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Is near light speed achievable? 
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#1
Jan711, 07:20 AM

P: 83

assume if enough fuel is there is it possible to achieve near light speed?
what are the factors preventing this? how we can link relativistic mass to the thrust of rocket at near light speed? assume mass of spacecraft is constant and enough fuel is there then can chemical rockets can reach near light speed? if not does relativistic mass has anything to do with this at higher speed does more energy is required to expel gases? if constant thrust is given is it possible to achieve near light speed? What is the relativistic version of thrust equation? 


#2
Jan711, 02:37 PM

P: 674

Hi sr241,
First note that according to relativity, no matter how fast the rocket goes, the people inside always see light going at lightspeed (the constant c). This is elementary relativity. Second, to the people outside the rocket, the major obstacle to reaching relativistic speeds is the amount of energy needed. Machines such as the LHC are required to send subatomic particles to near lightspeed, so we can't really imagine a realistic machine that could send a macroscopic object to such speeds, much less a humancarrying rocket. 


#3
Jan711, 11:40 PM

P: 83

actually my friend is working on an a rocket engine that doesn't work on mass expulsion
so, if constant thrust is given is it possible to achieve near light speed? What is the relativistic version of thrust equation? 


#4
Jan711, 11:57 PM

P: 15,319

Is near light speed achievable?
Very simplistically: If x thrust over time t gets you to .9c, then a greater thrust over more time will get you to .99c, and a still greater thrust over still more time will get you to .999c. No amount of thrust over any length of time will get you to c. 


#5
Jan811, 12:01 AM

Mentor
P: 22,243

There may be an automatic calculator out there for it but I'm not sure (you can google). 


#6
Jan811, 12:11 AM

P: 15,319

True, he said "rocket", but that is obviously not the word he should have used (since a rocket is defined by mass expulsion). He hould have simply said "a propulsion unit" that does not expel mass. The fact that his friend is almost surely building a pipe dream does not negate the principle. Just because you and I are both pretty sure the poster is barking up a whimsical tree doesn't mean we shouldn't give him accurate answers. We just need to qualify them. 


#7
Jan811, 10:41 PM

P: 83

what are the safety hazards in sending a fission reactor to space



#8
Jan811, 10:45 PM

P: 15,319

So... no hazards that aren't manageable. (They're not technically nuclear reactors. That would be too heavy. They're radioisotope thermoelectric generators.) 


#9
Jan911, 07:20 AM

P: 83

is fission thermo electric reactors are viable for electric propulsion like MPD or ion drive?
does their weight to energy or weight to thrust ratio allow space mission 


#10
Jan911, 07:37 AM

P: 3,387




#11
Jan911, 08:43 AM

Sci Advisor
PF Gold
P: 1,776

Remember that no matter how much one accelerates one is traveling at speed 0 relative to one's own frame and that the speed of light in that frame is just as far away as ever. To be more accurate one should speak of the deltaV of a spacecraft relative to the starting frame.
For a nonrelativistic rocket this is dictated by the Tsiolkovsky rocket equation: [tex]\Delta V = V_{exhaust} \cdot \ln\left( \frac{m_0}{m_1}\right)[/tex] where V_exhaust is the exhaust velocity of the ejected reaction mass, and the mass ratio is the initial mass divided by the final mass of the rocket. There is a relativistic version: [tex]\Delta V = c \cdot\tanh\left( \frac{V_{exhaust}}{c}\ln\left( \frac{m_0}{m_1}\right)\right)[/tex] To achieve a "delta Vee" approaching lightspeed one needs either nearlightspeed reaction mass exhaust, lightspeed reaction "mass" as in a photonic drive, or external reaction "mass" as with a lightsail or a Bussard ramscoop. If one has a photonic drive (eject electromagnetic radiation out the back of your ship via lasers, masers, or a radio antenna) then the exhaust velocity is c and the relativistic deltaV equation becomes: [tex]\Delta V = c \cdot\tanh\left( \ln\left( \frac{m_0}{m_1}\right)\right)=...= c\cdot\frac{m_0^2m_1^2}{m_0^2+m_1^2}[/tex] To achieve a DeltaV of say 50% c you would need to "burn" about 42.3% of your initial mass directly to energy. (say by matter antimatter annihilation. Many problems there including finding a lightweight means of directing the gamma ray energy emitted by such a reaction in one direction.) It is useful to look at the lightdrive case because any other reaction mass method will be less efficient and so still requires either direct conversion of mass or externally supplied power. The Bussard ramjet idea developed by Robert W. Bussard, assumes we crack fusion power and involves using very large magnetic fields to collect and compress interstellar hydrogen. The hydrogen is then fused for energy and ejected for reaction mass. Since it burns interstellar hydrogen both for reaction mass and energy it has theoretically unlimited Delta V (as a % of c) if one neglects issues of drag vs thrust. Another advantage of this idea is the magnetic scoop would shield the payload from the impact of high energy protons at relativistic speeds since these are what are being diverted for use as fuel. Another possibility: Lightsails also do not need to carry fuel but rather use the momentum of reflected light for relative motion and thus theoretically could achieve a deltaV close to c. They of course depend on a source of light and using stars has the added difficulty of their gravity being proportional to their light output at a given distance. Ideally one would push these with a directed beam of electromagnetic radiation, say a bank of orbital mirrors at home focusing sunlight toward the sail, or artificially transmitted EM radiation. One interesting and potentially viable idea is the Starwisp concept. It is a microwave "light"sail pushed by orbital transmitters back home. It can be powered by the very microwaves which are used to push it. This idea seems to me to be the most viable means for us to send probes to the nearest stars. 


#12
Jan911, 09:12 AM

P: 3,387




#13
Jan911, 09:51 AM

Mentor
P: 22,243




#14
Jan911, 09:57 AM

Mentor
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#15
Jan1011, 07:18 AM

P: 83

please tell me what is the fundamental law or laws that prevents nonmass expulsion type propulsion device? the propulsion device must use energy stored inside space craft only.
ignore solar sail or laser propulsion where energy source is from out side. you must agree that an efficient nonmass expulsion type propulsion device is the next best thing to wormholes. 


#16
Jan1011, 08:04 AM

P: 3,387

http://en.wikipedia.org/wiki/Reactionless_drive
If they worked, they would be one of the best ways to travel around the universe. But they don't. The "fundamental laws" that prevent such devices are the basics  Newtons laws of motion. 


#17
Jan1011, 08:44 AM

P: 15,319




#18
Jan1011, 08:47 AM

P: 3,387




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