Author Topic: Some space science Qs  (Read 1872 times)

0 Members and 1 Guest are viewing this topic.

Offline Nibelung44

  • Commander
  • *********
  • Posts: 302
Some space science Qs
« on: October 17, 2013, 10:05:48 AM »
Hey,

I have a few lingering questions on 'real science', so perhaps some of you have an answer on them.

First... I don't get why it's not possible to get a very high speed even with a chemical engine. I read somewhere the exhaust speed for the gas was around 1 to 2 km/s. So within some days, why a shuttle traveling to Mars can't reach say 10% or 20% the speed of light? I'm speaking after having escaped Earth attraction.

Corollary, I thought that in absence of thrust, a spaceship would maintain almost in-definitively, baring the attraction of nearby planets and suns, the same speed. This seems false so...  For example in the Honor Harrington series, once you reach a certain speed, when you cut off engines, you maintain said speed. So with enough time, and even with crappy acceleration, you can get close speed of light. (and when you approach it, your mass tends to goes to the infinite, so you never reach it).
 

Offline Charlie Beeler

  • Registered
  • Vice Admiral
  • **********
  • Posts: 1381
  • Thanked: 3 times
Re: Some space science Qs
« Reply #1 on: October 17, 2013, 12:36:55 PM »
Current chemical rockets are extremely inefficient.  The fuel required to achieve the speeds your suggesting is prohibitively high.  Even if we could functionally reach those speeds, roughly the same amount of fuel would be required to decelerate at the destination.  This doesn't even take into account the fuel mass for the return trip. 

The next issue has to do with the fact that space is not empty.  We currently do not have technology to protect a ship from damage from C-fractional impacts.
Amateurs study tactics, Professionals study logistics - paraphrase attributed to Gen Omar Bradley
 

Offline xeryon

  • Captain
  • **********
  • Posts: 581
Re: Some space science Qs
« Reply #2 on: October 17, 2013, 02:15:34 PM »
Gravitational pull from the Sun is a pretty powerful force.  It requires quite a bit to effort to escape it's pull heading out of the system.  Likewise it takes a lot to slow down and stop heading into the system.  You don't want to get going too fast or you will expend huge amounts of energy just to stop. 
 

Offline sublight

  • Pulsar 4x Dev
  • Captain
  • *
  • s
  • Posts: 580
  • Thanked: 7 times
Re: Some space science Qs
« Reply #3 on: October 17, 2013, 07:44:48 PM »
Clearly, this thread needs more math.  ;D

In theory a ship accelerating at several times standard gravity for several days would indeed be traveling at a large fraction of light speed. However, in practice a rocket accelerating at 2 or 3Gs will run out of fuel in maybe 10 minutes. Heavy rockets burn fuel fast.

State-of-the-art rockets appear to peak at 3 km/s exhaust in vacuum. Going from Low Earth orbit to a Phobos - distance Martian orbit at chemical acceleration requires, optimistically, 3.6 to 5.6 km/s delta V depending on aerobraking.

The Rocket Equation, rewritten, shows that M = e^(dV/Vr) =  2.71828^(5.6/3) = 6.5x payload if no aerobraking. That is: for ever ton of payload, engine, fuel tanks, etc the spaceship will need to burn 5.5 tons of fuel.

For example, if we wanted to send some people inside a BA330 module (30 tons with supplies?) into Martian orbit as a research station we would need about 200 tons assembled in LEO. Maybe more. According to Wikipedia, the cheapest way to do this would be 1x Ariane 5ECA for the habitat plus 11x Falcon 9 launches for the fuel and supplies. Estimated launch cost: $836,000,000 USD. Plus research and development.

What if we wanted to bring them back to Earth again? 30x6.5x6.5 = about 1,270 tons. Estimated launch cost: $55,000,000,000 USD. Plus research and development.


Electric propulsion systems are probably a better bet. The acceleration is too slow to take advantage of the oberth gravitational boost effect, but the higher exhaust speeds more than compensates. The limiting factor is power.

Suppose we want a space ship that is a fifth the size of our chemical rocket. Say, 2.5 ton power/propulsion and 5 ton fuel per ton payload. The lower acceleration nearly doubles the DV requirement to about 20 km/s round trip.

V_exhaust = 20 km/s / ln(8.5/3.5) = 22.5 km/s, easily in the ion thruster range. I think current technology offers something like 5 MW/ton ion output at 70% efficiency with 0.3 MW/ton solar power generation. Thus this space ship would have something like 0.0005G acceleration and require over 2 weeks just to reach Earth escape velocity.


Conclusions:
Going to Mars Orbit is probably no more than twice as expensive as building the International Space Station at worst. If the planetary space exploration budget were to suddenly double I bet we could be there by 2030. More likely, a bunch of smart people will figure out a way to go cheaper, or a bunch of crazy people we volunteer to skip the cost of a return trip. Or both.

Getting up to 10% light speed isn't possible. Best ion exhaust speeds may exceed 200 km/s, but that is a long way short of the 300,000 km/s that roughly equals light speed. A ship that is 99% fuel/reaction mass with near-future technology would only hit 0.3% light speed and take over a millennium to reach the closest neighboring star.

Sometimes I get carried away when thinking about space travel.
 

Offline xeryon

  • Captain
  • **********
  • Posts: 581
Re: Some space science Qs
« Reply #4 on: October 18, 2013, 07:24:01 AM »
In light of Sublights awesomely number filled forum post:



I love space.  I love science.  The mathematical equations that explain them are so far over my head that the light from them will take a couple years to reach me.
 

Offline Nibelung44

  • Commander
  • *********
  • Posts: 302
Re: Some space science Qs
« Reply #5 on: October 18, 2013, 08:14:29 AM »
Thanks for the interesting answers. So that's a combination of gravitic pull from the sun mostly, plus the high mass of fuel needed for chemical engines, that would prevent a decent speed for Mars or beyond. And also that you need a kind of mushroom cap at the front of the ship, to prevent it from being destroyed by particles, micro or bigger...

I read an article dealing with Mars colonization, and some discussions hover around: should we use a chem engine or a nuclear one. The nuclear one is superior over the chem one, but it still has to be designed whereas chem engines are already available and proved.

So for the sake of discussion, lets say we can now have effective fission reactors for a big shuttle, perhaps derived from what we use for nuclear CVs... What speed can we realistically achieve? Even 10.000 km/s after 3 weeks of constant acceleration would be awesome! Is it realistic within 20 years?
 

Offline Paul M

  • Vice Admiral
  • **********
  • P
  • Posts: 1259
  • Thanked: 22 times
Re: Some space science Qs
« Reply #6 on: October 18, 2013, 09:44:34 AM »
As a soviet scientist who specializes in this said at a confernce one time:  If you want an ion drive with 1 G performance you need a nuclear reactor on the spacecraft.

Large ion drive systems can do better than 0.0005 G but not if your fart around with solar cells.  Ion sources for other applications can deliever MeV beams with 10s of Amperes of current.  Basically what is used now is limited by the power the solar cells produce.  I've run the number out of interest sake and you could for several thousand tonnes of payload get at least 0.1 G without too much R&D beyond what is used for other applications.  That is probably sufficient to reduce the transit time to a few weeks.

Nuclear rockets give higher acceleration but you will have to plan to boost at high G then coast in zero G.  For humans this is not so good from a health point of view.

The issue is far less money then desire to do so.  Also for reasons best known to those responsible the costing of things for space exploration have been inflated significantly.  I suspect the golden hammer syndrome plays a huge role.
 

Offline Hawkeye

  • Vice Admiral
  • **********
  • Posts: 1043
  • Thanked: 2 times
Re: Some space science Qs
« Reply #7 on: October 18, 2013, 10:59:03 AM »
Thanks for the interesting answers. So that's a combination of gravitic pull from the sun mostly, plus the high mass of fuel needed for chemical engines, that would prevent a decent speed for Mars or beyond. And also that you need a kind of mushroom cap at the front of the ship, to prevent it from being destroyed by particles, micro or bigger...

I read an article dealing with Mars colonization, and some discussions hover around: should we use a chem engine or a nuclear one. The nuclear one is superior over the chem one, but it still has to be designed whereas chem engines are already available and proved.

So for the sake of discussion, lets say we can now have effective fission reactors for a big shuttle, perhaps derived from what we use for nuclear CVs... What speed can we realistically achieve? Even 10.000 km/s after 3 weeks of constant acceleration would be awesome! Is it realistic within 20 years?

Now, I am a bloody amateur and don´t claime to understand most of the math behind this, but due to your question, I went to

http://www.projectrho.com/public_html/rocket/enginelist.php#table    ( I reccomend you have a look at that site)

and did a few calculations re. nuclear thermal rockets

delta v = Exhaust Velocity * In(Mass Ratio)

Mass Ratio = Mass of space ship with propellant / mass of space ship without propellant

Solid Core Nuclear Thermal Rocket, using H2 as propellant could achieve an exhaust velocity of 8093 m/s (assuming a core temperatur of 3200°K (you can go higher, but the problems at higher tempertures get VERY big as your engine starts to melt)

This means, if your ship consists to 75% of propellant (i.e. mass ratio of 4), it has a Delta-V of just 11219 m/s and remember, you need half of your delta-V to slow down at the target (actually, you can only use half of your delta-v to get to your target and slow there, if you want to go back to where you came from (yes, using swing-bys will reduce this, but let´s ignore this for the sake of argument).

Going for a mass ratio of 10 (i.e. only 10% of the mass is the actual ship, the rest is all propellant), gives you a delta-v of 18634 m/s. The increased mass of the fuel is quite restrictive on the delta-V, as much of the propellant is used up to accelerate just that propellant.


Now, an open cycle Gaseous core nuclear thermal rocket has a much higher exhaust velocity (around 35000 m/s). Of course, you are now blowing U235 out your space ships exhaust which is not reccomended near an inhabited planet (and proposing this would probably be political suicide).
With a mass ratio of 4, you can achieve a delta-v of 48520 m/s, much better than before, but nowhere near the 10,000,000 m/s you wanted.


The best I could find was the Nuclear Salt Water Rocket (which also apparently has a lot of engiering problems) and reaches an exhaust velocity of 66,000 m/s and gives a delta-v of 91495 m/s at a mass ratio of 4 or 137243 m/s at a mass ratio of 8.


To sum it up, a delta-v of 10,000 km/s seems to be way, way, WAY above anything we could achieve.
I haven´t looked into fusion/anti matter drives, as they are so far above our abilities, it isn´t even funny.


PS: Looking into Ion Drives, the exhaust velocity of 210,000 m/s looks very good, but the power requirements are staggering. I also found this:


Quote
And it suffers from the same critical thrust-limiting problem as any other ion engine: since you are accelerating ions, the acceleration region is chock full of ions. Which means that it has a net space charge which repels any additional ions trying to get in until the ones already under acceleration manage to get out, thus choking the propellant flow through the thruster.

The upper limit on thrust is proportional to the cross-sectional area of the acceleration region and the square of the voltage gradient across the acceleration region, and even the most optimistic plausible values (i.e. voltage gradients just shy of causing vacuum arcs across the grids) do not allow for anything remotely resembling high thrust.



You can only increase particle energy so much; you then start to get vacuum arcing across the acceleration chamber due to the enormous potential difference involved. So you can't keep pumping up the voltage indefinitely.

To get higher thrust, you need to throw more particles into the mix. The more you do this, the more it will reduce the energy delivered to each particle.

It is a physical limit. Ion drives cannot have high thrusts.


With a mass ratio of 2, which is _very_ good, I get a delta-v of 145,560 m/s and with a mass ratio of 4 it is 291,121 m/s. Due the the low thrust, it would probably take a loooooooong time to burn all that propellant though.

Ralph Hoenig, Germany
 

Offline Nibelung44

  • Commander
  • *********
  • Posts: 302
Re: Some space science Qs
« Reply #8 on: October 20, 2013, 02:48:52 AM »
Wow, awesome site, with a lot to read, thanks!  :)
 

Offline Paul M

  • Vice Admiral
  • **********
  • P
  • Posts: 1259
  • Thanked: 22 times
Re: Some space science Qs
« Reply #9 on: October 20, 2013, 02:06:43 PM »
The stuff on ion drives is wierd.  The current density extracted from an ion source is space charge limited to a value proportional to V^1.5 until you reach the plasma limit (that is your beam current density = the density of the plasma in the ion source).  Afte that you can't get any more current for an increase in extraction voltage.  You can still increase the voltage up to what the grids will hold but grids can tolerate rather substantial voltages across small gaps.  The question of space charge limited current is true...but that is generally speaking only relevant to the point where you have no extraction voltage at all (such as thermal electron emission)...but even 90 V is suficient to extract amperes of current.  I've never in my entire career ran into a situation where the overall current of ion source was blocked by space charge.

Large area ion sources can deliver currents of 100 A (positive ions) or 40 A (negative ions).  Current technology is for an acceleration potential of 1 MeV.  The grids need to be well designed.   The SINGAP accelerator was gridless so basically I don't see an issue with using something like that to give you 1 MeV beam (positive ions or negative ions).  Those values are for sources of about 0.25 m2 for positive ions and 2 m2 for negative ions.

What they are saying is basically that the total beam current from the ion source = extraction area* beam current density.  Lets assume you are in the space charge limited region of the expression and so current density is proportional to extraction voltage to the 3/2.  I just don't see where that is a problem.  The cross sectional area is essentially freely choosable.  The grid voltage can be up to 1 MeV without an issue.  So how does this become a limit?     

Exhaust velocity, v = sqrt(2mE)  which is for 1 MeV Xenon gives extremetly high velocities, especially since a large fraction of the Xenon will be at more than a charge state of +1.  Note you don't extract at 1 MeV...you can extact at 10 kV and then boost it upwards succesively.  Via a 2 or 3 grid system usually.

Typical high power large area postive ion sources have 200 mA/cm2 of ion current density*10^4 cm2 extraction area = 2000 A of Xeon or 2x1022 xenon ions (assuming they are all +1) per second at whatever velocity the other formula gives per square meter of extraction area of ion drive.   Xenon is 133 grams per mol I think and so you are looking at about 0.003 kg/s of Xenon.  If you then take the 210, 000 m/s exhaust velocity...you have 600 N of thrust.  Or sufficient thrust to move 60 kg at 10 m/s2 or 600 kg at 1 m/s2  or 0.1 G.   And I'm rather dubious that v corresponds to 1 MeV.

Current ion drives use mA currents and a few kV beam voltage.  They are limited by the plasma source (which is low power) and so they are extracting beam densities that are, even for typical small ion sources, rather low.  But they have to do all this with no power so it is a lot more impressive then it sounds. You aren't going to get high thrust from a 10 cm ion thruster running on 5 kW from a solar panel...but that isn't what I'm talking about.

The power requirement is the critical issue.  Probable what is more critical is the cooling of the whole system.  There is also the fact you have to keep the ship neutral with respect to the surrounding space plamsa as well.  I'd be worried a lot about static charge build up.
 

Offline Nibelung44

  • Commander
  • *********
  • Posts: 302
Re: Some space science Qs
« Reply #10 on: October 23, 2013, 12:14:09 AM »
Another question.


It's about radio frequencies and the possibility of advanced life in the galaxy.

Can the radio frequencies emitted by an advanced civilization absorbed by stars, nebulae, anything, or alternatively, do they dissipate over distance?

If yes, then I can understand we might be unable to detect the existence of an advanced civilization in our own galaxy. If no, then it probably means we are alone in this galaxy at least... Why do I say that?


1. Universe is 13.82 BY old.
2. a sun like our is 4.6 BY old, and that's enough to get from nothing, to molten planetoids to advanced civilization on a planet.

from that, we can assume with a very high probability that if life is common, we are not the first in the galaxy with the capacity to emit radio frequencies. Because there is plenty of time since the beginning of the universe to have several generations of full grown stars, each with quite enough time to get advanced life.

And yet, we hear nothing. So either (A) radio frequencies are dissipated and can't reach us, or (B) advanced life is a super rare, miraculous phenomenon.

Now, I don't think that (A) is true, so this leaves (B). To my dismay...
 

Offline niflheimr

  • Martian Union
  • Lieutenant
  • *
  • n
  • Posts: 164
Re: Some space science Qs
« Reply #11 on: October 23, 2013, 03:39:41 AM »
Radio dissipates by the inverse square law , ie. 1/r^2 . What that means is that at twice the distance the intensity of the radio transmission reduces by four. When you consider an omnidirectional transmission well , past a few light years you are lucky if you get anything at all.

There are things that can absorb them as well , or swamp them with static - the actual base static is quite big . If you have a radio-emitting pulsar in line you won't be able to read any other signal.
 

Offline MarcAFK

  • Vice Admiral
  • **********
  • Posts: 1769
  • Thanked: 69 times
  • ...it's so simple an idiot could have devised it..
Re: Some space science Qs
« Reply #12 on: October 23, 2013, 06:05:27 AM »
Project ozma sentand monitored transmissions into/from space in a frequency named the 'water hole' in the understanding that extra terrestrials would know that this frequency would be a logical place to put extra terrestrial contacting messages. However nothing was found. Everything out there generates different radio frequencies, and it's notable that due to the expansion of the universe as well as the motion of celestial bodies frequencies of radiation visable from earth can be somewhat skewed from it's origenal frequency.
" Why is this godforsaken hellhole worth dying for? "
". . .  We know nothing about them, their language, their history or what they look like.  But we can assume this.  They stand for everything we don't stand for.  Also they told me you guys look like dorks. "
"Stop exploding, you cowards.  "
 

Offline TallTroll

  • Sub-Lieutenant
  • ******
  • T
  • Posts: 101
Re: Some space science Qs
« Reply #13 on: October 23, 2013, 06:42:07 AM »
>> due to the expansion of the universe as well as the motion of celestial bodies frequencies of radiation visable from earth can be somewhat skewed from it's origenal frequency.

Unless the redshift isn't due to expansion, of course. Just because it's a logical explanation, it doesn't have to be right

>> Can the radio frequencies emitted by an advanced civilization absorbed by stars,

I've always been faintly puzzled by SETI looking for radio waves. We are in danger of falling into an anthropocentric trap, I think, assuming that because we still use radio waves, all advanced civilizations do too. Consider that there are 3 basic possibilities

1) We are alone. We can't hear anything, because there are no advanced (in this case, radio-capable) civilizations out there

2) There are other civs out there, but we are listening on the wrong frequencies, or their radiation hasn't reached us yet. Seems a bit unlikely... one assumes that aliens observing the same universe we do derive a different model of physics from the same data, the other assumes that a now advanced civ arose almost simultaneously (in cosmic terms) with us

3) There are other civs out there, but they don't use radio any more, or at least not enough to be "heard" at any great distance. We are heading this way already. The original founders of SETI etc assumed that as a race advanced, it would produce more and more radiated radio energy, as their access to power grew, thus making them easier and easier to find with radio telescopes. But they never considered fibre optic cables, for example. Radio is a 19th century technology. It seems likely we won't be using it for much longer, so perhaps we shouldn't be too surprised if we can't find traces of others using it
 

Offline sloanjh

  • Global Moderator
  • Admiral of the Fleet
  • *****
  • Posts: 2697
  • Thanked: 48 times
Re: Some space science Qs
« Reply #14 on: October 23, 2013, 07:40:27 AM »
3) There are other civs out there, but they don't use radio any more, or at least not enough to be "heard" at any great distance. We are heading this way already. The original founders of SETI etc assumed that as a race advanced, it would produce more and more radiated radio energy, as their access to power grew, thus making them easier and easier to find with radio telescopes. But they never considered fibre optic cables, for example. Radio is a 19th century technology. It seems likely we won't be using it for much longer, so perhaps we shouldn't be too surprised if we can't find traces of others using it

About 20 years ago, when I was a physics post-doc at Ohio State, Jill Whatshername gave a colloquium on SETI.  In the Q&A I asked her how close an alien civ that was putting out as much radio as we were would need to be in order for SETI-at-the-time to detect them (similar to the Aurora principle of "design your AMM to be able to shoot down your own ASM").  Her answer (IIRC): ~2 light years. Even if you add a couple of orders of magnitude in sensitivity and alien civ output, they'd still need to be ridiculously close (i.e. 100s of light years) in order to be seen.  So it's not surprising to me at all that they've not heard anything.

This still leaves the Fermi paradox out there.  My suspicion on that is that it means that interstellar travel is just too energetically (and elapsed time) expensive for intelligent life to do it.  You would have to have organisms with lifetimes 100s or 1000s of times longer than ours in order for the beings that funded the travel to economically benefit from it at the speeds that are realistically obtainable.

John
 

 

Sitemap 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52