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.