- #1
Enthalpy
- 667
- 4
Dear visionary inventors, megalomaniac engineers and audacious explorers,
Chemical rocket engines aren't up to our desire to hop in the Solar system: go quickly to Mars, deviate Earth-threatening objects, and so many more missions. We need a higher ejection speed to save propellant mass, but this takes more energy than chemical reactions bring. One possibility is to tap Sunlight instead of transporting the energy.
I suggest - as others did - to heat the propellant with Sunlight; this thread shall describe it now and here with more details. The conversion to electricity first enables even higher ejection speeds, but direct heating is energy-efficient, so for a long weak thrust, the collector area is feasible - smaller and of cheaper materials than Solar cells or even a heat sink.
Material sublimation limits thermal designs to <3000K, so only hydrogen improves the ejection speed over a combustion. My plan is to dissociate a part of this hydrogen to increase the ejection speed a bit more.
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The heater that catches concentrated Sunlight and transfers heat to hydrogen is of tungsten alloy. At 2800K =2527°C, sublimation thins it by 45µm in 14 days, from Plansee's doc.
30mbar in the heating chamber let 23% of injected H2 split to atomic H* for performance. Expansion to 1Pa in the nozzle leaves 15.6MJ/kg from 92.8MJ/kg in the chamber, for isp = 1267s = 12428m/s.
Mean 2800K is already a lot, as sublimation is very sensitive to it. The nozzle can't grow much because the mean free path is already ~10mm. A lower chamber pressure dissociates more hydrogen and improves the ejection speed, but needs much more heating power, as the nozzle gets inefficient - recombining hydrogen takes a big expansion. But more pressure bring during some flight sequences more thrust traded against ejection speed; for instance, 0.8bar and 2093K from the same Sunlight concentrator push *2.1 times stronger with isp=800s.
Marc Schaefer, aka Enthalpy
Chemical rocket engines aren't up to our desire to hop in the Solar system: go quickly to Mars, deviate Earth-threatening objects, and so many more missions. We need a higher ejection speed to save propellant mass, but this takes more energy than chemical reactions bring. One possibility is to tap Sunlight instead of transporting the energy.
I suggest - as others did - to heat the propellant with Sunlight; this thread shall describe it now and here with more details. The conversion to electricity first enables even higher ejection speeds, but direct heating is energy-efficient, so for a long weak thrust, the collector area is feasible - smaller and of cheaper materials than Solar cells or even a heat sink.
Material sublimation limits thermal designs to <3000K, so only hydrogen improves the ejection speed over a combustion. My plan is to dissociate a part of this hydrogen to increase the ejection speed a bit more.
--------------
The heater that catches concentrated Sunlight and transfers heat to hydrogen is of tungsten alloy. At 2800K =2527°C, sublimation thins it by 45µm in 14 days, from Plansee's doc.
30mbar in the heating chamber let 23% of injected H2 split to atomic H* for performance. Expansion to 1Pa in the nozzle leaves 15.6MJ/kg from 92.8MJ/kg in the chamber, for isp = 1267s = 12428m/s.
Mean 2800K is already a lot, as sublimation is very sensitive to it. The nozzle can't grow much because the mean free path is already ~10mm. A lower chamber pressure dissociates more hydrogen and improves the ejection speed, but needs much more heating power, as the nozzle gets inefficient - recombining hydrogen takes a big expansion. But more pressure bring during some flight sequences more thrust traded against ejection speed; for instance, 0.8bar and 2093K from the same Sunlight concentrator push *2.1 times stronger with isp=800s.
Marc Schaefer, aka Enthalpy