Minimum usable acceleration for a spacecraft

[MichaelPoole]

Hello, apologies if this is in the wrong section, it's hard SF related but it didn't seem to fit any other part of the forum for me.

So, when reading up on rocketry on Projectrho, I came upon this interesting bit http://www.projectrho.com/public_html/ro...#lasersail

As an important point, the practical minimum acceleration for a spacecraft is about 5 milligees. Otherwise it will take years to change orbits. Photon sails can only do up to 3 milligees, but a laser sail can do 5 milligees easily.

Is this true? If yes, how can it be true since the Dawn probe did not take years to enter Vesta or Ceres's orbit (it took years to get there but IIRC a chemical rocket would not be capable of even having enough delta v for the mission, and the quoted claim is about orbit leaving and insertion) at a much lower acceleration? Many concepts for realistic though currently impossible craft on the site also have engines that generate below 5 miligees but are still projected to be able to reach Mars in 3 months to several weeks. What truth is there to that quote, if any?

[Drashner1]

Looking at the statement on the Project Rho site, I'm really not impressed by it. It is vague and problematic on multiple fronts. Specifically:

a) 'Practicality' is a rather subjective concept and often context dependent. What is impractical in one situation is fully practical (or at least the best option available) in another. In this context, questions of mission parameters, mission goals, and what other options are or are not available would all play a role in determining whether or not a given propulsion system is 'practical'.

b) On a related note - given that most of what Project Rho covers are futuristic space propulsion systems based on technology we haven't achieved yet, making 'practicality' the hill to die on in this specific instance seems...odd and even out of character. At a guess, this was either written a long time ago or more recently by 'new management' (I have heard the original founder of the site had passed away) with a different take on things or vision or the like.

c) Light sail spacecraft have existed as a concept for decades and been studied and written about by multiple people. If their acceleration was an issue making them unworkable, I suspect we would have heard about it by now.

d) It's not clear what the statement is referring to. The mention of 'orbits' is odd. Yes, orbits play a role in interplanetary travel, but I've not seen acceleration mentioned in the same breath. Normally it's a matter of achieving a certain velocity and getting to a certain place by a certain time to match velocity with the destination and the like. All of that routinely takes years, so I'm not sure why that's an issue here. Unless we're talking orbiting satellites and changing their orbits - but I can't recall ever hearing or reading about solar sails being used that way.

e) Using one of the many available acceleration calculators online, it comes up that a constant acceleration 3 milligee boost gets you across an AU of distance in about 52 days. The reality is much more complicated as I indicate above, what will needing to match velocity with your destination and solar sails needing to spiral out from Earth orbit for a good bit before really getting going - but even if all that takes 'years' - it's not really all that different from various other methods.

f) While the Dawn spacecraft used ion propulsion rather than light sails, an experimental light sail has been deployed - LINK - even if it only operated in orbit.

Putting all this together, I think the statement is imprecise, unclear, may have logic problems, and doesn't offer any supporting evidence for itself nor does it offer counterarguments against past research, math and even some limited RL activities.

Hope this helps,

Todd

[AstroChara]

Winchell Chung, the original writer of the Project Rho, is still alive btw.

[Drashner1]

Winchell Chung, the original writer of the Project Rho, is still alive btw.

Hm. I thought I read here or on the discord that he had passed away.

[AstroChara]

Winchell Chung, the original writer of the Project Rho, is still alive btw.

Hm. I thought I read here or on the discord that he had passed away.

Nah, the news was that they were diagnosed with a type of cancer that has high mortality rate, so people went around gathering well wishes from sci fi folks on a doc a while ago. Many people were also confused back then and thought they passed away, but nope they're still retweeting things today.

Also it seems your message was duplicated?

[Drashner1]

Fixed.

[stevebowers]

Glad to see that Winchell is still active.

[MichaelPoole]

e) Using one of the many available acceleration calculators online, it comes up that a constant acceleration 3 milligee boost gets you across an AU of distance in about 52 days. The reality is much more complicated as I indicate above, what will needing to match velocity with your destination and solar sails needing to spiral out from Earth orbit for a good bit before really getting going - but even if all that takes 'years' - it's not really all that different from  various other methods.

Why would 0.003 g take "years" to spiral out of Earth orbit or any orbit at all or to match velocity with the target? Dawn probe was able to enter and leave the orbit of Vesta and then enter Ceres orbit at 0.000007541 G much quicker than that plus the same ProjectRho site has concept studies of fusion spacecraft with about 0.003 g acceleration with much less than a year time to get even to Jupiter.

Page 33 Microsoft Word - Z-Pinch_20101130.docx (nasa.gov) this one takes over a year to get to Jupiter (still absolutely fantastic compared to "high thrust" chemfuel) but can handle Mars in 30 or 90 days depending on how much fuel is one willing to spare. The ship in question has an acceleration of 0.004381 G in the heavier "Mars in 30 days" configuration per info here Realistic Fusion Designs - Atomic Rockets (projectrho.com) , below the supposed 0.005 G limit below which supposedly spacecraft take years just to change orbits. IIRC there is one ion drive probe that did take around a year to just get to the moon (the European SMART probe) but it had an ion drive weak even by modern standards, and the typical ion drive IIRC has accelerations of just a few microgees to tens of microgees so about 100-1000x less than the thrust regime we're discussing here.

[Drashner1]

Why would 0.003 g take "years" to spiral out of Earth orbit or any orbit at all or to match velocity with the target? Dawn probe was able to enter and leave the orbit of Vesta and then enter Ceres orbit at 0.000007541 G much quicker than that plus the same ProjectRho site has concept studies of fusion spacecraft with about 0.003 g acceleration with much less than a year time to get even to Jupiter.

I'm not saying that it would, I'm saying that even if it did, it wouldn't matter. Most solar sail missions that I've ever read about involve moving cargo around in support of some other crewed mission or base (since sails are comparatively cheap, reusable and don't consume reaction mass) or being used on probe missions where spending years is likely not a problem (based on our RL experience).

Solar sails are not the fastest idea ever suggested, but when considering the range of tech options that we are currently capable of and/or have some idea of how to actually build (speaking conservatively), nothing else significantly outperforms them AFAIK. I'm not counting various flavors of beamed energy/mass atm (sort of outside the context of this), and we have no actual clue how to build a fusion drive or the like, so not counting that either.

In practice, I strongly suspect a solar sail ship would be much faster and wouldn't need anything like 'years', but I don't have the math or background to be able to figure out how long a solar sail based mission would take across all the 'parts' that such a mission would consist of. So again, being deliberately conservative for the reasons above.

Hope this helps,

Todd

[MichaelPoole]

I'm not saying that it would, I'm saying that even if it did, it wouldn't matter. Most solar sail missions that I've ever read about involve moving cargo around in support of some other crewed mission or base (since sails are comparatively cheap, reusable and don't consume reaction mass) or being used on probe missions where spending years is likely not a problem (based on our RL experience).

Solar sails are not the fastest idea ever suggested, but when considering the range of tech options that we are currently capable of and/or have some idea of how to actually build (speaking conservatively), nothing else significantly outperforms them AFAIK. I'm not counting various flavors of beamed energy/mass atm (sort of outside the context of this), and we have no actual clue how to build a fusion drive or the like, so not counting that either.

I understand but tbh I took the quote to apply in general, my inquiry was not specifically about solar sails but under 0.005 g thrust in general. But the quote does seem wrong to me, but I don't have the math to disprove it so I hope someone can

EDIT:

The whole thing may have been caused by the misuse of centigee as 0.001 g (really should be milligee) and milligee as 0.0001 g (which is really 1/10th of a milligee).

I was searching something about solar gravity at Earth orbit and found this:

https://space.stackexchange.com/question...of-the-sun

Sun's gravity in the neighborhood of earth is 6 millimeters/sec^2. Out in the Main Asteroid Belt it is less than 1 millimeter/sec^2. So for heliocentric orbits a little further out, ion might be adequate for an impulsive burn injection into an elliptical transfer orbit.

6 mm/sec2 = 0.0006 G.