You are not logged in.
We talk a lot on here about thermal energy storage, but I haven't yet seen it suggested as a means of powering a rocket...
Boron melts at 2349K and has a heat of fusion of 4.6MJ/kg. This could be used to heat a propellent such as water, as in a nuclear or solar thermal rocket. If an exhaust velocity of 3km/s can be attained, each newton-second of thrust would cost 1.5kJ; at a 67% efficient conversion of heat to exhaust energy, a kg of liquid boron would be able to provide 2 kN-s of thrust. If 10% of the mass of a vehicle was such a boron battery, it would have a total delta-V budget of 200 m/s. The battery would be recharged using a simple resistance heater.
200 m/s is not a lot, but consider. It is easier in space to provide extra energy than extra propellent. There are applications where such a high thrust moderate isp drive with a very moderate delta-V budget per pulse would be useful. Inspecting spaceships/stations, for example. A liquid boron drone would be able to recharge its batteries from the stations power a lot easier than it would be to refill its propellent tanks, and something like water is far less nasty to deal with than typical hypergolics. And away from Earth, water is far easier to come by than hydrazine. The liquid boron engine would be capable of wilderness refuelling.
It could also be used on planetary surfaces. Robert Zubrin has proposed using a nuclear thermal rocket with CO2 propellent as a surface-to-surface transport on Mars, as well as a shuttle to and from orbit. A liquid boron rocket hopper would not be capable of orbit, but it could manage maybe 2 minutes of flight at a time between charges, during which it would refill its propellent tanks. If it is fixed wing, the rocket would enable VTOL operation, which on a planet with no runways and limited knowledge of ground conditions would be very useful.
Use what is abundant and build to last
Offline
Like button can go here
This post is reserved for an index to posts that may be contributed by NewMars members over time.
The topic itself appears to me to have significant upside potential. Best wishes for success!
(th)
Offline
Like button can go here
This may be a useful addition to the topic.
https://en.m.wikipedia.org/wiki/Rocket_sled_launch
It is noted within that the space shuttle used 1/3rd of its fuel mass accelerating to 1000mph (450m/s). So a rocket sled equipped with a low performance but easily reusable propulsion system could eliminate the need for a lower stage or increase payload by a large fraction. If the sled can be rapidly decelerated at the end of the ramp, then it could support a rapid launch schedule.
Last edited by Calliban (2025-01-17 13:51:27)
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
Offline
Like button can go here
Lithium Hydride melts at 690 celcius with a heat of fusion of 2.9 MJ/kg.. This is a similar temperature to the exhaust of a hydrogen peroxide monopropellent rocket. A steam rocket using molten lithium hydride might get an isp of 140 based on the performance of peroxide, though the lighter exhaust (pure H2O instead of H2O/O2 mix) should improve on this. As a replacement for cold gas thrusters I think it has potential, given the simplicity of the design. Each kg of LiH might be enough for 1500 N-s of thrust.
On earth applications could be in rocket assist to achieve short takeoff and landing -- or, in the case of paratroopers, very rapid entry without a parachute. In the latter case the ability to recharge and refill the system would be less important than having something that can be easily throttled and has very high reliability. In the former though, a system that runs on electricity and water would be logistically trivial to provide compared to any other form of rocket.
Use what is abundant and build to last
Offline
Like button can go here
Key highlights
PBR-20 (Water-based Resistojet Thruster) is a low-pressure (<60 kPa) propulsion system with a scalable water tank and a redundant flow control system with a fail-safe valve. The thruster unit is modular and it is possible to expand the overall system by clustering multiple units and scaling the propellant tank as needed. The limits of such clustering or scaling are determined by the mass, volume or power of a spacecraft.
The individual units provide a total impulse of >220 Ns and a specific impulse of >70 s, with a thrust of 1 mN. The first model was demonstrated aboard a 3U ISS-deployed CubeSat in 2019 and two flight model thrusters are to be delivered in 2021 and launched by SLS and Falcon-9, respectively. The thruster operated in low earth orbit in March 2023 on board the Sony "EYE" satellite.
TECHNICAL SPECIFICATIONS
Wet Mass: < 1.5 kg
Power: 20 W
Thrust: 1 mN
Envelope: 1 U
Total Impulse: 220 Ns
ApplicationsOrbit insertion
Orbit maintenance
De-orbit
Collision avoidance
Formation flying
https://satsearch.co/products/pale-blue … t-thruster
Assuming the energy cost per N-s for a lithium hydride thermal rocket is 2kJ, we would need 440kJ to match this for total impulse. That would be about 150g of LiH, with double the Isp. But of course, we wouldn't be using anywhere near that much, since the point is to regenerate with solar power. Such a storage system would be useful for raising orbits through perigee kicks, not something feasible with solar electric systems.
Use what is abundant and build to last
Offline
Like button can go here
LiH reacts explosively with water. Maybe we could put the LiH in stainless steel balls and pass saturated steam through a pebble bed of SS spheres.
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
Offline
Like button can go here
A YouTube video showing reactions of LiH with a variety of substances is available.
The demonstrations are certainly entertaining. The material used to hold molten LiH was an expensive quartz glass.
That said, the application for a fusion reactor as described in the web site linked from the opening post appears to call for 100+ cubic meters of LiH to absorb thermal energy.
A system designed to use this material for station keeping needs to be able to heat the stainless steel spheres using electricity and then (somehow) make them available to heat water.
Would cylinders of stainless steel holding LiH offer the needed combination of capabilities?
The electric heating could be done at the ends, and the water heating could be done at the cylinder walls.
Update: Could the resistance heater run through the cylinder? That would provide faster heating, I would think.
(th)
Offline
Like button can go here
LiH doesn't appear to have any particular materials compatability issues. The reference below discusses problems with its synthesis, which are mostly due to the corrosiveness of liquid lithium. Ionic LiH is compatible with 304SS.
https://juser.fz-juelich.de/record/8480 … Condon.pdf
The issue of rapidly heating the LiH to melting point, suggests that a pebble bed design may not be the optimum configuration. Instead, we could build the rocket as a cylindrical tank of LiH. Heating elements could be slotted into stainess steel tubes within the tank. Propellant would also flow through SS tubes through the tank, exiting into an expansion chamber. The optimum propellant is probably water. At temperatures ~700°C, corrosiveness of steam may be a life limiting problem.
Propellant would be stored as saturated water. The temperature of the water is a design consideration. The higher the water temperature, the higher its mass energy density. However, saturation pressure increases non linearly with temperature, which increases the mass of the storage vessel. There is likely a sweet spot that balances energy density against stage dead weight for optimum performance. I have no idea where the sweet spot is.
The stage itself never has to leave the ground. It could be used to accelerate an upper stage on a rocket sled travelling up a slope. Even relatively modest launch assist velocity can have a large impact on payload to orbit. Additionally, the Starship lower stage is a substantial fraction of the capital cost of the rocket. So launch assist using a rapidly rechargeable propulsion system could have a sizable impact on launch costs.
Last edited by Calliban (2025-01-21 03:05:33)
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
Offline
Like button can go here