"Tim Attwood" <tim### [at] comcastnet> wrote:
> I'm of the opinion that a large enough power source, such as
> a nuclear reactor, could produce enough electric power to fuel a large
> ion thuster. If a thrust to power ratio of around 50 mN/kW is assumed, and
> a total ship weight of 10 tons then we're looking for a power source of
> about 2 gigawatts. This would let a ship accelerate a little over 1G for
> extended periods. Such a ship could make a round trip to Alpha Centari A in
> about 9 years. If such a vehicle was unmanned then little of the radiation
> shielding would be needed, which is what turns a reactor into a big building
> with 6ft thick cement walls.

Does this estimate take relativistics effects into account?  (I've long
forgotten how to do the math.)  A vehicle that gets to Alpha cen in 4.5
years has to go pretty close to the speed of light.  The relativistic mass
gain would surely affect the thruster's ability to maintain acceleration.

Post a reply to this message

Cousin Ricky wrote:
> gain would surely affect the thruster's ability to maintain acceleration.

It depends on whether you measure the acceleration inside or outside the 
ship. The passengers can continue to experience 1G indefinitely.

-- 
   Darren New / San Diego, CA, USA (PST)
     Scruffitarianism - Where T-shirt, jeans,
     and a three-day beard are "Sunday Best."

Post a reply to this message

"Cousin Ricky" <ric### [at] yahoocom> wrote in message 
news:web.458015c7ff75702c85de7b680@news.povray.org...
> "Tim Attwood" <tim### [at] comcastnet> wrote:
>> I'm of the opinion that a large enough power source, such as
>> a nuclear reactor, could produce enough electric power to fuel a large
>> ion thuster. If a thrust to power ratio of around 50 mN/kW is assumed, 
>> and
>> a total ship weight of 10 tons then we're looking for a power source of
>> about 2 gigawatts. This would let a ship accelerate a little over 1G for
>> extended periods. Such a ship could make a round trip to Alpha Centari A 
>> in
>> about 9 years. If such a vehicle was unmanned then little of the 
>> radiation
>> shielding would be needed, which is what turns a reactor into a big 
>> building
>> with 6ft thick cement walls.
>
> Does this estimate take relativistics effects into account?  (I've long
> forgotten how to do the math.)  A vehicle that gets to Alpha cen in 4.5
> years has to go pretty close to the speed of light.  The relativistic mass
> gain would surely affect the thruster's ability to maintain acceleration.

You're thinking about this way too hard! ;-)

Jon

Post a reply to this message

"Jon S. Berndt" <jsb### [at] hal-pcorg> wrote:
> "Cousin Ricky" <ric### [at] yahoocom> wrote in message
> news:web.458015c7ff75702c85de7b680@news.povray.org...
> >
> > Does this estimate take relativistics effects into account?  (I've long
> > forgotten how to do the math.)  A vehicle that gets to Alpha cen in 4.5
> > years has to go pretty close to the speed of light.  The relativistic mass
> > gain would surely affect the thruster's ability to maintain acceleration.
>
> You're thinking about this way too hard! ;-)

Well, he *did* say "realistic."  It's details like this that cause
expensive, taxpayer-funded space probes to vanish without a trace.

Post a reply to this message

"Cousin Ricky" <ric### [at] yahoocom> wrote:
> "Jon S. Berndt" <jsb### [at] hal-pcorg> wrote:
> > "Cousin Ricky" <ric### [at] yahoocom> wrote in message
> > news:web.458015c7ff75702c85de7b680@news.povray.org...
> > >
> > > Does this estimate take relativistics effects into account?  (I've long
> > > forgotten how to do the math.)  A vehicle that gets to Alpha cen in 4.5
> > > years has to go pretty close to the speed of light.  The relativistic mass
> > > gain would surely affect the thruster's ability to maintain acceleration.
> >
> > You're thinking about this way too hard! ;-)
>
> Well, he *did* say "realistic."  It's details like this that cause
> expensive, taxpayer-funded space probes to vanish without a trace.


taxpayer-funded space probes to vanish without a trace.
Ok there was a trace :-)

Stephen

Post a reply to this message

"Stephen" <mcavoys_AT_aolDOT.com> schreef in bericht 
news:web.45810d28ff75702cf1cb1e660@news.povray.org...
 Ok there was a trace :-)
>
trace( ) ??  :-)

Thomas

Post a reply to this message

"Thomas de Groot" <t.d### [at] internlDOTnet> wrote:
> "Stephen" <mcavoys_AT_aolDOT.com> schreef in bericht
> news:web.45810d28ff75702cf1cb1e660@news.povray.org...
>  Ok there was a trace :-)
> >
> trace( ) ??  :-)
>
> Thomas




"Hanging is too good for a man who makes puns; he should be drawn and


"Puns are the last refuge of the witless."
"Immanuel doesn't pun; he Kant."


Stephen

Post a reply to this message

"Stephen" <mcavoys_AT_aolDOT.com> wrote:
> "Cousin Ricky" <ric### [at] yahoocom> wrote:
> > "Jon S. Berndt" <jsb### [at] hal-pcorg> wrote:
> > > "Cousin Ricky" <ric### [at] yahoocom> wrote in message
> > > news:web.458015c7ff75702c85de7b680@news.povray.org...
> > > >
> > > > Does this estimate take relativistics effects into account?  (I've long
> > > > forgotten how to do the math.)  A vehicle that gets to Alpha cen in 4.5
> > > > years has to go pretty close to the speed of light.  The relativistic mass
> > > > gain would surely affect the thruster's ability to maintain acceleration.
> > >
> > > You're thinking about this way too hard! ;-)
> >
> > Well, he *did* say "realistic."  It's details like this that cause
> > expensive, taxpayer-funded space probes to vanish without a trace.
>

> taxpayer-funded space probes to vanish without a trace.

don't forget to put povray units in the mix too and get strange results for
things really far too big and far away... thankfully, no tax paying... ;)

Post a reply to this message

> Does this estimate take relativistics effects into account?  (I've long
> forgotten how to do the math.)  A vehicle that gets to Alpha cen in 4.5
> years has to go pretty close to the speed of light.  The relativistic mass
> gain would surely affect the thruster's ability to maintain acceleration.

I think 9 years is more likely to be the one-way trip, assuming you
accelerate at 1g to halfway and then turn round and decelerate ass-first
the rest of the way. A round trip would be more like 16-18 years. The crew
would only see it as 7-8 years, of course.

This is based on this page:
http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html

I haven't checked the maths but it looks right!

The mass gain is irrelevant, really. From the ship's point of view,
acceleration is constant and the universe deforms. :-)

Bill

Post a reply to this message

"Stephen" <mcavoys_AT_aolDOT.com> wrote:
> "Cousin Ricky" <ric### [at] yahoocom> wrote:
> >
> > Well, he *did* say "realistic."  It's details like this that cause
> > expensive, taxpayer-funded space probes to vanish without a trace.
>
> No it’s getting Imperial and Metric units mixed up that costs
> taxpayer-funded space probes to vanish without a trace.
> Ok there was a trace :-)

Or relying on the atmospheric pressure for altimeter readings for when to
cut the landing thrusters (a possible fate of Beagle).

Or forgetting to take the Doppler effect into account in radio
communications.  This almost happened to Cassini-Huygens; luckily (or
prudently), the error was discovered during the dry run when Cassini
returned to Earth for a gravitational assist.

Or sending your lander into the kitchen without building it to stand the 740
kelvin and show grace under 9100 kilopascals (all of the early Veneris
missions).  (Kudos to the former Soviets as the only country to
successfully land on Venus, and the only country other than the USA to land
*anywhere* successfully.)

Or typing a period in a computer program where there should have been a
comma.  Er... OK, so it *could* have happened if NASA hadn't caught the bug
in time.  Unfortunately, they didn't catch the missing hyphen in Mariner 1's
backup guidance software...

And the most grievous error of all:  Sending your space ship to Mars!  After
Spirit and Opportunity successfully docked with the Angry Red Planet, NASA
convened a study to find out what went *right*. :-O

Post a reply to this message