Re: [PLUG] "Nearby"

[christopher barry <christopher.r.barry@gmail.com>]

On Fri, 3 Mar 2017 08:59:14 -0500
Timothy Jones <ti.do.jo@gmail.com> wrote:

> 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

you mean vger?

> 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,

or microwaves...
https://en.wikipedia.org/wiki/RF_resonant_cavity_thruster

> 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
> > happen).
> >
> > 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.
> >
> > --
> > Rich
> > ____________________________________________________________
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