excess fuel as a percentage of the mix in exhaust gases, but there is some, by design. Maybe a few percents. Why do we want some fuel in the exhaust? Burning an oxydizer-rich mix is not a good idea, especially with a reusable rocket. You'll have a very hot gas that is going to burn-corrode your engine. Metals can burn in an oxygen-rich air. Now imagine that mix at 3,600 K and 300 atm pressure. You'll have what is called in the industry an 'engine-rich' burn. You don't want that. Also, I don't know about this rocket but sometimes you want to cool down your nozzle and you can do that with unburnt fuel that you inject from the side. Running a stochiometric mix puts you also at risk of damaging your engine (some parts of your mix might be oxygen-rich), on top of combustion temperature being higher. Now, burning a fuel-rich mix make these problems go away. And does not decrease much your thrust: heated at very high temperatures, this residual fuel will expand and contribute to some part in the force generated. Last, a fun fact: you can detect when then rocket leaves the upper atmosphere with the flame color! That's when the flame goes from being orange (high temp residual fuel meets oxygen from the atmosphere and burns) to being grey (flame meets the atmosphere but not enough oxygen to relight it, and soot is grey). You can clearly see it happen around 40km altitude here: https://youtu.be/xY96v0OIcK4?t=15881 https://youtu.be/xY96v0OIcK4?t=15881
- corbulo 4y ago
  Thanks for making HN awesome

Why is there so much excess fuel to begin with?

by adzm 4y ago

Why is there so much excess fuel to begin with?

tuatoru 4y ago

Contingency planning.

Symmetry 4y ago

Physical systems aren't perfectly predictable, and so require feedback. You include a little more fuel than you think you need so that if you use more than expected, say because solar conditions make the atmospheric drag a bit higher, you won't run out with the payload just shy of the intended orbit. But during re-entry there's no reason to keep that with you and if by some chance your stage arrives intactish at the ground it might cause a fire. Hence the venting before re-entry.

pixl97 4y ago

In case something doesn't work as efficiently as possible. Since these rockets are not specialty designed for each launch you only save a very tiny amount of money/weight by not filling it all the way up.

LeonM 4y ago

To add to the other (good) answers: it is also because this makes (re)starting the engines in zero gravity easier. Liquids behave very weird in 0g, trying to suck fuel from a nearly empty tank in 0g can be challenging. Leaving more fuel in the tanks makes this less of a challenge.

Tuna-Fish 4y ago

That's not all that much fuel. There's always some margin in the tanks to make sure that the turbopump won't ingest a bubble, and probably a little more so that if the rocket underperforms slightly on ascent they won't lose the mission because they skimped on 50kg of fuel.

0xDEF 4y ago

There isn't that much excess fuel. The reason it looks so big and dramatic is because of water vapor and fumes reflecting sunlight.

BenoitP 4y ago

I wouldn't say there is a lot of excess fuel as a percentage of the mix in exhaust gases, but there is some, by design. Maybe a few percents. Why do we want some fuel in the exhaust? Burning an oxydizer-rich mix is not a good idea, especially with a reusable rocket. You'll have a very hot gas that is going to burn-corrode your engine. Metals can burn in an oxygen-rich air. Now imagine that mix at 3,600 K and 300 atm pressure. You'll have what is called in the industry an 'engine-rich' burn. You don't want that. Also, I don't know about this rocket but sometimes you want to cool down your nozzle and you can do that with unburnt fuel that you inject from the side. Running a stochiometric mix puts you also at risk of damaging your engine (some parts of your mix might be oxygen-rich), on top of combustion temperature being higher. Now, burning a fuel-rich mix make these problems go away. And does not decrease much your thrust: heated at very high temperatures, this residual fuel will expand and contribute to some part in the force generated. Last, a fun fact: you can detect when then rocket leaves the upper atmosphere with the flame color! That's when the flame goes from being orange (high temp residual fuel meets oxygen from the atmosphere and burns) to being grey (flame meets the atmosphere but not enough oxygen to relight it, and soot is grey). You can clearly see it happen around 40km altitude here: https://youtu.be/xY96v0OIcK4?t=15881 https://youtu.be/xY96v0OIcK4?t=15881

corbulo 4y ago
Thanks for making HN awesome