# Why can't we reach to Speed of light at Space?

1. Mar 4, 2015

### Majid1986

a question has made my mind busy....it is told that there is no frictions at space...frictions of air...friction of gravity and etc...none of these does exist in the space (outside the earth)...so we can launch a spacecraft with a primary speed(orbit speed of earth) and equip it with a engine....with a primary speed , and with a engine , and without any friction....theorically we can increase the speed and acceleration....and then reach to a ultra high speed until a speed like light speed....but WHAT IS THE LIMITATIONS???

2. Mar 4, 2015

### Jobrag

The problem is that as the speed of an object increases so does the mass at normal speeds this is pretty insignificant but as yuo approach the speed of light the mass of the object approaches infinity.

3. Mar 4, 2015

### ZapperZ

Staff Emeritus
You don't have to go into outer space, nor do you need to consider something as big as a spacecraft. Just look at what is happening at particle accelerators around the world. The LHC can only get the protons to 0.999c traveling inside the vacuum pipes. If something that small (when compared to your spacecraft) inside a vacuum line required such huge amount of energy to "just" get to that speed, how much more of an effort do you think it will take to do the same on your spacecraft?

There are a lot of things you can learn from things we already know here on Earth.

Zz.

4. Mar 4, 2015

### phinds

Not really. It has the energy equivalent of increased mass but not an increase in actual mass.

Think about it this way: An object traveling at .999c relative to the Earth is also traveling at .8c relative to some other frame of reference and .1c to yet another and very slowly relative to something traveling almost along-side it. If the object actually had some mass based on it's speed relative to Earth, it would have to have different masses relative to each of those other frames of reference. Now you can't have the same object having different inherent masses, thus, clearly, the object itself does not have any increase in mass.

This concept of "relativistic mass" has long been deprecated in Physics, it's just that the word hasn't yet gotten to pop-sci writers.

5. Mar 4, 2015

### Chronos

In rocket propulsion, matter is forcefully ejected from the engine, producing an equal and opposite reaction. Under Newtons 3rd law, it appears rocket velocity cannot exceed its exhaust velocity, but, the exhaust velocity of a LOX/H2 rocket [used to launch satellites] is only 15,000 mph whereas earths escape velocity is 17,500 mph. Tsiolkovsky, legendary rocketry expert, worked out the math in 1903. $$\Delta v = v_e \ln \frac{m_0}{m_1}$$.
= initial mass
= final mass [ less expended mass - i.e. fuel and boosters]
= rocket exhaust velocity
= maximum change in rocket velocity - assuming no external forces

Even putting relativistic corrections aside, you would need stupendous exhaust velocity to achieve anything close to relativistic velocities.

6. Mar 4, 2015

### Staff: Mentor

I don't really like any of the answers (sorry). Based on the premise of the question (the total irrelevancy of friction), it doesn't appear the OP even understands Newton's laws. In that context, the critical reason our real rockets can't get anywhere close to C is f=ma and the fact that rockets have to carry a lot of fuel to generate thrust. So they can't accelerate very fast or for very long. Different propulsion technologies, such as nuclear fuel, may result in much more efficient and therefore much faster rockets, but nothing on the horizon even gets us close to where Relativity matters.

Chronos, your answer in particular is very confusing. It implies a conflict due to the speed of the rocket exceeding the speed of the exhaust. This isn't the case. They are not tied to each other in any direct way -- that's why the rocket speed isn't in the equation. Getting a rocket to move much, much faster than its exhaust velocity can be done by firing the rocket for a long time and using multiple stages -- which would be similar to just arbitrarily changing reference frames (such as from earth stationary to milky-way stationary). The (Galilean) principle of Relativity demands that acceleration is independent of reference frame.

Last edited: Mar 4, 2015
7. Mar 5, 2015

### Quds Akbar

As you approach the speed of light your mass increases and slows you down, and with more energy you have more mass (E=mc2) so you would never actually approach the speed of light.

8. Mar 5, 2015

### Chronos

Russ. Are you disputing the Tsiolkovsky equation, or what? I dont get it.

9. Mar 5, 2015

### phinds

When responding in a thread, it is a good idea to read the other responses before you respond, so you can be sure you are not simply saying the same thing over again or even worse, as in your case, continuing to provide incorrect information. Please read post #4

10. Mar 5, 2015

### Staff: Mentor

No, certainly not. I was disputing something implied by your description/application of it: that rockets are typically limited to roughly the speed of their exhaust.

11. Mar 5, 2015

### Chronos

12. Mar 5, 2015

### stehfahknee

I will not pretend that I am an expert, but I do know that the laws of physics break down at that speed correct?

13. Mar 5, 2015

### phinds

No, absolutely not. The laws of physics are quite clear about what happens. Please read the other responses in this thread.

14. Mar 5, 2015

### stehfahknee

I have. Many disputes mostly and no way to know who is correct or incorrect. Which means more research to learn, but that's actually a good thing.
I have always read that when you reach the speed of light things get weird.
I'm off to read some more then.

15. Mar 5, 2015

### DaveC426913

There is no dispute. Special relativity is extremely well-studied, and well-understood.
That is not to say it is not difficult to describe in few words - as we are seeing here.

Weird is subjective.
It is certainly very different from our usual day-to-day experiences, but it quite faithfully obeys our laws of physics.

16. Mar 5, 2015

### stehfahknee

You have a point. I'm reading up on it now, and getting pretty into it. Which means I will probably be preoccupied for the next...well... who knows how long... I tend to start reading about one thing, see something mentioned and go off on that branch, which leads to another and so on and so on. I can stay up for days on these tangents...

Last edited: Mar 5, 2015
17. Mar 6, 2015

### D H

Staff Emeritus
The first link is, to use Russ Waters' words, rough. There's nothing there that I can see that says maximum rocket velocity is twice its exhaust velocity. That is not the case. If it was, we wouldn't be able to vehicles into orbit. The best specific impulse fuel that is capable of launching a vehicle into orbit is liquid hydrogen as propellant and liquid oxygen as oxidizer. This has a vacuum exhaust velocity of 4.4 to 4.5 km/s. Performance in an atmosphere is less than that. Double that and you get at most 9 km/s. Getting into low Earth orbit requires 10 to 11 km/s delta V. Escaping the Earth's gravitational field from the surface of the Earth takes roughly twice that.

Given that, how do we send vehicles into orbit, and then towards other planets? (That's a rhetorical question.)

The answer is that a factor of two isn't quite right. A single stage rocket whose initial mass is 90% fuel will achieve 2.3 times the exhaust velocity, which is already over your limit of twice the exhaust velocity. That's something even amateur rocketeers can accomplish. A single stage rocket whose initial mass is 95% fuel will achieve 3 times the exhaust velocity. This still doesn't get a vehicle to the ~20 km/s Δv needed to escape the Earth's gravitational field. The solution is simple: Don't use a single stage rocket. For example, the Saturn V+Apollo vehicle used to take astronauts to the Moon and back was essentially a six stage rocket. (Note well: Nobody has made a six stage rocket since Apollo was cancelled.)

Nonetheless, what Russ wrote was essentially correct. He said "roughly". Rockets are indeed roughly limited by the speed of their exhaust (to within an order of magnitude).

Getting back to the original question,
Aside: I'm being nitpicky here, but it's "what **are** the limitations," not "what is the limitations." You are posting from an English-speaking country. You should know the rules. Use them.

The ideal rocket equation establishes one of the limitations. Making a vehicle that is 99% fuel is an engineering impossibility. Even if we could make such a vehicle, the Δv would only be 4.6 times the exhaust velocity. Play games with making a multistage vehicle and you might get another factor of 3 or so on top of that. You would need a vehicle whose exhaust velocity is beyond anything achievable or imaginable, and you still wouldn't get anywhere close to the speed of light. The amount of energy needed to make a tiny proton get close to the speed of light is immense. The amount of energy needed to make a decent-sized spacecraft get close to the speed of light is beyond immense. A decade's worth of the world's entire energy output might do the trick.

Even then, there are other problems. Suppose our children's children's children do find some magic that enables a spacecraft to 90% of the speed of light. Space is not quite empty. There's gas and dust in interstellar space, and this will slow them down (a lot). Interstellar space also contains other occasional obstacles. At 90% the speed of light, a collision with a tiny one gram chunk of stuff would unleash the equivalent of multiple nuclear bombs worth of energy on the spacecraft.

18. Mar 6, 2015

### Majid1986

thanks all...but none of you friends understood my question...
lets give you a clearance about what is across my mind....first i am not from an english country and miss-spelling may occur in my post...i have a basic information about physics and not involved with equations and formulas
second
suppose we launch a spacecraft with primary speed (for example 1000km/h or mils/h)...in the space where no friction exists, we turn the engine ON and naturally the speed must be increased as in our example to 1100 km/h.....we turn the engine OFF....now the speed must be 1100km/h......after a while we turn the engine ON and our speed reaches to 1200km/h....we turn the engine OFF...turn it ON again and so on...we continue this manner for 1 year...2 years..3 years and more(suppose our spacecraft has enough fuel to last all the years)....after many years theorically we must reach to the speed of light....but why we can't reach??? this is my question....

19. Mar 6, 2015

### Chronos

To quote from http://www.hq.nasa.gov/pao/History/SP-4026/noord12.html

"... If the v/c ratio becomes greater than 1 (the travel velocity exceeds the velocity of expulsion), the efficiency of the reaction is diminished again and, finally, for v/c=2 it again goes through zero and even becomes negative (at travel velocities more than twice as large as the velocity of expulsion).

The latter appears paradoxical at first glance because the vehicle gains a travel velocity as a result of expulsion and apparently gains a kinetic force as a result! Since the loss of energy, resulting through the separation of the expulsion mass loaded very heavily with a kinetic force due to the large travel velocity, now exceeds the energy gain realized by the expulsion, an energy loss nevertheless results for the vehicle from the entire process despite the velocity increase of the vehicle caused as a result. The energy loss is expressed mathematically by the negative sign of the efficiency. Nonetheless, these efficiencies resulting for large values of the v/c ratio have, in reality, only a more or less theoretical value.

It can, however, clearly and distinctly be seen from the table how advantageous and, therefore, important it is that the travel velocity approaches as much as possible that of the expulsion in order to achieve a good efficiency of reaction, but slight differences (even up to v=0.5 c and/or v=1.5 c) are, nevertheless, not too important because fluctuations of the efficiency near its maximum are fairly slight. Accordingly, it can be stated that the optimum travel velocity of a rocket vehicle is approximately between onehalf and one and onehalf times its velocity of expulsion."

Thus, v/c = 2 is not asserted as an absolute limit on rocket velocity. The exhaust velocity of LOX/LH rocket is approximately 15,000 mph and escape velocity of earth is about 25,000 mph. So, achieving escape velocity is within v/c=2. From http://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html it is noted

" If the radius of our planet were larger, there could be a point at which an Earth escaping rocket could not be built. Let us assume that building a rocket at 96% propellant (4% rocket), currently the limit for just the Shuttle External Tank, is the practical limit for launch vehicle engineering. Let us also choose hydrogen-oxygen, the most energetic chemical propellant known and currently capable of use in a human rated rocket engine. By plugging these numbers into the rocket equation, we can transform the calculated escape velocity into its equivalent planetary radius. That radius would be about 9680 kilometers (Earth is 6670 km). If our planet was 50% larger in diameter, we would not be able to venture into space, at least using rockets for transport. "

Assuming a proportionate mass increase, escape velocity would increase to about 36,000 mph at r = 9680 km, which exceeds v/c = 2. I therefore concluded v/c = 2 was reasonably correct.

20. Mar 6, 2015

### Chronos

Majid, using the Tsiolkovsky equation, and assuming your rocket has a realistic fuel to payload mass ratio of 24 to 1 [96% fuel], you could never increase rocket velocity to more than about 3.17 times the fuel exhaust velocity. So, even ignoring relativistic corrections, you need stupendous exhaust velocity to rocket your way even remotely near light speed. Even with a ludicrous fuel to payload mass ratio of 9999 to 1 [99.99% fuel] you would still only add about 9.21 times fuel exhaust velocity to your rocket. An ion thruster, the most powerful known rocket engine, is only capable of achieving around 200,000 mph exhaust velocity - which is still a very long way from light speed. At some point you must make a concession to the material science gods and worry about what kind of exhaust velocity can be tolerated by the material employed by your chosen rocket design. It is generally agreed that interstellar travel via rocket propulsion is a fail.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook