4 ms·

You're describing a brachistochrone trajectory. The "magic" in sci-fi isn't the drugs, it's rocket engines that are actually that efficient. If you accelerate/d

You're describing a brachistochrone trajectory. The "magic" in sci-fi isn't the drugs, it's rocket engines that are actually that efficient. If you accelerate/decel at a sustained 1 g, you can make it to Mars in about 2 days.

zaroth 6y ago
> If you accelerate/decel at a sustained 1 g, you can make it to Mars in about 2 days. This is the key calculation where you realize that quick inter-planetary travel is totally within the realm of physics. > Using Days and AU (astronomical units) we can see 3 days will get about 2.5 AU (halfway to Jupiter). 4.5 days will get you 5 AU (halfway to Saturn). 9 days will get you 20 AU (more than halfway to the Kuiper belt) Obviously inter-stellar travel seems like a totally different matter, but actually at 1g constant acceleration it works out to about “1 year + the number of light years” to an outside observer. Less time passes for the traveler. Invent an engine efficient enough to allow constant 1g acceleration for years at a time and humans really could become interstellar. The best ion engines get around 200,000 Isp. If you had a 500t ship burning 1t of mass per day, you would need an Isp of ~4.5m to achieve 1g over that first day (it gets easier as you get lighter). The theoretical limit (ejecting mass out the back at 1c) for specific impulse is 300 million. https://space.stackexchange.com/questions/840/how-fast-will-1g-get-you-there https://space.stackexchange.com/questions/840/how-fast-will-...
CubicsRube 6y ago
At first I was like: Nah, it can't be right... High-school math, napkin, and 5 minutes later: Wow, it's only 42 hours!
ubermonkey 6y ago
Thanks for providing that term. I did not know it. I'm surprised to learn that Mars is (or can be) so "close" in terms of a sustained 1g acceleration.
AtlasBarfed 6y ago
We just need to work some more on the dark matter repulsion drive, or the parallel universe reaction mass extraction drive. Or, the "belch of god". Still figuring out what the belch or the god part of that one actually are. Or you fire off refueling packs ahead of the trip.

zaroth 6y ago

> If you accelerate/decel at a sustained 1 g, you can make it to Mars in about 2 days. This is the key calculation where you realize that quick inter-planetary travel is totally within the realm of physics. > Using Days and AU (astronomical units) we can see 3 days will get about 2.5 AU (halfway to Jupiter). 4.5 days will get you 5 AU (halfway to Saturn). 9 days will get you 20 AU (more than halfway to the Kuiper belt) Obviously inter-stellar travel seems like a totally different matter, but actually at 1g constant acceleration it works out to about “1 year + the number of light years” to an outside observer. Less time passes for the traveler. Invent an engine efficient enough to allow constant 1g acceleration for years at a time and humans really could become interstellar. The best ion engines get around 200,000 Isp. If you had a 500t ship burning 1t of mass per day, you would need an Isp of ~4.5m to achieve 1g over that first day (it gets easier as you get lighter). The theoretical limit (ejecting mass out the back at 1c) for specific impulse is 300 million. https://space.stackexchange.com/questions/840/how-fast-will-1g-get-you-there https://space.stackexchange.com/questions/840/how-fast-will-...

CubicsRube 6y ago

At first I was like: Nah, it can't be right... High-school math, napkin, and 5 minutes later: Wow, it's only 42 hours!

ubermonkey 6y ago

Thanks for providing that term. I did not know it. I'm surprised to learn that Mars is (or can be) so "close" in terms of a sustained 1g acceleration.

AtlasBarfed 6y ago

We just need to work some more on the dark matter repulsion drive, or the parallel universe reaction mass extraction drive. Or, the "belch of god". Still figuring out what the belch or the god part of that one actually are. Or you fire off refueling packs ahead of the trip.