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

[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.

 

[Ibix]

limited to

I hope you mean "limited by" here. A rocket isn't limited to the speed of its propellant - delta-v can easily exceed the exhaust velocity. The exhaust velocity is, however, a component in the calculation of the speed increase a rocket is capable of, so it is limited by it.

 

[AlexB23]

I hope you mean "limited by" here. A rocket isn't limited to the speed of its propellant - delta-v can easily exceed the exhaust velocity. The exhaust velocity is, however, a component in the calculation of the speed increase a rocket is capable of, so it is limited by it.

Yeah, I meant "limited by". It is one part of the rocket delta-v equation, along with fuel mass.

 

[hutchphd]

I think the destinction is important because it is a common misconception that rockets are constrained to travel slower than the exhaust velocity of the propellant from the engine.....this seems alluringly reasonable at first blush. Many people get hung up on this hence the emphasis for educational reasons.

 

[phinds]

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

Glad to hear I'm not the only one

 

[PeterDonis]

Even a tiny acceleration would slash the journey times.

And tiny accelerations are possible--just not easy with current technology and nobody seems to be investing much in making it better.

Ion drives, for example, are an obvious technology to pursue for putting out a small but consistent acceleration over an extended period of time in vacuum. There just doesn't seem to be any real investment in pursuing them. At some point that will likely change.

 

[berkeman]

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.

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.

Yes, please start a new thread if you want to discuss that perspective. It is off-topic in this thread. Thank you.

 

[AlexB23]

And tiny accelerations are possible--just not easy with current technology and nobody seems to be investing much in making it better.

Ion drives, for example, are an obvious technology to pursue for putting out a small but consistent acceleration over an extended period of time in vacuum. There just doesn't seem to be any real investment in pursuing them. At some point that will likely change.

Yeah, to be honest, I thought the Lucy asteroid mission would use ion drives, as the Dawn asteroid spacecraft used ion drives, but Lucy only uses hydrazine. Come on NASA, invest in ion drives. :)

 

[nightvidcole]

Another thing to keep in mind is that any fuel that could allow you to reach a relativistic speed before running out of fuel (in practice this would be nuclear or matter/antimatter), would be potentially very destructive due to the emission of large amounts of ionizing radiation. This radiation can not only damage living things, but could also damage the solid materials that compose the rocket engine, capsule, and radiation shieldings, and particularly the electronics used to control the reaction and other functions of the rocket/spaceship.

 

[DaveC426913]

I see the ideal rocket equation includes the parameter g.

Is that because it applies to rockets in the influence of a gravitational body (such as Earth)? but that it also covers scenarios outside of gravitational influence (by setting g to zero - or at least zero-ish?)

 

[Ibix]

I see the ideal rocket equation includes the parameter g.

Is that because it applies to rockets in the influence of a gravitational body (such as Earth)? but that it also covers scenarios outside of gravitational influence (by setting g to zero - or at least zero-ish?)

It doesn't really include $g$. It includes the exhaust velocity $v_e$, but some sources prefer to use the specific impulse defined as the impulse change per unit weight on Earth provided by propellant, which is $I_{sp}=v_e/g$. So the $g$ comes in if you use that to eliminate $v_e$, and it's because of the "weight on Earth" unit definition.

Some sources prefer to define specific impulse as impulse per unit mass, in which case it's just equal to $v_e$. So watch your units carefully.