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

[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.

 

[Santa Susana Brat]

I thought I saw the Space Ex mars trip had a new projected 64 k mph trip speed with using refilled space ex fuel tanks in space, reducing the trip time from 9 months to 3 months. If you have 3,6,9 refueled rocket tanks in space traveling at 17,000 miles per hour does that not increase the projected spacecraft speed. Using multiple launches to increase the potential fuel in space? Possibly, launching a small nuclear engine to send a cell phone sized probe on top to increase final speed and find and send back earth like planet information? To see if an earth like planet is worthy of more inspection. The question is what is the most efficient burn rate for maximum velocity for fuel mass.
Santa Susana Brat.

 

[phinds]

I thought I saw

This is a science forum. That "reference" is about as useful as "I heard some guy on a bus say that ... ".

We have no way of knowing whether

• You remember something that is true
• You "remember" something incorrectly
• You were misinformed
• etc

 

[Santa Susana Brat]

Thought we were having a open and explorory conversation about science so, after reading 3 pages of old rocket formulas form the 50’s. But according to NASA the estimated speed on the Mars mission is 24,600 miles per hour or 39,600 kph. This is down from 9 months estimated trip time to 7 months. Yet Space Ex estimated Mars trip speed is 62,700 miles per hour or 3 months trip time. That’s three times your current thinking. What is the differance of the two Mars mission speed and time estimates does anyone know? Could it be outside of the box engineering, earth based launches vs. one spaced based refueled tanks with relaunchable rocket launches. After reading three pages on rocket formulas to find a max cheminical rocket volicity the answer was depends! But I did see it, did you?
Santa Sunansa Brat

 

[renormalize]

Yet Space Ex estimated Mars trip speed is 62,700 miles per hour or 3 months trip time.

Please cite the specific reference where SpaceX states this.

 

[Ibix]

Thought we were having a open and explorory conversation about science so, after reading 3 pages of old rocket formulas form the 50’s. But according to NASA the estimated speed on the Mars mission is 24,600 miles per hour or 39,600 kph. This is down from 9 months estimated trip time to 7 months. Yet Space Ex estimated Mars trip speed is 62,700 miles per hour or 3 months trip time. That’s three times your current thinking. What is the differance of the two Mars mission speed and time estimates does anyone know? Could it be outside of the box engineering, earth based launches vs. one spaced based refueled tanks with relaunchable rocket launches. After reading three pages on rocket formulas to find a max cheminical rocket volicity the answer was depends! But I did see it, did you?
Santa Sunansa Brat

As others have said, it's difficult to comment on numbers that may or may not be accurately remembered from a source that may or may not be authoritative. And there's also the question of speed relative to what - Earth, Mars or the Sun. All three references are reasonable and will all give different speeds, and if the sources are using different references they could be talking about similar trips in different terms. That's why we ask for references.

Extra fuel allows you to burn for longer which does let you get there faster, yes. That comes at a greater cost and a brutal law of diminishing returns - simply doubling your fuel comes nowhere near doubling your peak speed. Better propellant (one with a higher specific impulse) would help, but I've no idea what the mission planners are thinking on that.

You certainly could partially construct a vessel in orbit, at least in principle. That might give you more flexibility in design, but it likely comes at a cost in reliability because it's yet another new technology that you'd want to test thoroughly. I doubt it does anything revolutionary to your fuel efficiency.

 

[ketoenol]

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.

Plenty of real spacecraft use ion drives, including SpaceX's Starlink satellites which are a majority of spacecraft in Space right now. NASA's Psyche spacecraft is using them, as is the mission the UAE is designing to go to the asteroid belt.

An ion drive doesn't really change the fundamental problem; they manage to throw their fuel out the back faster, so less fuel gets you more delta-V, but at best you're really only increasing the specific impulse by an order of magnitude (often less)

 

[PeterDonis]

Plenty of real spacecraft use ion drives

Only for station keeping, though, correct? In other words, the ion drive will do an occasional short acceleration to correct orbital parameters. It won't be running continuously for a long period of time. The latter usage, for example to make interplanetary trips in significantly shorter times than by current free-fall orbit methods, is what I was talking about.

An ion drive doesn't really change the fundamental problem

It doesn't change the rocket equation, of course. But it does potentially offer a way of providing a small but constant acceleration over a long period of time, something which is not possible with chemical rockets.

 

[ketoenol]

Only for station keeping, though, correct? In other words, the ion drive will do an occasional short acceleration to correct orbital parameters. It won't be running continuously for a long period of time. The latter usage, for example to make interplanetary trips in significantly shorter times than by current free-fall orbit methods, is what I was talking about.

I would say using ion thrusters only for station keeping is actually the rarer use case. The Starlink ion thrusters are used to raise the orbits from the initial low insertion orbit to their final operational orbit, so they are firing continuously for weeks or months slowly gaining speed. The interplanetary spacecraft that have them are absolutely using them long-term, slowly gaining speed. The Wikipedia page for the Dawn spacecraft has a good image showing this; they're not operated absolutely continuously, but they are operated for large chunks of the time, very different than traditional spacecraft trajectory maneuvers.

 

[PeterDonis]

the Dawn spacecraft

Yes, you're right, that's a good example of a long term constant thrust use of ion drives, but it's very small--a dry mass of 747 kg according to the Wikipedia page you referred to [1].

The real breakthrough will be using such a drive for a craft large enough to carry humans. For example, the dry mass of the Apollo CSM was about 12,000 kg.

[1] https://en.wikipedia.org/wiki/Dawn_(spacecraft)