# Is near light speed achievable?

by sr241
Tags: lightspeed, rocket engine
 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?
 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 human-carrying rocket.
 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?
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Is near light speed achievable?

 Quote by sr241 actually my friend is working on an a rocket engine that doesn't work on mass expulsion
Well then it isn't a rocket...

 Quote by sr241 so, if constant thrust is given is it possible to achieve near light speed?
Yes.

 Quote by sr241 What is the relativistic version of thrust equation?
Use the Lorentzian transformation to calculate the addition of velocities.

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.
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P: 22,281
 Quote by sr241 actually my friend is working on an a rocket engine that doesn't work on mass expulsion
At face value that sounds like a violation of the laws of physics.
 so, if constant thrust is given is it possible to achieve near light speed?
Um, well yes, of course if we throw the laws of physics out the window we can do whatever we want. But this is a forum where we discuss reality so your question was answered in a realistic way: the answer is no.
 What is the relativistic version of thrust equation?
What's the "thrust equation"? I think you may mean the relativistic rocket equations: http://math.ucr.edu/home/baez/physic...SR/rocket.html

There may be an automatic calculator out there for it but I'm not sure (you can google).
P: 15,319
 Quote by russ_watters At face value that sounds like a violation of the laws of physics.
Russ, there are lots of designs for propulsion that don't involve mass expulsion. Solar sails are an obvious one.

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.

 Quote by russ_watters Um, well yes, of course if we throw the laws of physics out the window we can do whatever we want. But this is a forum where we discuss reality so your question was answered in a realistic way: the answer is no.
The answer is yes. If constant thrust were provided, one would approach light speed. An example of this is a Bussard Ramjet.

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.
 P: 83 what are the safety hazards in sending a fission reactor to space
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 Quote by sr241 what are the safety hazards in sending a fission reactor to space
In fact, it has been a popular choice for powering our deep space probes for quite some time now.

So... no hazards that aren't manageable.

(They're not technically nuclear reactors. That would be too heavy. They're radioisotope thermoelectric generators.)
 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
P: 3,387
 Quote by sr241 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
sr241, (assuming I have the right thread) please take note of my email.
 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 delta-V of a spacecraft relative to the starting frame. For a non-relativistic rocket this is dictated by the Tsiolkovsky rocket equation: $$\Delta V = V_{exhaust} \cdot \ln\left( \frac{m_0}{m_1}\right)$$ 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: $$\Delta V = c \cdot\tanh\left( \frac{V_{exhaust}}{c}\ln\left( \frac{m_0}{m_1}\right)\right)$$ To achieve a "delta Vee" approaching light-speed one needs either near-light-speed reaction mass exhaust, light-speed reaction "mass" as in a photonic drive, or external reaction "mass" as with a light-sail or a Bussard ram-scoop. 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 delta-V equation becomes: $$\Delta V = c \cdot\tanh\left( \ln\left( \frac{m_0}{m_1}\right)\right)=...= c\cdot\frac{m_0^2-m_1^2}{m_0^2+m_1^2}$$ To achieve a Delta-V of say 50% c you would need to "burn" about 42.3% of your initial mass directly to energy. (say by matter anti-matter annihilation. Many problems there including finding a light-weight means of directing the gamma ray energy emitted by such a reaction in one direction.) It is useful to look at the light-drive 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: Light-sails also do not need to carry fuel but rather use the momentum of reflected light for relative motion and thus theoretically could achieve a delta-V 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.
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 Quote by jambaugh 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.
From just that rather short wiki entry alone, it doesn't seem particularly viable to me.
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 Quote by DaveC426913 Russ, there are lots of designs for propulsion that don't involve mass expulsion. Solar sails are an obvious one. 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.
I have a very good crack-dar, Dave. The fact that he called it a "rocket engine" implies to me that he means a reactionless rocket PMM, not a solar sail.
 The answer is yes. If constant thrust were provided, one would approach light speed. An example of this is a Bussard Ramjet.
The Bussard Ramjet is still just theoretical/science fiction.
 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.
I gave him the same answer you did, Dave, my qualifications were just different!
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P: 22,281
 Quote by sr241 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
It can be done, sure. I think for missions closer to the sun solar panels are typically used, but for something deeper into space, some form of nuclear power would be the way to go.
 P: 83 please tell me what is the fundamental law or laws that prevents non-mass 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 non-mass expulsion type propulsion device is the next best thing to wormholes.
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http://en.wikipedia.org/wiki/Reactionless_drive
 A reactionless drive or inertial propulsion engine (also reactionless thruster, reactionless engine, bootstrap drive, and inertia drive) is any form of pseudoscientific propulsion not based around expulsion of fuel or reaction mass.
 In spite of their physical impossibility, such devices have been often proposed in recent history and are a staple of science fiction.
sr241, I have explained why your design won't work. If you would like to get a second opinion, please post the design here and allow others to comment on it. I assure you, it will be the same response.

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.
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 Quote by sr241 please tell me what is the fundamental law or laws that prevents non-mass expulsion type propulsion device? the propulsion device must use energy stored inside space craft only.
There is no fundamental law. We just don't have any viable way of doing it.
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 Quote by DaveC426913 There is no fundamental law. We just don't have any viable way of doing it.
Just for clarity, my post refers specifically to sr241's design itself regarding Newtons Laws, and specifically conservation of momentum.

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