Posted by rbanffy 10/26/2024
“The paper defined an ideal power plan that can output 1 kW per kg of weight.
This is currently well outside the realm of possibility, with the best ion thruster power sources coming at something like 10 W per kg and even nuclear electric propulsion systems outputting 100 W per kg.”
I wished I remember where I read about this engineering problem, because it's an entertaining one. The main constraint on your [kW/kg] past a certain point is heat dissipation—the mass of the radiators rejecting waste heat into the vacuum of space. Thermal radiation scales like [temperature^4] (a very fast-growing function), so that parameter's obvious—you have to scale your exhaust temperature as high as you can engineer. That's how you shrink the radiator size. And you still need to run a heat engine—you need a significant temperature gradient, on top of the already-high exhaust temperature, to get useful work out of it. That's the temperature output of your primary heat source. So, the high-level design solution is: you have an array of infrared radiators glowing red, and a nuclear fission reactor glowing orange. That's the way to get a high power/mass ratio in space.
Also, everything's built around pipes of molten metals of different species (optimized heat transport), and the heat engine is like a steam turbine that spins on boiling molten potassium metal. (I think?) They're really exciting-looking machines. I wish someone would build one!
Or to put it another way, nuclear-thermal rockets might have an Isp of 3000 or so, which is amazing compared to a chemical rocket which might have 1/10 of that ... but an ion engine can have an Isp of 30000-70000, which is what makes a 550-AU trip even remotely practical.
I don't have the math right now to estimate how hot a conventional combustion chamber would have to be to get ion-engine-like exhaust velocities through a rocket nozzle/bell, but I think the answer is probably "ridiculously hot".
https://en.wikipedia.org/wiki/De_Laval_nozzle#Exhaust_gas_ve...
(I'm assuming hydrogen at 200,000 K becomes fully dissociated atomic hydrogen radicals, which act as a helium-like, monoatomic ideal gas—γ = 5/3).
If we beef up the chemical stage, e.g. by launching on Starship and re-fuelling in LEO, can we make do with 100 or even 10 kW/kg?
(Also, to put 550 AU in perspective, Voyager 1 is 165 AU out [1]. At 38,000 mph Voyager 1 [2] travels about 3.6 AU/year [a]. Going straight out, it would reach the Solar gravitational lens in 2131 [b].
[1] https://science.nasa.gov/mission/voyager/where-are-voyager-1...
[2] https://en.m.wikipedia.org/wiki/Astronomical_unit
[a] (38026.77 x 24 x 365) / (9.2956 x 10^7)
[b] 2024 + (550 - 165) / a
We’re not getting to 550 AU with chemical rockets alone. Nuclear, ion and/or solar sails will be needed.
This is pure science fiction, with no real practical benefit.
This is super exciting and needs to be made a higher priority than it is considering our only other way around this is to build gigantic telescopes far beyond our current capabilities.
Transmitting power is not a new idea: lasers are the go-to example for this. Powering the craft with solar energy is another theoretical way of doing it.
My idea on the other hand is different. Imagine spooling a long wire behind the space ship and just transmit electricity to it the same way you transmit power to your hoover. Except instead of sending power up a wire, send it up as bolts of lighting through the ionised gas trail your ship is trailing behind it.
You have a small spacecraft (USB memory stick sized). Do a sub-orbital launch to get it out of the atmosphere. Then deploy the solar sails and fire ground based lasers at the craft to accelerate it. IIRC the theoretical maximum acceleration gets a you to 550AU in a few months.
https://www.space.com/laser-propelled-spaceships-solar-syste...
I can however imagine a proper ion-thrusted satellite with enough Antenna gain to communicate from 550AU back with real science.
When Kennedy committed us to go to the moon, the first American hadn’t even orbited the Earth.
Previous generations just used to get shit done.
One order of magnitude in propulsion.
When Kennedy made his “We choose to go to the Moon” speech [1], our most powerful rocket was the Saturn I. Its H-1 engines thrusted at 200k lbf [2]. The Saturn V’s F-1s did 1.5mm lbf [3]. (The Saturn V, similarly, could lift an order of magnitude more mass to LEO than the Saturn I.)
It wouldn’t surprise me to find 100x increases in some material’s performance, et cetera.
[1] https://en.m.wikipedia.org/wiki/We_choose_to_go_to_the_Moon
Saturn V had a specific impulse of 263 s [1]. NSTAR did 3,000 s [2]. That’s an order of magnitude improvement in efficiency in 30 years of low-effort improvements.
Starship should demonstrate in-orbit refuelling next year. That’s another 10x technology. Add on a solar sail and you’re in the realm of two orders of magnitude of efficiency gains with known technologies. (Three from Apollo.)
[1] https://en.m.wikipedia.org/wiki/Saturn_V
[2] https://en.m.wikipedia.org/wiki/NASA_Solar_Technology_Applic...
The first Apollo test (unfueled) had three astronauts die due to an electrical fire! At the time Apollo 10 launched the lunar module wasn't finished yet (they wouldn't have been able to get back from the moon because it was too heavy). Apollo 16 and 17 by chance missed the 1972 solar storm by months - if an astronaut had been outside the Earth's magnetic field during the storm they have received a potentially lethal dose of radiation. One reason why after Apollo 17 the rest of the flights were cancelled even though the rockets etc. were already ordered or even built and why it took until 1981 (apart from Apollo–Soyuz) to get humans into space again.
Of course, I am an optimist, but one cannot relate historical circumstances in the same way. I will be glad if it does happen of course, but I do not expect it to be so based on past performance.
Here's a list of them, I'm too tired at the moment to figure out which one it is specifically.
>2.5% of the US GDP ($26 trillion in 2023) would be 600 billion. At its peak in 1967, the Apollo program budget was 3 billion, while the US GDP was about 850 billion. So 0.35 percent. US government spending in 1967 was about 112 billion, so closer to 2.5 percent of the federal budget, not the GDP. Converting to today’s 6,000 billion federal budget, about 150 billion today, or not quite 20% of the defense budget, the largest federal expenditure after Social Security (the defense budget is essentially tied with Medicare).
I’m not sure we want those sort of expenses anymore.
Personally, I'd rather fix healthcare if we're going to spend political capital. https://youtu.be/goh2x_G0ct4 remains as relevant as ever
It's not a generational thing, it's that the Moon landing was a top priority Cold War effort to beat the Soviets and show that Capitalism is the best. This mission in the other hand would be neat but has limited political value. What money were are willing to spend on space will mostly be spent on having a permanent moon base before the Chinese.
Or so it seemed.
> will mostly be spent on having a permanent moon base before the Chinese.
At least it could be launched from the Moon with a magnetic rail in addition to whatever extra propulsion it could carry onboard.
The term 'capitalism' is often used as a smear by the adherents of 'opposing' ideologies like socialism and communism so let's agree on a definition using Britannica Money's example [1]: capitalism, economic system, dominant in the Western world since the breakup of feudalism, in which most means of production are privately owned and production is guided and income distributed largely through the operation of markets..
Given this definition and weighing the positives and negatives it still seems to be the best system, something which I do not see changing as long as humans remain in control of society.
Do you have any examples which show where another system has been proven to be superior at a large scale? That - scale - is an important factor here since there is a direct relation between the scale of the group and the applicability of economic systems.
I agree. Humans can be bad, and bad people can participate in markets.
Until you get rid of bad people (that's a scary thought on what that would entail), all forms of economic systems can be used for bad things.
Especially after 2000, there has been an extreme consolidation in almost all markets, so that they are controlled by quasi-monopolies and the operation of those markets resembles more and more every year with the economies of the former socialist countries.
The economies of the former socialist countries were pretty much identical with the end stage of evolution of the capitalist markets, when a true monopoly controls each market.
When the dominance of Russia has collapsed after 1990, for about a decade there was a huge hope of improvements that would lead to economies everywhere functioning according to the ideal "free markets" theories. However, to the dismay of those liberated from the Russian influences, the Western countries have evolved since then to resemble more and more the systems that they were formerly criticizing, not only in the monopolistic markets, but also in mass surveillance, whistleblower punishments, great discrepancies between what the politicians say and what they do, politically-controlled Supreme Courts of Justice and so on.
SPOILER ALERT
lindburg just shows up unanounced at the cape ,security did not call ahead just escorted him up to the main deck everything stops,he hangs out for a bit,heads on his way and then they get back to work
there might not even be a photo,and so you have to trust that it happened,and in that is a large part of how shit got done,on trust
Just like 20 years ago.
Admittedly it's hypothetical and no one's built one but it would be quite similar to the thing Helion is building which is theoretically supposed to make electricity this year. Although many are skeptical about that.
There's a photo here of an earlier Helion thing https://www.linkedin.com/posts/helion-energy_first-frc-in-ou...
(Caution, many spoilers in the Wikipedia article) https://en.wikipedia.org/wiki/Project_Hail_Mary
It’s not as if our species doesn’t have millennia of experience with sailing ships to compare to when evaluating how to manage navigating to a specific point using either a powered or sailing vessel, or decades of experience maneuvering spacecraft at high velocities with high precision using both thrusters and gravitational slingshots within the solar system.
It might even be possible to use a modulated retroreflector on the craft-- e.g. we fire a high power low divergence beam at the craft and it reflect it back. By modulating the reflection in can send back data with practically no power usage.
The angles accepted by the reflector could be fairly hard and so the difficult pointing problem would only exist on our end.