Rich Freeman on 3 Mar 2017 05:16:19 -0800

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Re: [PLUG] "Nearby"

On Fri, Mar 3, 2017 at 1:53 AM, Steve Litt <> wrote:
> On Wed, 22 Feb 2017 15:25:11 -0500
> 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
> ^^^^^^^^^^^^^^^
> 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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