wumpus

Depending on your values for "never", you need either Earth escape velocity or Solar escape velocity. If you stick to Earth escape velocity, expect the arrow to return to Earth within 100s-1000s of years. Solar escape velocity means there won't be an Earth if the arrow ever gets near the brown dwarf that used to be the Sun (and it almost certainly will be collected by some other star system before that). File this under "things I didn't really understand before playing KSP".

Thus the invention of the crossbow. Oddly enough, when I was taking archery merit badge (and thus ages ago), my state required all crossbows to be registered (but not rifles). Either the powers that be were afraid of a weapon that is relatively silent, [soft] armor piercing, and has zero muzzle flash (but you can follow the tail of the arrow back to the crossbowman); or they simply could regulate crossbows while rifles had historical, legal, and political defenses. Obviously crossbows are obsolescent if not obsolete (and a rate of fire worse than a muzzleloading rifle/musket). But often easier to manufacture than rifles.

Here's one that goes to great lengths to accurately simulate an English longbow (war bow) vs. French armor at the battle of Agincourt. https://www.youtube.com/watch?v=DBxdTkddHaE&t=1555s WARNING: it is 30 minutes long (largely because they want to point out just how far they went to make it as accurate as possible). Spoiler: the arrows don't penetrate the armor (breastplate, anyway). Soft body armor simply isn't good for arrows. Hard metal trauma plates will certainly stop it, and are overkill. Not surprisingly, you want armor much closer to something Sir Lancelot might have worn (well more like be illustrated in, the legend probably predates plate armor) than anything issued to cops. Even ancient shields appear to allow arrows to significantly penetrate, and use a metal "boss" to protect the hand that holds it. https://www.youtube.com/watch?v=y6IlEUm_Eo4&t=336s (the video uses a shield strapped to the arm, but shows why boss grips might be more popular).

Or an even thinner protective coating further away. This is the preferred means of protecting against tiny, high energy collisions and simply lets the "protective" coating get a tiny hole punched through it while the tiny object hit is spread out over a large area. Unfortunately for those traveling at >>.2c, your "tiny" object is often individual hydrogen atoms (or possibly molecules, not sure). Don't expect it to work well here. And your "thick protective coating" will have the same issues, but might possibly work ok as multiple thin layers presumably in hopes of deflecting the impact away from the ship.

At what point do you come to the conclusion that one of the providers *is* the issue?

That's not required to make potassium nitrate anymore...

There would be funding, but you wouldn't start to make a profit until you produced all your return propellant on Mars and cutting costs in similar fashion. And even then you'd probably be too deep in debt to ever make a profit on those things. NASA is planning some sort of "Mars rock return" mission, don't even think of looking at the price tag vs. the total weight of cargo returned. 16th Century Spain had effectively infinite amounts of gold and silver thanks to the new world. But the biggest effect of all that "wealth" was to crash their economy. So even if Mars had all that it would be of limited use (although industrial uses for gold and platinum are probably worth it).

Except that the asteroid belt isn't sitting deep in a gravity well, so once you mine an asteroid you can easily ship it where needed. Granted, this is going to require either expensive ore or cheap robotics (technically near-zero interest rates would help, but that's almost certainly an illusion or hidden subsidy), because the *time* till your return on investment is going to be brutal. It is entirely possible that you might send multiple probes out to locate ideal asteroids, unfurl a solar sail, reverse slingshot around Jupiter to Venus, reverse slingshot at venus, and finally capture at Earth. Some of those asteroids would effectively break the current market for that ore, and only be valuable as either some sort of monopoly (like DeBeers) or finding new uses for said ore (like having platinum catalysts available everywhere, or replace copper conductors with gold). - never heard of a golden asteroid, but you *know* you would get investors to go grab it. It wouldn't matter how irrational, you'd get investors.

You had to serve in the *government*. This should have been clear in the book and was repeated by RAH plenty of times. Of course, if you sign a hitch in the government, there is always the chance that after your physical you will get slotted as 11b (infantryman MOS) instead of census records sysadmin. The bit about limiting a franchise to those willing to fight was quite true, just limiting it to those who actually fought. RAH seemed to like the idea of a "opt-in" social contract, often with areas designated for those who "op-out". Requiring some sort of service beyond "opting in" goes even further. The Old Ones. And I'm pretty sure the Old Ones were the ones that destroyed the planet formerly between Mars and Jupiter. The living were neither consulted nor likely needed. - Note I don't recommend taking his post-stroke attempts to unify works meant to stand on their own. And as far as I can tell, it seemed like a short-lived fad among old authors at the time. PPS: For a better way to counter Starship Troopers, see Joe Haldeman's Forever War. Same basic book, except one was written by a career naval officer who was kick out due to a medical condition before Pearl Harbor, the other was drafted into Viet Nam, and set off airport metal detectors thanks to carrying too much sharpnel even before 9/11.

From your linked article This sounds like what you are asking for. Looking up the footnote we get: https://www.amazon.com/Classical-Mechanics-Modern-Perspective-2nd/dp/0070037345 And checking this link: https://www.worldcat.org/title/classical-mechanics-a-modern-perspective/oclc/357081 I found it was supposed to be in most nearby colleges with pretensions of teaching physics. Unfortunately, only one of them had the book on the shelves, but the nearest one (admittedly in a different city) had two copies. YMMV.

True, and after reading it I have to assume that it is entirely based on beamed power, although it looks like you are supposed to decelerate via a solar sail receiving Earth-based power. My guess is that you are supposed to somehow drag an even larger mirror (presumably mylar or similar) and kick it slightly ahead of you before stopping. Hopefully the laser will stop the spacecraft before the mirror goes too far past the Alpha Centauri system as it will be traveling significantly higher than .4c (how do you protect mylar from spacedust traveling at >>.2c?). Perhaps a series of mirrors? There is a throwaway line suggesting that the lasers might be powered by anti-matter. Should you have such magic available, there are far, far easier means than using Terawatt lasers. It also isn't clear where the lasers are stationed. My guess is some sort of body of ice or rock would make sense (for a heatsink), but without any atmosphere (to block the laser). I'm guessing a large asteroid, possibly with rotation manually stopped and under human control. Again, the real question is if you have other uses for the >>26TW power supply (and presumably the laser).

Drop the idea of using crewed interstellar spacecraft, unless you are serious about life imprisonment for the crew. No existing tech will work. Sorry, haven't had time to go over the full paper. Maybe in a day or so. Solar sails: By definition you can't significantly exceed Solar escape velocity with a solar sail. Assuming you had to spiral out with a solar sail, you won't get significant braking with a solar sail. And of course, all available sail should be used for both leaving and braking (in reality this means you ditch the solar sail when leaving the solar system, and don't have any available sail for braking). Having a 6TW laser in space largely depends on what you would do with the even more powerful power supply once the starship leaves. Photon rocket: Infinite Isp, but nearly the worst possible energy efficiency known to man. Expect to carefully design your radiators to assist in selectively radiating black body radiation in the correct way. Also expect to use them to start braking *immediately* on exiting the solar system if you want to brake in time... On the other hand, this might be useful in an uncrewed system that would continue to accelerate until the Americanium RTG expired (a century or two), but otherwise shouldn't be taken seriously. Fusion rocket: pretty much necessary and doesn't require "solved fusion power generation". Still a lot of work needs to be done to make it happen.

Was liquid hydrogen *that* hard to work with that Glushko preferred his exotic hypergolics, or was he only interested in missile engines and demanded a high shelf life for all his fuels? Hydrolox typically beats all but nukes on that list.

Wasn't it Apollo-Soyuz where the Apollo spacecraft had a hypergolic (not sure fuel or oxidizer) leak that injured the crew? I imagine a generation of Russian spacecraft designers heard a whole lot of how bad hypergolics were in the crew stages. Seeing how much Spacex uses nitrogen RCS (instead of hypergolics), my guess is that Elon has a problem with them as well (probably having everyone involved in physically preparing for reuse being HAZMAT certified and all that PPE). Some things just call out for hypergolics, and LES (and LEM) call out for them.

That's one option. One is to simply get enough delta-v in enough atmosphere so the extra .5-1.0 km/s won't kill you (but yes, you really want the high Isp last and the low Isp first, this is a bit of a kludge). Or you could have a low-thrust methlox engine and vacuum bell to get you the final kick (possibly tuned, RS-25 style to use for landing. Somehow I doubt that an airbreathing rocket works well upside down in an atmosphere). Or you could simply ignore the whole issue and stick with two stages. I suspect that making a spacecraft that can be mated together inexpensively might be easier to solve than SSTO.

"Interesting you will be shot out of it by ridiculous speed." I'd assume that the exit speed depends entirely on the ratio between the distance of the last tick of time before you reached the center vs. the last time after you reach the center. If the later click was closer, you will presumably slow down and get a second (and possibly third or more) try to have your "final acceleration" higher than the "first decceleration". Does Kerbol's interior have an atmosphere?

You'd need to throttle if you wanted a retropropulsive SSTO landing, and you'd still only need about 25% throttle (basically the inverse of your mass fraction, maybe less depending on how hard you want to hoverslam). And of course if the thing is crew-rated, you'd have to keep TWR from creeping much past 3 (but that's far less throttle than needed for a hoverslam, somewhere between 50%-75% and probably something the RS-25 can do). Even if it can't SSTO, I have to wonder how hard (assuming the engines were already developed) it would be to design a booster to lift Starship (or the upper stages of New Armstrong or similar) and return to launch. Certainly you'd need the 10% throttle (possibly going the way of Merlin and only using a single engine), but it would allow a heavier booster to still be efficient and maintain costs (assuming that mating them together each time didn't break the bank).

600s seems a little low for a SSTO candidate, but considering GNOM had 550s I suspect that most of the advantage is from air augmentation. Perhaps they are using intake air as oxidizer, but it can't be much with that Isp (much, much less than X-43). The unavoidable problem with a SSTO is that you essentially subtract the mass of your high-thrust booster engines (plus their propellant tanks) from your final payload (with traditional engines this means a trivial payload, with 600s Isp it might be enough to matter). It isn't clear how light and powerful these engines will be.

And once you cross the event horizon, all physics is undefined. "Reaching the event horizon" pretty much has to mean "reaching the black hole", because after that you might as well argue how many angels dance on the head of a pin*. You need a pretty massive black hole to survive tidal forces just to get to the event horizon (no idea about surviving Hawking radiation. Presumably your spacesuit has lots of shielding). Scott Manley (of course) has some interesting videos of what the requirements are to orbit a black hole. * To answer a related debate, I'd declare that God can indeed create an object so massive he himself can not lift it. Exhibit A: black holes. PS: I've been told that using the point where escape velocity = c is cheating and that you're supposed to tensor calculus and general relativity to calculate it, but ve=c works and is trivial (and I don't know the first thing about tensor calculus). Although according to Scott Manley, if your Ap is less than twice the radius of the event horizon you are absolutely doomed, and it is only a matter of time before you enter the black hole (no matter how much acceleration your spaceship has).

Don't forget the whole thrust issue for hydrolox first stages. On the other hand, Delta-IV appears to have a TWR similar to Falcon 9 (~1.3) and a much longer first stage burn time 245s vs 180s. So you can build a higher performance rocket than falcon 9 with hydrolox (and presumably make a profit with DoD/NASA pricing).

Atkin's laws of rocket design: 39. Any exploration program which "just happens" to include a new launch vehicle is, de facto, a launch vehicle program. https://spacecraft.ssl.umd.edu/akins_laws.html Making it a launch vehicle program simply gives it more ways to delay, and makes it that much easier for the Senate to take over the program. Also, the thing has consumed the lion's share of NASA's non-SLS budget. Do you really want to risk it on the maiden voyage of some "cheaper and faster" booster? There's a reason that SpaceX launched a wheel of cheese and Elon's old Tesla on the first flight.

Compared to full CDRs of all the landers and other non-SLS crewed vehicles? And you get to do it in full parallel? Sounds like the least of the schedule's issues. But I don't see a lot of construction upgrading the pad, so I imagine the 2024 schedule only gets lip service.

Earth's radius: 6,380km Earth's radius+LEO: 6,700km Half of Earth's circumference: 20,000km Half of Earth's circumference (at LEO): 21,300km So going low will save you less than 10% of your distance (10% is less than *sea level*), and require a slightly slower speed. I'd recommend at least staying out of the atmosphere for the duration. Probably closer to 90 degree, there's no reason to build your ICBMs further south than necessary*. Also assumes they are for going the quickest route, instead of lofting high and coming down fast (to avoid ABMs). * Brits threatening Argentina? That's the biggest cross-hemisphere danger I can think of, and it doesn't seem realistic.

Doesn't follow. Current terrestrial predators didn't have to compete with dino predators (possible exception: moas of New Zealand). Rodents have proven brutal for just about any ecosystem previously missing them, and odds are they would be harsh on dinos (although primitive rodent-like mammals coexisted with dinos for millions of years and didn't appear to thrive like you'd expect). Dino evolution: 165 million years (not including birds) Mammal evolution (post dino): 65 million years It is foolish to assume "newer == better", when the dinos had so much more time to optimize to their environment (but I wouldn't bet against rodents. They are especially lethal against modern dinos and responsible for plenty of bird extinction). Also, you'd probably want to bring a full suite of invasive species. It isn't so much that "invasive == dangerous", but if you throw a whole ecosystem's worth of species at a new ecosystem, you are bound to get a few species better adapted for at least few niches than existing ones.

I'm assuming that most of the space tourism folks look to the expansion of the air industry for examples of progression (and how to pay for what they want to do). The earliest flyers kept there planes running by barnstorming and taking up passengers for a quick flight. This looks like the stage we are in. Mail transport is probably what really got the ball started, but that isn't an option here (although historians are likely to show parallels between early air mail and communication satellites). Actual passenger airlines took awhile, had to compete with equally glamorous oceanliners, and were rather dangerous (think old Ford Tri-motors). Presumably Starship could get the price under $10k, but that assumes that Starship is sufficiently ready to launch and land full of ordinary passengers. Something has to pay for all those flights until Starship (or New Armstrong or whatever) is ready to carry passengers.