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Should we send interstellar probe to Alpha Centauri?

by Urvabara
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FredGarvin
#19
Jul14-08, 04:03 PM
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Quote Quote by Oberst Villa View Post
I am not very impressed by statements like "could be solved within 30 years". I recently read an article about rail-gun development from the 1980s (when Reagan's SDI had renewed interest in this things). In this review a paper from the early 1940s was cited, where a german scientist had been quite optimistic about rail-gun development if only the 3rd Reich would give him enough ressources for his research. Then, again in the 1980s article, much optimism for the next decades. Now we have 2008 and the only operational rail-gun I ever saw was in an Arnold Schwarzenegger movie ! (I know some guys built various prototypes in their labs, but the predictions were about operational guns.) So much for "can be solved within x0 years".
Whether you are impressed or not, you have to realize that being technically able to do something means nothing in reality. There is a lot of other things that have to happen. This is where 99% of failed ventures get tripped up. We went to the moon not only because we had a lot of talented technical people, but mostly because Kennedy set the government on the path that it would be done and gave NASA the support they needed.

Like Asto pointed out, there is an operational rail gun being tested for the Navy.
russ_watters
#20
Jul14-08, 04:23 PM
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This is the engineering forum, so no one commented on this:
Quote Quote by Urvabara View Post
There could be a Earth-like planet in the Alpha Centauri system.
Extremely unlikely because:

a: If there was, we'd probably have detected it already.
b: It's a triple-star system so such a planet would probably not have a uniform/stable orbit.
Based on theoretical computer simulations, other planetary astronomers consider that any potential terrestrial planets that did once orbit near the stars' habitable zones are now likely no longer located there. The loss several billion years ago of these small bodies probably happened during the system's formation. All may have since been ejected by significant disruptions caused by strong gravitational or perturbation effects generated between the two main stellar components.
http://en.wikipedia.org/wiki/Alpha_Centauri

That's a lot of effort to answer a question that's already got a "probably not" answer.
ray b
#21
Jul14-08, 04:43 PM
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I wonder how we would react to a probe coming in
that was useing H-bombs to slow down
I think we need a better system before we try to explore
DaveC426913
#22
Jul14-08, 04:53 PM
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Quote Quote by ray b View Post
I wonder how we would react to a probe coming in
that was useing H-bombs
"They're insane! We surrender!"
Footfall - Larry Niven
D H
#23
Jul14-08, 05:24 PM
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Quote Quote by DaveC426913 View Post
According to Wiki, only 3 out of 27 (~10%) doomed Mars probes failed on landing. 16 (~60%) failed somewhere between Earth orbit and Mars orbit. The other 8 failed on launch (~30%).

It is called the Mars Curse.
I count at least six landing failures amongst the vehicles that were supposed to land.
  1. Mars 2. "Crash landed on surface of Mars".
  2. Mars 3. "Success (First successful landing)". That mission was a failure. It ceased transmission seconds after landing. The mission did not do any of the planned surface science, including use of its rovers, and only managed to transmit part of one rather useless picture of the Marse surface (low resolution and very low illumination).
  3. Mars 6. "Partial success". Partial success my ***. The Mars 6 lander mission ended with "direct proximity to the surface" at 61 m/s. Stripping away the bureaucrateze, Mars 6 ended with a crash landing.
  4. Mars Polar Lander. The most likely cause of the MPL failure was software interprations of vibrations that made the engines cut out 40 meters above the Martian surface.
  5. Deep Space 2. "The probes reached Mars apparently without incident, but communication was never established after landing."
  6. Beagle 2. "Lost contact while landing; Assumed to have crash landed".
I count 15 lander missions. Of those,
  • Two failed in the vicinity of Earth (Sputnik 24 and Mars 96).
  • One failed on route to Mars (Phobos 1).
  • One failed at Mars orbit insertion (Mars 7).
  • One failed at Mars entry (Phobos 2).
  • Six failed shortly before, at, or very shortly after landing (above).
  • Four succeeded (Viking 1, Viking 2, Pathfinder, Phoenix).
Landing failures accounts for six (seven if you count Phobos 2) out of the eleven failed lander missions.
DaveC426913
#24
Jul14-08, 06:32 PM
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Quote Quote by D H View Post
I count at least six landing failures amongst the vehicles that were supposed to land.
  1. Mars 2. "Crash landed on surface of Mars".
  2. Mars 3. "Success (First successful landing)". That mission was a failure. It ceased transmission seconds after landing. The mission did not do any of the planned surface science, including use of its rovers, and only managed to transmit part of one rather useless picture of the Marse surface (low resolution and very low illumination).
  3. Mars 6. "Partial success". Partial success my ***. The Mars 6 lander mission ended with "direct proximity to the surface" at 61 m/s. Stripping away the bureaucrateze, Mars 6 ended with a crash landing.
  4. Mars Polar Lander. The most likely cause of the MPL failure was software interprations of vibrations that made the engines cut out 40 meters above the Martian surface.
  5. Deep Space 2. "The probes reached Mars apparently without incident, but communication was never established after landing."
  6. Beagle 2. "Lost contact while landing; Assumed to have crash landed".
I count 15 lander missions. Of those,
  • Two failed in the vicinity of Earth (Sputnik 24 and Mars 96).
  • One failed on route to Mars (Phobos 1).
  • One failed at Mars orbit insertion (Mars 7).
  • One failed at Mars entry (Phobos 2).
  • Six failed shortly before, at, or very shortly after landing (above).
  • Four succeeded (Viking 1, Viking 2, Pathfinder, Phoenix).
Landing failures accounts for six (seven if you count Phobos 2) out of the eleven failed lander missions.
OK, you've interpreted them differently and divided them up differently. For example: MPL "lost contact before arriving". In my books, that's an "en route" failure, not a "landing" failure. It failed before successfully achieving orbit.


Regardless, your results miss the point: you're only counting lander attempts. All lander attempts on either Mars or A. Centauri must first get there - which means you include the en route failures too.

I am trying to point out that landing is a relatively minor problem in the scope of long missions. The bigger problem is just getting there in the first place.


Or put another way: If 98 out of 100 men died crossing the desert, and one of the two remaining died by drinking from a poisoned well, would you be claiming that 50% of the failed desert crossings were due to poisoned water?
D H
#25
Jul14-08, 06:51 PM
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Quote Quote by DaveC426913 View Post
OK, you've interpreted them differently and divided them up differently. For example: MPL "lost contact before arriving". In my books, that's an "en route" failure, not a "landing" failure. It failed before successfully achieving orbit.
That is not what the accident review board said.
From http://oig.nasa.gov/old/inspections_assessments/MPL.pdf,
Quote Quote by Office of Inspector General
The MPL's three landing legs were kept in a stowed position during launch and the spacecraft s journey to Mars. They were designed to deploy into position for landing when the MPL had descended to 1,500 meters above the surface of Mars. Each leg had a magnetic sensor to detect when it touched the planet s surface. When the sensors detected touchdown, the flight software would send a signal to shut down the MPL s thrusters. During MPL development, tests of the landing legs found that the magnetic sensors usually generated a false touchdown signal when the legs were deployed. To solve the problem, MPL systems requirements stated that all touchdown signals generated before the spacecraft descended to 40 meters above the surface should be ignored. However, the flight software requirements did not properly implement this system requirement. The result was that when the spacecraft descended to 40 meters above the surface, the touchdown signal from leg deployment was still in the system, causing the engine to shut off. At Mars, this would have resulted in the MPL hitting the surface at approximately 22 meters per second (50 mph), causing the loss of the spacecraft. Review teams have determined that this was the most probable cause of the loss of the MPL.


Regardless, your results miss the point: you're only counting lander attempts.
You're missing my point. Vehicles that aren't designed to land can't have landing failures. (They might have planetary impact failures, such as Mars Climate Observatory, but that is a different issue.)

I am trying to point out that landing is a minor problem in the scope of long missions. The bigger problem is just getting there in the first place.
That might well be the case for a mission to A. Centauri, but not for a mission to Mars. The vast majority of Mars flyby, orbiter, and lander mission failures occurred during active flight phases: launch, TMI, MOI, entry, descent, landing.

I've ignored the stuff about a mission to Alpha Centauri because, to be blunt, the concept is entirely ludicrous. A lander, doubly so.
DaveC426913
#26
Jul14-08, 07:00 PM
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Quote Quote by D H View Post
You're missing my point. Vehicles that aren't designed to land can't have landing failures.
I get that. But it is not the point.

The claim was that "the most dangerous part of a mission is the landing". I am refuting that claim. Landing or no, most missions have been lost in transit.
This is relevant to any long-term mission. Again, there's little point in worrying about landing issues if the craft never makes it there.

Again, you're concentrating on the poisoned well, and regarding the 98 dead men as immaterial to the mission success ratio.

Quote Quote by D H View Post
I've ignored the stuff about a mission to Alpha Centauri because, to be blunt, the concept is entirely ludicrous. A lander, doubly so.
Then you've ignored the intent of the post, meaning this is a derailment.
D H
#27
Jul14-08, 10:27 PM
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Quote Quote by DaveC426913 View Post
Or put another way: If 98 out of 100 men died crossing the desert, and one of the two remaining died by drinking from a poisoned well, would you be claiming that 50% of the failed desert crossings were due to poisoned water?
That is not a good analogy for space mission failures. A better analogy: 100 men set off on a journey across a desert. Thirty three die drinking from a poisoned well at the start of the journey, another thirty three die drinking from a poisoned well at the end of the journey. Thirty three make it to the destination because only one dies while crossing the desert.

Nearly all of the problems with space missions occur at the start or at the end of the journey. Very few missions fail en route.

A mission to Alpha Centauri is a different beast. Assuming present-day technologies such a journey would take hundreds of years or more. In this case we would have to worry about things going wrong en route. The vehicle would have to expend a lot of energy just to stay warm and a would have to expend a whole lot of energy to communicate with Earth. The propulsion, avionics, communications, and sensors systems would have to be extremely redundant to accommodate the inevitable failures that would occur in such a long mission.

Assuming future technologies casts the problem into the realm of science fiction. This is the easy way out because it sweeps all of the hard problems under the rug.
Urvabara
#28
Jul15-08, 11:18 AM
P: 100
Ok. I just calculated that about 99.78% of the mass of the probe will be fuel, if assuming [tex]\Delta v = 29979245.8 + 29979245.8 = 59958491.6[/tex] m/s and maximum exhaust velocity of hydrogen nuke Orion of [tex]v_{e} = 9800000[/tex] m/s. That's a big fraction of fuel, I know...

Off topic. I also know you think I am crazy. Maybe I am but how about those who believe in ghosts, angels or god? I have never seen a scientific proves of the existence of super natural forces! I am just believing that engineers/scientists can overcome the problems of space exploration...
DaveC426913
#29
Jul15-08, 11:58 AM
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Quote Quote by Urvabara View Post
Off topic. I also know you think I am crazy.
Nobody thinks you're crazy.
It's just that this forum mainly deals with present-day, established physics and technology, and tends not to deal with speculation so much.

Ok. I just calculated that about 99.78% of the mass of the probe will be fuel ...That's a big fraction of fuel, I know...
Oh I dunno, the Saturn V was 85% fuel by mass, and another 12% of launch mass was jettisoned just to deliver the remaining 3% payload to low orbit. That's only an order of magnitude diff (3% : .22%)

Quote Quote by Urvabara View Post
if assuming [tex]\Delta v = 29979245.8 + 29979245.8 = 59958491.6[/tex] m/s and maximum exhaust velocity of hydrogen nuke Orion of [tex]v_{e} = 9800000[/tex] m/s.
How did you calculate that? Are you assuming full thrust for the duration of the trip?
Urvabara
#30
Jul15-08, 12:18 PM
P: 100
Quote Quote by DaveC426913 View Post
How did you calculate that? Are you assuming full thrust for the duration of the trip?
I just used the Tsiolkovsky's rocket equation and put the numbers ([tex]\Delta v[/tex] and [tex]v_{e}[/tex]) in.

I don't know how long a time the accelerating takes exactly. If a = 10g = 98,1 m/sē, then accelerating to 0.1c takes about 85 hours.

I think the magnitude of the fuel fraction maybe somewhat correct, but the exhaust velocity might be too optimistic. I took it from here.
Urvabara
#31
Jul15-08, 12:40 PM
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Quote Quote by ray b View Post
I wonder how we would react to a probe coming in
that was useing H-bombs to slow down
My reaction would be: "So, there is a civilization that found a good use for their H-bomb stockpiles!"
Oberst Villa
#32
Jul15-08, 03:50 PM
P: 145
Quote Quote by Astronuc View Post
I saw operational railguns 20+ years ago (with muzzle velocities of 3 km/s), and the Navy is now testing one for naval artillery.

Here's a Popular Mechanics article on the BAE system delievered to the US Navy.
http://www.popularmechanics.com/tech...w/4231461.html
First thing, thanks a lot for the link, I was not aware of this latest development, I have to admit that it makes my ranting a bit pointless.

As for the railguns that were operational 20+ years ago, probably I have communicated my thoughts quite badly. English is not my native language and I sometimes use words sloppily even in german. The heart of the problem seems to be my (possibly wrong) usage of the word "operational". What I had in my mind when saying that I have not seen an operational railgun yet was something like "a weapon system that is ready to be used in the field right now", (as opposed to a prototype in the lab). The best I could find in this context was a definition of "Initial Operational Capability", don't know whether it's an offical definition of this term, but anyway this is quite precisely what I meant:

"The first attainment of the capability to employ effectively a weapon, item of equipment, or system of approved specific characteristics, and which is manned or operated by an adequately trained, equipped, and supported military unit or force."

Now I have the impression that when YOU say "operational" you mean "something that works" (as opposed to something that is not working yet) and OK, this might just be the normal meaning of the word.

Anyway, maybe I should have said "I have not seen an operationally deployed railgun yet", then I could have avoided this misunderstanding, sorry for this.
Oberst Villa
#33
Jul15-08, 04:15 PM
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Quote Quote by FredGarvin View Post
Whether you are impressed or not, you have to realize that being technically able to do something means nothing in reality. There is a lot of other things that have to happen. This is where 99% of failed ventures get tripped up. We went to the moon not only because we had a lot of talented technical people, but mostly because Kennedy set the government on the path that it would be done and gave NASA the support they needed.
Hmm, I don't disagree with you on this. But I think it's not very probable that such a foccussing of the nations scientific and industrial ressources on a single goal will be repeated in the near future. There is nobody in sight against whom the US would have to compete like they did against the Soviets.
Oberst Villa
#34
Jul15-08, 05:01 PM
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Quote Quote by Urvabara View Post
I don't know how long a time the accelerating takes exactly. If a = 10g = 98,1 m/sē, then accelerating to 0.1c takes about 85 hours.
10g is muuuuuuuuch to high.

First thing, for the probe to tolerate such an acceleration without breaking apart, you would need a massive support structure, which would add a lot of mass to your probe => Kiss your 99,78% goodbye.

Second thing, there is no need for such a high acceleration. You need 40 years as a minimum, so would it hurt much to add e.g. 850 days for the acceleration phase ?
D H
#35
Jul15-08, 06:06 PM
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Quote Quote by Urvabara View Post
Ok. I just calculated that about 99.78% of the mass of the probe will be fuel, if assuming [tex]\Delta v = 29979245.8 + 29979245.8 = 59958491.6[/tex] m/s and maximum exhaust velocity of hydrogen nuke Orion of [tex]v_{e} = 9800000[/tex] m/s. That's a big fraction of fuel, I know...
An Orion-type rocket will need a *lot* of mass to protect the payload from the explosions.

Quote Quote by Urvabara View Post
I just used the Tsiolkovsky's rocket equation and put the numbers ([tex]\Delta v[/tex] and [tex]v_{e}[/tex]) in.
You are hypothesizing relativistic velocities, so you need to use the relativistic rocket equation:

[tex]\frac{\Delta v}{c} = \tanh\left(\frac {v_e}{c}\,\ln\frac {m_0}{m_1}\right)[/tex]

You chose an extremely unrealistic rocket. A more realistic choice of rocket technology will yield a very different mass ratio. For example, a VASIMR engine has an effective exhaust velocity of up to 300,000 m/s. The fuel mass ratio for a VASIMR engine is 99.999...%: 87 nines! There is no room for vehicle structure here. We need something a bit more exotic than VASIMR engine but something that has a ghost of a chance. How about injecting small amounts of antimatter into a plasma, forcing some plasma to escape at a high velocity, and just making a fly-by? Ths gets us down to 99.999....% fuel mass (13 nines). There's still no room for structure, but an improvement.

Note well that both of these rockets are way out there in terms of specific impulse. Nonetheless, neither a 200 fold or 600 fold increase in Isp will enable us to send a payload to Alpha Centauri.
DaveC426913
#36
Jul15-08, 09:03 PM
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Quote Quote by Oberst Villa View Post
10g is muuuuuuuuch to high.

First thing, for the probe to tolerate such an acceleration without breaking apart, you would need a massive support structure, which would add a lot of mass to your probe => Kiss your 99,78% goodbye.

Second thing, there is no need for such a high acceleration. You need 40 years as a minimum, so would it hurt much to add e.g. 850 days for the acceleration phase ?
1] I am fairly certain a probe could be built to withstand much 10g's or more.
2] You don't need to add mass, you just need a good shock absorbing system.


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