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28

There is Carnot's theorem for the theoretical maximum efficiency of heat engines. It is valid not only for mechanical engines like steam engines or Stirling engines but also for solid state devices like the thermocouples used in RTGs. The Carnot efficiency depends on the upper and lower working temperature. $$ \eta = 1 - \frac {T_c}{T_h} $$ Tc is the cold ...


12

All heat engines, whether mechanical or solid state, produce work based on heat flow across a temperature difference. The maximum efficiency of a heat engine depends on how large that difference is.


7

Solar Irradiance Mars has seasons, just like Earth. While Mars weather is significantly less interesting than Earth's, due to the thinner atmosphere, the average temperature difference between summer and winter can be more than 50 C. The difference between daytime high and low can be more than 120 C. If we consider the three forms of heat transfer, we can ...


6

Yes, it can. There's also a proposed hybrid nuclear-electric system, described here, where hydrogen is run through the reactor and heated to drive a turbine to produce electricity, then run through a second time to be heated for exhaust as in the basic nuclear-thermal rocket, then further boosted with an electromagnetic accelerator stage.


5

To generate power from a turbine, you need a temperature gradient. Using a nuclear reactor as a heat source is straightforward enough, but you would then need to reject that heat after it has passed through the steam turbine. Terrestrial nuclear power stations have enormous evaporative cooling towers for this purpose. For long-duration space missions, you'...


5

Shuttle used both AC and DC power. DC, because that is what the fuel cell power plants generated, and AC, to enable the use of compact three-phase electrical motors. The AC was generated from the DC using inverters. For details on the shuttle electrical power system, read section 2.8 of the Shuttle Crew Operations Manual and/or the Electrical Power System ...


5

I'll elaborate further on some of the points in other answers and provide some additional background. To put it as simply as possible, a good white spacecraft paint such as AZ93 (ZnO pigment in a silicate binder) is very white in the visible where you eye responds. Also where most of the energy from the sun is located. So the white paint reflects sunlight ...


5

We assume a transmitter with 20 W on one side and a receiving antenna with 3 m diameter on the other side. When we increase the power from 20 W to 200 W, we have 10 times more power. We may reduce the area of the antenna to 1/10 and its diameter to 1/sqrt(10). Instead of 3 m we may use now 0.95 m. We need 2 KW instead of 20 W to reduce the diameter by 1/...


5

Each of the eight power channels originally shipped with six Nickel-Hydrogen Battery ORUs (Orbital Replacement Units), making 48 battery ORUs. These are gradually being replaced with Lithium Ion Battery ORUs, at a ratio of 1:2. Six channels have been completed to date, making the total number of active batteries 30 as of the date of this answer. By the ...


5

This is a bit of a breadcrumb trail, but page 2-99 of NASA MSC-01372-1 "Apollo Operations Handbook Extra Vehicular Mobility Unit" (March 1971) discusses the power used for Apollo 15-17: The dual mode is the normal operating position of the switch. In this mode, the EVC-2 transmits a 0.3- to 2.3-kHz voice signal and two interrange instrument group (IRIG) ...


4

Yes, part of NASA's function is public outreach. As such they will use units such as "Aircraft Carriers$^1$" "School Buses$^2$" and "horsepower$^3$" While not precise, expressing thrust in terms of horsepower notionally allows people to understand it more. 1: [...] horsepower [...] 2: [...] horsepower [...] 3: [...] horsepower [...]


4

You have it correct, but you are looking at a very narrow range of ISP. The BepiColombo mission to mercury uses an ion thruster with a ISP of 4,200 seconds(41,202 M/s). The same formula yields a mass ratio of 11.325. The NEXIS ion thruster can achieve 8000 seconds (78,200 m/s) The European DS4G innovative thruster manages 19,300 seconds (188,200 m/s) ...


4

To my knowledge no mission has attempted to generate electricity from Titan/Saturn's magnetic fields (or Jupiter's for that matter), a list of missions to-date includes: https://en.wikipedia.org/wiki/Pioneer_11 https://en.wikipedia.org/wiki/Voyager_1 https://en.wikipedia.org/wiki/Voyager_2 https://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens Planned ...


4

Are you burning the hydrogen in a fuel cell to get the power for an ion drive? I don't think that's wise. You'd get more delta/v using the hydrogen and oxygen in a conventional rocket. A nuclear reactor fuel mass I don't think would be prohibitive. According to this site, generating 200 MW for a year would require about 5 tons of fuel. You're bringing ...


4

This 2018 release from Georgia Tech describes one such - claimed to be more practical than previous methane-fueled cells because it operates at a lower temperature. It is a solid-oxide cell. It appears to work by catalytically cracking the methane to hydrogen, and it seems that the advance is in the cracking part. https://www.news.gatech.edu/2018/10/29/...


3

Yes, ion engines are somewhat of a false economy in the sense that they rely on electrical power on top of their mass and their propellant (e.g. Xenon). So, compared to a chemical rocket engine where engine mass, thrust, specific impulse, plus propellant (and the mass of the tanks to store it) is about everything you need to plan an hypothetical spaceship, ...


3

Batteries, fuel cells, radioisotope thermoelectric generators and solar panels provide DC. To get several different DC voltages AC and a transformer with several secondary windings is needed followed by a recifier and smoothing filter. A switching circuit converts DC to AC. A higher switching frequency allows the use of very small and lightweight ...


3

There are some examples of space reactors: 3kW@385kg: BES-5 5kW@1000kg: TOPAZ At your 24400kg weight budget that scales to 100-200kW electrical output. But these are thermionic(3% efficiency) and you may be able to do better with gas turbine (30%-50% efficiency), or not (secondary loop, turbine and generator machinery are heavy). Let's make a rough ...


2

One mission profile thought up for the VASIMR involves initially travelling closer to the sun to take advantage of the stronger solar flux to run the engine at high power, releasing a payload on a long coasting trajectory and slowing the launching vehicle back down to return to Earth. The idea is still a little half-baked at present (the engine performance ...


2

Selecting the ideal propulsion system for a mission is closely tight to your requirements. You are typically interested in providing a given $\Delta V$ for a vehicle using an amount of mass (both the dry mass of the propulsion system $m_d$ and propellant $m_p$) and power $P$ over a period of time $\Delta t$. Considering these parameters you select the ...


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