B What limits a rocket's max speed if in space drag is zero?

[user079622]

What limit rocket max speed if in space drag is zero?
If drag is zero and thrust is constant that mean rocket accelerate all the time?

 

[fresh_42]

What limit rocket max speed if in space drag is zero?
If drag is zero and thrust is constant that mean rocket accelerate all the time?

Yes, but where do you get infinite thrust from? It can only carry a limited amount of energy. And the faster it gets, the more energy it needs to accelerate even more.

The drag is not zero either, very close to zero, but positive, a proton per cubic meter or something like that.

 

[erobz]

Doesn't relativity come into play as well?

 

[user079622]

Yes, but where do you get infinite thrust from? It can only carry a limited amount of energy. And the faster it gets, the more energy it needs to accelerate even more.

The drag is not zero either, very close to zero, but positive, a proton per cubic meter or something like that.

So rocket accelerate until has fuel or this few protons limit rocket speed even before all fuel is used?

 

[berkeman]

What limit rocket max speed if in space drag is zero?
If drag is zero and thrust is constant that mean rocket accelerate all the time?

Yes, it keeps accelerating, but its speed will asymptotically approach the speed of light in a vacuum ("c"). This is described by Special Relativity. Here is a graph showing the speed of a rocket in space over time as it has a constant "1g" acceleration (9.81m/s^2):

https://commons.wikimedia.org/wiki/File:1gRocketGraph.png

You can read more about Special Relativity here:
https://en.wikipedia.org/wiki/Special_relativity

 

[user079622]

Doesn't relativity come into play as well?

No, I talk what in practice limit rocket speed..

 

[fresh_42]

No, I talk what in practice limit rocket speed..

The amount of energy you can load. And in case you have to protect lifeforms on board, the type and mass of the rocket and therefore energy again, e.g. nuclear bombs need a lot of shielding material.

 

[user079622]

The amount of energy you can load. And in case you have to protect lifeforms on board, the type and mass of the rocket and therefore energy again, e.g. nuclear bombs need a lot of shielding material.

Why rocket need shielding?
So cosmic microwave background and these few protons/m3 dont limit our rockets speed?

 

[berkeman]

So cosmic microwave background and these few protons/m3 dont limit our rockets speed?

You did see my post #5, right?

 

[Filip Larsen]

I talk what in practice limit rocket speed

For a classical rocket the available speed ("delta V") can be modeled using the rocket equation. This mean the speed is limited by the total mass of propellant (the stuff the rocket ejects) relative to its total initial mass, as well as the effective speed of the ejected propellant relative to the rocket. For multi-stage rockets the equation become a "staged" version of the rocket equation.

 

[fresh_42]

Why rocket need shielding?

I said in case you have living beings on board. Otherwise, radiation will kill it sooner or later without shielding it.

 

[DaveC426913]

The OP seems to have an idea of rocketry that the rest of us do not share, and it sort of feels like we're having to guess what the OP is thinking.

So I'd like to ask @user079622 for their thoughts - to describe what they are expecting to happen, given enough fuel, and maybe we can correct some misunderstandings, if there are any.

 

[user079622]

You did see my post #5, right?

Yes, you want to say if we have enough fuel ,rocket speed is limited only by relativistics effects?

 

[berkeman]

Yes, you want to say if we have enough fuel ,rocket speed is limited only by relativistics effects?

Yes. The small resistance from the single particle per cubic meter is very small compared to relativistic effects.

 

[DaveC426913]

Yes, you want to say if we have enough fuel ,rocket speed is limited only by relativistics effects?

Yes.

Sure, there are a lot of practical and engineering hurdles to take into account (such as radiation and fuel) but nature doesn't set to upper limit on speed until you get near c.

 

[berkeman]

It, too, becomes a problem at high speeds.

Yeah, I guess they're a lot like flourescent bugs hitting the windshield...

https://starwars.fandom.com/wiki/Lightspeed/Legends

 

[fresh_42]

Sure, there are a lot of practical and engineering hurdles to take into account (such as radiation and fuel) but nature doesn't set to upper limit on speed until you get near c.

I don't think this is true. Since space isn't empty, IIRC, we will get effects like bow waves long before we come close to c.

 

[phinds]

Doesn't relativity come into play as well?

No, I talk what in practice limit rocket speed..

@user079622 I assume you now understand that that response was not correct and @erobz has it right, as explained by @berkeman in post #5

 

[Filip Larsen]

@user079622 I assume you now understand that that response was not correct and @erobz has it right, as explained by @berkeman in post #5

For what its worth, I understand the OP's question as to what does in practice limit the speed of a practical rocket (i.e. rocket technology in practical use). If that is indeed the intended question then it is perhaps not totally surprising if OP express some bafflement hearing about relativistic effects only.

 

[Ibix]

For what its worth, I understand the OP's question as to what does in practice limit the speed of a practical rocket

Depends on how literally you take "thrust is constant", I think.

Eventually a rocket will run out of fuel and stop accelerating. That's the hard limit in practice. Depending on how unrealistically powerful your rocket is you may find that you travel slower than a naive $v=at$ calculation would predict due to relativity, but you have to reach sizeable fractions of $c$ for that to be a major concern. For reference, the Apollo missions reached about 0.00004c.

 

[user079622]

@user079622 I assume you now understand that that response was not correct and @erobz has it right, as explained by @berkeman in post #5

It seems that in real life limits by relativity effects dont play at all.

In this answer if we use entire mass of universe for fuel we get only 0.2 c

Answer from Jack Fraser

https://www.quora.com/What-is-the-maximum-speed-a-chemical-rocket-can-achieve

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[user079622]

Depends on how literally you take "thrust is constant", I think.

Eventually a rocket will run out of fuel and stop accelerating. That's the hard limit in practice. Depending on how unrealistically powerful your rocket is you may find that you travel slower than a naive $v=at$ calculation would predict due to relativity, but you have to reach sizeable fractions of $c$ for that to be a major concern. For reference, the Apollo missions reached about 0.00004c.

Basically my question is; does our rockets/probes in space, whenever engine is working they accelerate?

 

[Ibix]

I haven't checked his maths, but he's specifically talking about chemical rockets, and his point is that they suck. You can do better with a more efficient fuel, although even with an idealised 100% efficient matter/antimatter powered photon rocket the numbers are still depressing - still billions-to-one dry mass-to-fuel ratios for relativistic travel, but not as bad as his calculations. Or (as he notes) you can use tricks so you don't have to carry all your reaction mass.

You are likely to get more helpful answers if you can be more specific in your questions. You're getting different answers because there are several different interpretations of your question being considered. Can you say why do you want to know? Are you talking about current technology, or current technology if you got rid of economics and just went for it? Or are you interested in plausible near-future technology, or idealised perfect models?

 

[user079622]

Can you say why do you want to know?

Because in my logic in space there is zero drag(if it is?) when engine working(const. thurst) rocket must accelerate all the time?
So I ask my self , why our rockets/probes travel so slow (400 000mph) if nothing stops them.....Why we dont get more speed, lets say 0.8c etc etcI am not interested in Einstein theoretical limits.

 

[Ibix]

Basically my question is; does our rockets/probes in space, whenever engine is working they accelerate?

Yes, always.

(One can imagine situations where they're exposed to a force by the solar wind and they use engine power to hold position, but that's very much "I'm just saying this because I'm a pedant" answer, since the kind of forces are tiny.)

So I ask my self , why our rockets/probes travel so slow (400 000mph) if nothing stops them.....Why we dont get more speed, lets say 0.8c etc etc

Because they only carry a small amount of fuel, and when it's gone, it's gone, and they can't accelerate any more. You could always build bigger rockets that carry more fuel, but that costs more, so if you aren't in a hurry, why bother? That goes double if you want to slow down at the other end of your trip. You have to accelerate all the fuel you need to stop again, so the faster you go the more fuel you need to stop and the more fuel you need to get going in the first place.

 

[Filip Larsen]

why our rockets/probes travel so slow (400 000mph) if nothing stops them

Because of the rocket equation when applied to chemical rockets that has to be assembled and launched from the Earths surface within a realistic budget. That is, the limiting factor is the amount of fuel you can bring along while still having room for a sensible payload.

 

[PeroK]

Because in my logic in space there is zero drag(if it is?) when engine working(const. thurst) rocket must accelerate all the time?

It's essentially impossible to accelerate in a vacuum.

 

[phinds]

It's essentially impossible to accelerate in a vacuum.

Uh ... HUH ???

 

[user079622]

Yes, always.

Ok then everything is clear.

 

[PeroK]

Uh ... HUH ???

It's a more realistic starting point, than "you just fire the engines"! The Mars missions have only the capability to "tweak" the flight path a few times en route. If you could accelerate/decelerate at even $1m/s^2$, you could get to Mars in a few weeks, rather than 7 months. It takes 7 months to get to Mars because ... once you have completed your initial acceleration, it's essentially impossible to keep accelerating in a vacuum. Even a tiny acceleration would slash the journey times.

Focusing on the "maximum speed" is pointless when there is essentially no means of acceleration, beyond an initial boost.

 

[Ibix]

It's a more realistic starting point, than "you just fire the engines"! The Mars missions have only the capability to "tweak" the flight path a few times en route. If you could accelerate/decelerate at even $1m/s^2$, you could get to Mars in a few weeks, rather than 7 months. It takes 7 months to get to Mars because ... once you have completed your initial acceleration, it's essentially impossible to keep accelerating in a vacuum. Even a tiny acceleration would slash the journey times.

Focusing on the "maximum speed" is pointless when there is essentially no means of acceleration, beyond an initial boost.

I must admit I was a bit confused by #27, too. Your expanded version makes a lot more sense!

 

[PeroK]

I must admit I was a bit confused by #27, too. Your expanded version makes a lot more sense!

As a concrete example. If I want to go round a corner in a car, I just turn the steering wheel. A steering wheel would achieve nothing in a vacuum!

 

[Ibix]

As a concrete example. If I want to go round a corner in a car, I just turn the steering wheel. A steering wheel would achieve nothing in a vacuum!

In space, no-one can hear your tyres screech.

 

[user079622]

once you have completed your initial acceleration, it's essentially impossible to keep accelerating in a vacuum.

If you shout down engines rocket will travel at const speed, you can accelerate only when engine is working.
Isnt it?

 

[Filip Larsen]

A steering wheel would achieve nothing in a vacuum!

I guess that is a good reason why practical rockets usually aren't equipped with a steering wheel? However, if used as an argument for why a rocket don't need speed beyond some point, it seems a bit weak to say steering wheels don't work well in space.

 

[Filip Larsen]

If you shout down engines rocket will travel at const speed, you can accelerate only when engine is working.

Yes, a rocket (or any mass really) not being affected by any force will as a whole not accelerate. However, a mass coasting in the Solar system will to some extend be affected by gravity and solar radiation.

 

[PeroK]

If you shout down engines rocket will travel at const speed, you can accelerate only when engine is working.
Isnt it?

Depends what the engine does. I suggest that you are missing the fundamental point about motion on the Earth's surface or in the atmosphere. An object doesn't move just because you switch on an engine. The vehicle must interact with either the ground or air to propel itself. If you take away the road, a car engine does nothing. And, if you take away the air, then a jet engine does nothing. So, although air drag limits the speed of a plane, without it the plane can't fly in the first place.

Your whole post is predicated on the assumption that if there is no drag, you can accelerate without resistance. You are, however, missing the fundamental point that you need to intetract with something external in order to accelerate in the first place. You need an external force, in other words.

A conventional rocket can get round this for a short period by firing expellant from the rear. This has severe practical limitations, as highlighted in the Quora post. But, if you think of the expellant as part of the rocket, then the centre of mass of your system is not accelerating. In other words, firing expellant is technically an internal force and results in zero acceleration of the centre of mass.

There are, of course, ideas for propulsion through a vacuum. Once you have a proposed system of propulsion, then you can estimate the maximum speed. The lack of drag in vacuum has little to do with the answer.

 

[user079622]

Yes when pilot move to the left, hang glider move to the right, but center of mass of the system remain in same place.

even this has nothing to do with my question

 

[DaveC426913]

even this has nothing to do with my question

How has your question not been answered?
Did you want to refine it now?

 

[user079622]

How has your question not been answered?
Did you want to refine it now?

My question is answered in post #25

 

[ohwilleke]

Basically my question is; does our rockets/probes in space, whenever engine is working they accelerate?

The engine only accelerates the rocket when it has fuel.

So, the maximum speed of a rocket depends upon (1) the amount of fuel relative to the payload, and (2) the velocity with which the engine ejects mass from the rocket (which, in turn, depends upon the design and efficiency of the engine and the energy density of the fuel).

All chemical fuels have energy densities of the same order of magnitude. Nuclear fuel is much, much more energy dense, so a comparable sized rocket with nuclear fuel can reach much higher speeds than a chemical fuel rocket, even though a rocket with nuclear fuel needs a bigger payload to shield the payload from the radiation of the nuclear blasts that move the rocket.

The tricky thing about a rocket is that the amount of fuel is not independent of the amount of fuel you need to reach a certain velocity (i.e. speed).

This is because until you burn it, your engine needs to push not only the rocket's payload but also the fuel you haven't used yet.

In the image above of a real historical rocket using chemical rocket fuels that was used to go to the moon, only the area circled in red is the payload. Everything else in the image is fuel (and temporary engines to use it until that segment's fuel is used up) that gets disposed of segment by segment as the fuel from that segment (also called a "stage" of the rocket) is used up.

As Wikipedia explains:

The classical rocket equation, or ideal rocket equation is a mathematical equation that describes the motion of vehicles that follow the basic principle of a rocket: a device that can apply acceleration to itself using thrust by expelling part of its mass with high velocity can thereby move due to the conservation of momentum. . . . The necessary wet mass grows exponentially with the desired delta-v.

The equation is as follows:

Where "e" is Euler's number (a mathematical constant equal to 2.71828 . . . .).

So, the faster you want your rocket to go, the bigger the percentage of the rocket that is used for fuel must be.

The only loopholes that can be used to reach a speed greater than the speed allowed by the ideal rocket equation involve using source of energy to achieve higher speed other than thrust from the rocket engine derived from carried fuel.

In practice, the two main exceptions to the ideal rocket equations are (1) to gain speed with a gravitational slingshot effect, and (2) to use a beam of energy like a laser (or solar energy) to transfer energy to the rocket that doesn't come from its own carried fuel. The laser allows for significantly higher acceleration than relying on solar energy (a design called a "solar sail"), but is limited by the need to have a line of sight between the laser and the rocket and the need to keep the laser beam perfectly focused over a long distance. But both methods accelerate the rocket above its maximum speed using carried fuel alone.

An artist's impression of a solar sail.

 

[sdkfz]

... In the image above of a real historical rocket using chemical rocket fuels that was used to go to the moon, only the area circled in red is the payload. ...

(Very small quibble but your red circle includes the Launch Escape System (discarded at about 90km up) and excludes the Lunar Module (older "LEM" in the diagram) ... which in turn illustrates your point in having a descent and ascent stage.)

 

[Devin-M]

On seeing this graph I couldn’t help but think “I’ve been accelerating at 1g for a lot more than 3 years…”

Here is a graph showing the speed of a rocket in space over time as it has a constant "1g" acceleration (9.81m/s^2):

https://commons.wikimedia.org/wiki/File:1gRocketGraph.png

You can read more about Special Relativity here:
https://en.wikipedia.org/wiki/Special_relativity

 

[Ibix]

On seeing this graph I couldn’t help but think “I’ve been accelerating at 1g for a lot more than 3 years…”

GR is more complicated than SR. I'd suggest starting a different thread if you want to discuss it.

 

[ohwilleke]

On seeing this graph I couldn’t help but think “I’ve been accelerating at 1g for a lot more than 3 years…”

You've been subject to a 1 g downward force for a lot more than 3 years, but the net force on you has been approximately zero, because it is matched by the force of the ground in the opposite direction of the gravitational force giving rise to an acceleration.

 

[ohwilleke]

(Very small quibble but your red circle includes the Launch Escape System (discarded at about 90km up) and excludes the Lunar Module (older "LEM" in the diagram) ... which in turn illustrates your point in having a descent and ascent stage.)

Fair points. My drawing precision is alas, still stuck somewhere around the level it was at when I was in kindergarten.

 

[PeroK]

You've been subject to a 1 g downward force for a lot more than 3 years, but the net force on you has been approximately zero, because it is matched by the force of the ground in the opposite direction of the gravitational force giving rise to an acceleration.

I guess that's true in the classical physics forum. If we were in the relativity forum it would be a different matter!

 

[Devin-M]

You've been subject to a 1 g downward force for a lot more than 3 years, but the net force on you has been approximately zero, because it is matched by the force of the ground in the opposite direction of the gravitational force giving rise to an acceleration.

I thought it was the ground accelerating me upward for more than 3 years at 1g, since if I were in freefall an accelerometer would measure 0g… interesting to think every 3 year old knows “what it feels like” to be accelerated to nearly the speed of light.

 

[Ibix]

interesting to think every 3 year old knows “what it feels like” to be accelerated to nearly the speed of light.

That graph is not appropriate for GR, so your conclusion here is unwarranted. Except in the trivial sense that "nearly the speed of light" is true of any state of motoon from some choice of frame.

 

[AlexB23]

What limit rocket max speed if in space drag is zero?
If drag is zero and thrust is constant that mean rocket accelerate all the time?

The maximum speed of the rocket is limited to the maximum amount of fuel the rocket can hold, as well as the velocity of the fuel being expelled.