Timothy Jones on 3 Mar 2017 05:59:20 -0800

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Re: [PLUG] "Nearby"

Once you reach a certain speed in space, unless you encounter a significant dust cloud or gravitational body, you won't slow down. In Earth's atmosphere you need constant thrust to compensate for air drag. In space, there is no drag force so once you accelerate to a certain speed and then stop firing your engines, you may end up going that speed forever and ever. This is why space probes that are meant to enter orbit around a planet apply breaking maneuvers (dipping into the upper atmosphere to slow via drag) and/or reverse thrust (for example, the Japanese probe Akatsuki failed to enter orbit around Venus because its reverse thrust engines didn't fire long enough to slow it down). So the good news is that you don't need a ton of fuel for an interstellar trip. New Horizons didn't enter orbit around Pluto because having enough fuel to slow it down would have made the mission very expensive as it used gravitational assist around Jupiter in addition to its original thrust to make it to Pluto. New Horizons had enough fuel to be directed to another body beyond Pluto, but after that flyby it will head off to interstellar space and there is nothing we can do now to prevent it from doing so. Voyager 1, Voyager 2, Pioneer 10, Pioneer 11 and New Horizons are all probes which will leave our solar system, of which Voyager 1 is already considered to be in interstellar space. No extra fuel is needed once it reaches enough speed to escape the gravitational pull of our solar system. So any generation ship sent on an interstellar mission would likely need a huge amount of fuel to accelerate the large mass of the ship, crew ,and supplies, but not an impossibly huge amount, and it would use a gravity assist from Jupiter to accelerate it even further. Here's the bigger problem though: slowing the ship down upon reaching the destination star. TRAPPIST-1 has a much weaker gravitational field than our star does, and a generation ship would need more much more fuel than it used to leave our solar system to reverse-thrust and get trapped in orbit around that star. However, between the time of their initial engine firings to accelerate fast enough to get out of our system, and the time they would need to start reverse thrust to obtain orbit around TRAPPIST-1, they wouldn't need to use a single drop of fuel except perhaps for occasional short trajectory corrections every few thousand years. 

On Fri, Mar 3, 2017 at 8:16 AM, Rich Freeman <r-plug@thefreemanclan.net> wrote:
On Fri, Mar 3, 2017 at 1:53 AM, Steve Litt <slitt@troubleshooters.com> wrote:
> On Wed, 22 Feb 2017 15:25:11 -0500
> bergman@merctech.com wrote:
>> In the message dated: Wed, 22 Feb 2017 15:11:00 -0500,
>> The pithy ruminations from Walt Mankowski on
>> <Re: [PLUG] "Nearby"> were:
>> => Sure, given current technology we're not going to get there anytime
>> => soon, but on the scale of the universe, it's practically next
>> door.
>> If the Apollo 11 crew had headed to those planets instead of the moon,
>> they'd have gotten there, done their 21.5 hours of exploration and now
>> they'd be 20% of their way home already.
> Either you began with a different set of numbers than I, or one of us
> slipped a decimal point.
> Lightspeed = 186,000mi/second = 669600000mi/hr
> Apollo speed = 24,000mi/hr
> ^^^^^^^^^^^^^^^
> https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=fastest+spacecraft&*
> Lightspeed/Apollospeed = 669600000/24000 = 27900 (constant)
> Light earth to 40lightyear time = 40 years
> Apollo to 40lightyear time = 40years * 27900 = 1116000 years

I haven't run the numbers, but these sorts of arguments tend to be
based on something like taking the thrust of a conventional spacecraft
and assuming that it was just applied continuously throughout the
entire trip.  I'm not sure if they even bother to account for the mass
of fuel needed to accomodate that.

Sure, if you accelerate constantly at 1G (which is a perfectly
reasonable figure for a conventional spacecraft) and keep that up for
decades you can travel to other stars in timelines that seem
reasonable (though, going 40 light-years in 50 years isn't going to

The problem is that conventional spacecraft do not carry anywhere near
the fuel needed to run their engines for decades, and their
acceleration would of course be miniscule if the same engines actually
had to push all that mass.  You'd need to scale up the entire
spacecraft to make it work.  You'd also need a bazillion stages with
something like an Apollo-style engine because the specific impulse of
the engine is "low"  (maybe "normal" is the better word, as opposed to
"exotic").  If you just make a single stage bigger and bigger the
total delta-v it changes asymptotically approaches a limit based on
its specific impulse, because you end up adding mass in the form of
fuel to keep accelerating it just as quickly as you're getting the
benefit out of that fuel.

And that is setting aside other issues like life support, reliability,
long-term exposure to radiation, and all that other stuff.  I'm sure
the Apollo engines were a marvel of engineering even by today's
standards, but nobody intended them to run nonstop for a decade.

That is why proposed interstellar ships tend to have fairly radical designs.

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