NASA Planning A New Mars Rover

"Fred J. McCall" wrote in message
...
bob haller wrote:

On Dec 11, 9:34 am, Fred J. McCall wrote:

Bobbert, please go read up on various proposals for manned Mars
missions before you defecate further on the newsgroup...


yep they are mostly minimalist ideas, but growth must occur for
mission safety


Your ideas are not 'minimalist', Bobbert. Your ideas are 'stupid'.

--
"Some people get lost in thought because it's such unfamiliar
territory."
--G. Behn

Besides "stupid," I'd throw in the following: lunacy, unrealistic, and
idiotic. Better yet, Fred: all of the above.

In sci.space.policy message , Mon, 10 Dec 2012
18:39:42, Wayne Throop posted:


A quick botec, assuming the ion engine can manage 1/1000 g
continuous acceleration for at least a couple weeks,
suggests several months rather than a few centuries.

Botes are not needed; try
http://www.merlyn.demon.co.uk/astron-3.htm#DAT ff.

At a guess, Mars is typically at about 1.25AU from Earth; try 3 months,
with turnover, 0.001g, reaches 1.02 AU, Allow another fortnight, get
about 1.40 AU.

--
(c) John Stockton, nr London, UK. Mail via homepage. Turnpike v6.05 MIME.
Web http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms and links;
Astro stuff via astron-1.htm, gravity0.htm ; quotings.htm, pascal.htm, etc.
No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News.

On 12/11/2012 12:12 AM, Fred J. McCall wrote:
(Wayne Throop) wrote:

::: How you plan on getting it there? It's around five times more
::: massive than even the Curiosity spacecraft.

:: it doesnt have to go fast, even a realtively small ion engine could
:: get it there given time.

: Sure, if you can wait a few centuries.

A quick botec, assuming the ion engine can manage 1/1000 g
continuous acceleration for at least a couple weeks,
suggests several months rather than a few centuries.

What are your assumptions that lead to centuries?


That you have to use something close to what actually exists. To get
travel time down to what you're talking about requires continuous
thrust at 0.001g for the entire trip. And 0.001g out of an ion drive
is a preposterously high level for anything that exists (where thrust
levels run in the milliNewtons); not "a relatively small ion engine"
at all.

What are you talking about using for a power source for this
(relatively) huge ion drive you're talking about?


Here are a few computations:

A small ion thruster: NSTAR
http://www.grc.nasa.gov/WWW/ion/past/90s/nstar.htm
(to which we must add a 2.3 kW power source):
Thrust = 92 mN
The mass of the thruster including propellant etc. should be about 100 kg.

I don't have the mass of the NRO telescopes, it should be in the same
ball park as that of Hubble: 11,100 kg. I'm not sure whether a NRO
telescope has a 2.3 kW power source, but if you have to a few kg for
power, it doesn't change the overall picture much.

So a total mass of about 11,200 kg, together with a thrust of 92 mN
gives an acceleration of 8.2 micro-m/s.


So, how much delta-V do you need from LEO to reach Mars with an ion
thruster? Well, technically one could use a Hohmann minimal delta-V
transfer orbit by firing the ion thruster only for short periods of time
near the perigee. Fred's estimate of centuries to reach Mars, would then
be an understatement. If the ion-drive is used in the normal continuous
operation the delta-V is greater. I integrated the equations and I get
that from a circular orbit with orbital velocity V0, one neads to add V0
delta-V to escape (that is at the limit when the thrust of the ion drive
tends to 0). So from LEO, at 8 km/s one neads the
ion-drive to add 8 km/s to escape Earth's gravity well. You could do it
with a little less using lunar gravity assist, but let's keep it simple
for now.

Then from solar orbit at Earth's distance from the Sun, to reach solar
orbit at Mars' distance from the Sun we compute the delta-V by first
computing delta-V for escape, as above this is equal to Earth's orbital
velocity, which is 29.78 km/s. But from a solar orbit at Mars' distance
to escape the delta-V is 24.08 km/s, this delta-V does not need to be
provided since we don't want to go beyond Mars. So from Earth's orbit to
Mars' the delta-V needed is 29.78-24.08=5.7 km/s.

Finally, to get to low Mars orbit, let's say an orbit with an orbital
velocity of 2.6 km/s, the ion drive needs to add another 2.6 km/s.

The total delta-V is (8+5.7+2.6) km/s = 16.3 km/s.

With the 8.2 micro-m/s acceleration, this gives 16.3 km/s / 8.2
micro-m/s which is about 2 billion seconds, or a little less than 63
years. This could of course be reduced a lot by using lunar gravity
assist and aerobraking. So Fred's claim that centuries would be needed
is a little off, nonetheless, one would expect that more than a small
ion thruster would be used. With a large ion thruster the travel
duration becomes more reasonable.


Alain Fournier

In article ,
says...

On 12/11/2012 12:12 AM, Fred J. McCall wrote:
(Wayne Throop) wrote:
What are your assumptions that lead to centuries?

That you have to use something close to what actually exists. To get
travel time down to what you're talking about requires continuous
thrust at 0.001g for the entire trip. And 0.001g out of an ion drive
is a preposterously high level for anything that exists (where thrust
levels run in the milliNewtons); not "a relatively small ion engine"
at all.

What are you talking about using for a power source for this
(relatively) huge ion drive you're talking about?

Here are a few computations:

A small ion thruster: NSTAR
http://www.grc.nasa.gov/WWW/ion/past/90s/nstar.htm
(to which we must add a 2.3 kW power source):
Thrust = 92 mN
The mass of the thruster including propellant etc. should be about 100 kg.

I don't have the mass of the NRO telescopes, it should be in the same
ball park as that of Hubble: 11,100 kg. I'm not sure whether a NRO
telescope has a 2.3 kW power source, but if you have to a few kg for
power, it doesn't change the overall picture much.

Computations snipped to save space...

With the 8.2 micro-m/s acceleration, this gives 16.3 km/s / 8.2
micro-m/s which is about 2 billion seconds, or a little less than 63
years. This could of course be reduced a lot by using lunar gravity
assist and aerobraking. So Fred's claim that centuries would be needed
is a little off, nonetheless, one would expect that more than a small
ion thruster would be used. With a large ion thruster the travel
duration becomes more reasonable.

63 years is most certainly well beyond the design lifetime of the
components which make up such a telescope. A quick look at what failed
on Hubble over the years would give you an idea of the challenges you'd
face, but the task would seem to be quite daunting, especially for
moving components like the gyros needed to achieve extremely accurate
pointing.

Also, I doubt using "lunar gravity assist" and "aerobraking" would be
viable when you're talking about such a low thrust system. For
aerobraking especially, you need a relatively high thrust engine to fine
tune your velocity for multiple dips into the atmosphere. I seriously
doubt that the ion system would have the necessary thrust.

Finally, I'd be extremely leery of using aerobraking on a telescope that
you want to keep the inside (optics) pristine. How do you keep the
"nasties" of an atmosphere out of it, when you deliberately plan on
subjecting it to Mars atmosphere? Any scheme to do this will
necessarily add mass to the telescope.

All in all, it's just *not* a workable idea.

Jeff
--
"the perennial claim that hypersonic airbreathing propulsion would
magically make space launch cheaper is nonsense -- LOX is much cheaper
than advanced airbreathing engines, and so are the tanks to put it in
and the extra thrust to carry it." - Henry Spencer

On 12/20/2012 8:40 AM, Jeff Findley wrote:
In article ,
says...

On 12/11/2012 12:12 AM, Fred J. McCall wrote:
(Wayne Throop) wrote:
What are your assumptions that lead to centuries?

That you have to use something close to what actually exists. To get
travel time down to what you're talking about requires continuous
thrust at 0.001g for the entire trip. And 0.001g out of an ion drive
is a preposterously high level for anything that exists (where thrust
levels run in the milliNewtons); not "a relatively small ion engine"
at all.

What are you talking about using for a power source for this
(relatively) huge ion drive you're talking about?

Here are a few computations:

A small ion thruster: NSTAR
http://www.grc.nasa.gov/WWW/ion/past/90s/nstar.htm
(to which we must add a 2.3 kW power source):
Thrust = 92 mN
The mass of the thruster including propellant etc. should be about 100 kg.

I don't have the mass of the NRO telescopes, it should be in the same
ball park as that of Hubble: 11,100 kg. I'm not sure whether a NRO
telescope has a 2.3 kW power source, but if you have to a few kg for
power, it doesn't change the overall picture much.

Computations snipped to save space...

With the 8.2 micro-m/s acceleration, this gives 16.3 km/s / 8.2
micro-m/s which is about 2 billion seconds, or a little less than 63
years. This could of course be reduced a lot by using lunar gravity
assist and aerobraking. So Fred's claim that centuries would be needed
is a little off, nonetheless, one would expect that more than a small
ion thruster would be used. With a large ion thruster the travel
duration becomes more reasonable.

63 years is most certainly well beyond the design lifetime of the
components which make up such a telescope. A quick look at what failed
on Hubble over the years would give you an idea of the challenges you'd
face, but the task would seem to be quite daunting, especially for
moving components like the gyros needed to achieve extremely accurate
pointing.

I agree.

Also, I doubt using "lunar gravity assist" and "aerobraking" would be
viable when you're talking about such a low thrust system. For
aerobraking especially, you need a relatively high thrust engine to fine
tune your velocity for multiple dips into the atmosphere. I seriously
doubt that the ion system would have the necessary thrust.

You just need to be more precise and plan your gravity assist from
further away and make your dips into the atmosphere shallower. It would
be a challenge, but if your foolish enough to go for a 63 years mission
why not be foolish enough to ask for high precision?

Finally, I'd be extremely leery of using aerobraking on a telescope that
you want to keep the inside (optics) pristine. How do you keep the
"nasties" of an atmosphere out of it, when you deliberately plan on
subjecting it to Mars atmosphere? Any scheme to do this will
necessarily add mass to the telescope.

You may be right about this. But whatever atmosphere enters the
telescope will vent rapidly. I'd be more wary of tempereture cycling
during the aerobraking.

All in all, it's just *not* a workable idea.

Well, I certainly don't suggest to use an ion engine for such a mission.
A 63 years mission for this is insane. I just wanted to know how much
insane it is. Fred was claiming centuries for the mission. My
computations say that the mission would be insane, but a little less
insane than what Fred was saying. And if you drop the words "relatively
small" from the phrase "even a realtively small ion engine could" and
use a big ion engine you can actually get a workable mission, maybe
stupid, but workable. I see no reason why one wouldn't use a chemical
rocket for this.


Alain Fournier

On 12/20/2012 11:47 AM, Alain Fournier wrote:
On 12/20/2012 8:40 AM, Jeff Findley wrote:
In article ,
says...

On 12/11/2012 12:12 AM, Fred J. McCall wrote:
(Wayne Throop) wrote:
What are your assumptions that lead to centuries?

That you have to use something close to what actually exists. To get
travel time down to what you're talking about requires continuous
thrust at 0.001g for the entire trip. And 0.001g out of an ion drive
is a preposterously high level for anything that exists (where thrust
levels run in the milliNewtons); not "a relatively small ion engine"
at all.

What are you talking about using for a power source for this
(relatively) huge ion drive you're talking about?


Also, I doubt using "lunar gravity assist" and "aerobraking" would be
viable when you're talking about such a low thrust system. For
aerobraking especially, you need a relatively high thrust engine to fine
tune your velocity for multiple dips into the atmosphere. I seriously
doubt that the ion system would have the necessary thrust.

You just need to be more precise and plan your gravity assist from
further away and make your dips into the atmosphere shallower. It would
be a challenge, but if your foolish enough to go for a 63 years mission
why not be foolish enough to ask for high precision?

I thought about the lunar gravity assist again. Not only is it possible,
but it is just about compulsory. When you get to 300,000 km, the moon
will have an important effect on your trajectory. If you don't plan for
that effect to be positive, you will get a random gravity push. If
that random push increases your velocity, you've had a gravity assist
despite not planing it. If the random push decreases your velocity, you
might lose a year or two, and during that time you will probably think
about how to avoid that from repeating the next time, therefore you will
plan for a gravity assist.


Alain Fournier