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Back in 2017 as a new member of this forum, I was suggesting a "modular approach" for getting to Mars--using at the time, "throwaway modules," that were fueled by hypergolics of MMH and NTO. Then Elon came along with the news of the Raptor using liquid Methane and LOX, and a fully reusable system in what was then being called the BFR.
Listening to Eon this past Thursday evening and his talk about the now properly named "Starship," and his talk ending with a concept of exploration of the outer Solar System and beyond, I decided to revise my modular scheme to something even more exotic and probably more useful in the long run.
This is borrowing another concept from Buzz Aldrin, that of a "cycler," but only as a module in my modular system: a nuclear thermal powered interplanetary stage, one that is only a power provider or "tug" to get a vehicle boosted into Hohman orbits using nukes, and then slowing and parking the landing vehicle into an orbit around Mars. The the stage separates and goes dormant until needed again. This way we get the additional boost provided from a high Isp power source and avoid all the radioactive products in a planetary landing or planetary departure into orbit. The orbiting "power module" is simply waiting to be used again--provided the propulsive media of liquified gasses isn't needing replenishment. But since the patiently waiting power module is parked in a stable orbit--it can also be refueled robotically by planetary tanker rockets, and periodically be "topped off."
By not using the Starship onboard methane and LOX for generating the Earth departure delta V, and not being used to do a braking into Mars (or other planetary body) orbit, the fuel is available for a safer propulsive landing and retaining some fuel for planetary departure.
At this point in time, we're looking at an absolute requirement for ISRU in order to return to Earth, and worries about the gee forces on the bodies of returning astronauts coming in at high velocities with massive deceleration gee forces playing roles. Using a slower deceleration into an Earth orbit an easier time would obtain for returnees from deep space journeys.
Last edited by Oldfart1939 (2022-02-18 10:40:51)
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After additional consideration of the concept proposed in post #1 of this topic, it would possibly make sense to return the power module on a slow-boat trajectory to Earth after slowing the landing Starship in Mars orbit. The earliest Starships landing on Mars will probably NOT be returning to Earth, as they will be cargo ships and will be one-way vessels. In this way, the next Hohman transfer window would already have a flight-proven deep space booster in Earth orbit and with plenty of time for refilling the tanks with cryogens, so the number of power modules would be far less than Starships. Only after 2 flights, would one be used to return a crewed Starship back to Earth on it's 2nd return journey.
Last edited by Oldfart1939 (2022-02-18 10:46:56)
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For OF 1939
The article below is about development of chips able to convert about 40% of thermal energy to electricity, if the source temperature is great enough.
Happily a small fission reactor (such as those Calliban has been describing) can operate at the needed temperature.
A space craft fitted with a system of energy conversion based upon this chip could (I presume) employ the 40% of energy directly converted to electricity to power all the support services of the vessel, as well as a part of the propulsion.
However, 60 percent of the output of the reactor needs to go somewhere useful, so I am wondering if hydrogen gas (or even just plain water) might be enlisted as a propellant that draws off the excess thermal energy in a useful way.
Because water is so much easier to package for use as a propellant, it's lower efficiency as a propellant may be less of a drawback than might at first seem to be the case.
Is there anyone in the forum who can speak (authoritatively) to this question.
I bring it up because a huge repository of water ice is on it's way for a visit to the Solar system interior.
An atomic propulsion system that can make good use of just plain old water would be at an economic advantage over all other competitive systems.
https://www.yahoo.com/finance/news/ther … 51572.html
Engadget
Thermophotovoltaic cell converts 40 percent of heat energy to electricityMIT
Steve Dent
Steve Dent·Contributing Reporter
Thu, April 14, 2022, 6:10 AM
Researchers have revealed a new thermophotovoltaic (TPV) cell that converts heat to electricity with over 40 percent efficiency, performance nearly on par with traditional steam turbine power plants. The cells have the potential to be used in grid-scale "thermal batteries," generating energy dependably with no moving parts.Thermophotovoltaic cells work by heating semiconducting materials enough to significantly boost the energy of photons. At high enough energies, those photos can kick an electron across the material's "bandgap," generating electricity. So far, TPV cells have achieved up to just 32 percent efficiency because they operate at lower temperatures.
By contrast, the new design from MIT and the National Renewable Energy Laboratory (NREL) takes power from white-hot heat sources between 1,900 to 2,400 degree Celsius (3,452 to 4,352 degrees F). To do that, it uses "high-bandgap" metal alloys sitting over a slightly lower-bandgap alloy.
The high-bandgap layer captures the highest-energy photons from a heat source and converts them to electricity, while lower-energy photons pass through the first layer and add to the voltage. Any photons that run the two-layer gauntlet are reflected by a mirror back to the heat source to avoid wasting energy.
This is an absolutely critical step on the path to proliferate renewable energy and get to a fully decarbonized grid.
Measuring the efficiency using a heat flux sensor, the team found that power varied with temperature. Between 1,900 to 2,400 degrees Celsius, the new TPV design produced electricity with about 40 percent efficiency.
Steam turbines can deliver the same efficiency, but are far more complicated and restricted to lower temperatures. "One of the advantages of solid-state energy converters are that they can operate at higher temperatures with lower maintenance costs because they have no moving parts," MIT Professor Asegun Henry told MIT News. "They just sit there and reliably generate electricity."
In a grid-scale thermal battery, the system would absorb excess energy from renewable sources like the sun and store it in heavily insulated banks of hot graphite. When needed, the TPV cells could then convert that heat to electricity and send it to the power grid. The experimental cell was just a square centimeter, so the team would have to ramp that up to around 10,000 square feet for grid-level power, but the technology already exists to create cells on that scale, Henry notes.
"Thermophotovoltaic cells were the last key step toward demonstrating that thermal batteries are a viable concept," he said. "This is an absolutely critical step on the path to proliferate renewable energy and get to a fully decarbonized grid."
(th)
Last edited by tahanson43206 (2022-04-14 10:02:42)
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As a follow up, here is a Google snippet that puts water into perspective as a propellant
About 28,300,000 results (0.76 seconds)
Water at that temperature is steam, and acts as a reasonably good propellant by itself, though has a low ISP of about 161. As an example, it was the propellant for the Bell Rocket Belt.Apr 24, 2016Is it possible to energize a steam rocket enough to go to space?
https://space.stackexchange.com › questions › is-it-possibl...
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The trick is turning on and off the nuclear tug as required to give a starship or any ship that push over time that we require.
https://en.wikipedia.org/wiki/Nuclear_thermal_rocket
https://ntrs.nasa.gov/api/citations/201 … 033337.pdf
Nuclear Thermal Propulsion: An Overview of NASA Development Efforts
The rocket equation is still the enemy.
The last design reference mission was version 5 (DRM5) which contains a great deal of this discussion.4
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