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Writing: Input Wanted Some problem solving -- probe is traveling to Alpha centauri

  1. Jul 9, 2017 #1

    I am trying to write a scene- story is set a bit in the future in which theoretically, using nuclear as a fuel is a possibility in space travel.

    The problem I'm trying to solve right now is if a probe is traveling to Alpha centauri (4.37) light years from the sun and the probe accelerates the first hour and decelerates the last hour of travel, how many g forces are placed on the probe if it reaches alpha centauri in 8 years? So basically i only need to know what the G forces would be in the first hour of acceleration. My character is trying to explain to my other character that the g forces needed to accelerate a probe that fast are physically impossible for a human to withstand.

    I hope my question makes sense but I don't know how to set up the problem to solve.
    Last edited by a moderator: Jul 10, 2017
  2. jcsd
  3. Jul 10, 2017 #2
    I will assume that the 8 years is measured by the travelers, not an Earth bound observer.
    So you will need to accelerate to 0.73764 c.

    Your will be obliterated.
    For every 100Kgs that you are accelerating, you will need to convert about 48Kg of mass into energy - assuming perfect efficiency. It would be like setting off a whole series of thermonuclear blasts over the 1 hour period.
    The only good news is that your nuclear reactor will never be able to put out that much power that quickly.
  4. Jul 10, 2017 #3

    Vanadium 50

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    It's several thousand gravities. Even a probe wouldn't support its own weight.
  5. Jul 10, 2017 #4


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    0.73764c is ~221 million metres per second. Accelerating from zero to this speed in one hour means each second the probe has to gain 2.21e8/3600 = ~61,000m/s, so every second the probe is speeding up by 61km/s. One g is 9.8m/s2; 61,000/9.8 = 6224g.

    Your probe is not going to survive that, let alone the waste heat released from the propulsion system. Is it really important for the probe to accelerate for only an hour?
  6. Jul 10, 2017 #5
    Hmm, no it does not necessarily need to accelerate for only an hour. I'm open to a more "realistic" acceleration rate. I suppose the one hour was my starting place, but yeah I could extend it to a day, or a week even.
  7. Jul 10, 2017 #6
    This would actually be as observed from Earth because there are no passengers, it's simply a probe. That said, I'm willing to work with a longer acceleration period.
  8. Jul 10, 2017 #7
    I appreciate everyone's input though. Thank you very much.
  9. Jul 10, 2017 #8

    stefan r

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    A nuclear reactor cannot convert 48kg per 100kg into mass energy. Hydrogen converted completely iron is only going to get around 1%. That is not practical.

    Try maybe 1015g launching a 1000 kg probe. Kick the helium out the back. Still has the problem of getting burned up in the hour. Why not accelerate for 1 year?
    Why the rush? They could send out lots of probes at lower velocity. More likely that one survives the interstellar dust.
  10. Jul 10, 2017 #9
    There's no huge rush. My main concern is, if we're accelerating for a year, then it's more logical to accelerate at 1g. At that point, it becomes more likely to be able to say, hey, yeah, we can send a human as easily as a probe. I'm trying to stray away from that concept, as I'm trying to portray that it's not probable to create a space craft traveling at 1g carry a human to another star system. Basically, I want to portray that it's one thing to send a probe to another star system fairly quickly, but not at all possible to send a human in the same span of time because the g force would kill them.

    Meanwhile, I want the story set in the future enough that sending a manned space craft out to neptune is a probable feat in a lifetime, but not to another star system. So that's the bottom line, I suppose. A probe can get to Alpha centauri in 8 years, but a human cant. But a human could explore the far reaches of our solar system.
  11. Jul 10, 2017 #10
    4.37 light-years, 8 years as observed from earth: 0.54625c
    Your Lorentz factor is 1.1938. For every 100Kg, you will need 42.5Kg of mass for acceleration and deceleration. This can not be done by any normal nuclear reactor. It cannot be done efficiently by any known reactor design, and when it is done inefficiently, the total mass skyrockets.

    Suggest you give it a century or more.
  12. Jul 10, 2017 #11
    I mean, I get that... but it's fiction and set in the future. So we can assume that we've come up with a way to actually do it efficiently. The probe gets there in eight years. That's fixed. Acceleration time and rate can vary. I'm looking for g-forces of acceleration assuming that the g forces are at least higher than 20 gees.
  13. Jul 10, 2017 #12
    Thank you for helping simplify some of the math down for me. I've figured out if the probe is traveling at 4.37c for an average velocity, if it accelerates in a day we're looking at 4,676 gees, but if I change acceleration to a day, than it's only 194 gees, and if I change acceleration to a week than it's only 27 gees, and if I changed acceleration to one year the gees would be .54 which makes sense given that if a ship were to travel at 1 gee for 1 year, it would nearly reach c. So if a probe is traveling at about half c, than it would require about half the gee's to get to that speed in one year.

    So realistically in the scope of my writing, I'd be looking at a more realistic acceleration period being between 1 day to a week that would not completely obliterate my probe. In terms of how that's achieved, well that's where fiction is fun and you can create things such as the flux capacitor etc. Thank you for all your help, this is exactly the kind of ballpark stuff I was looking for.
  14. Jul 10, 2017 #13

    stefan r

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    Must be typo. If it can fly that fast Han Solo would have stolen your ship.

    No amount of acceleration will ever reach c. 7/8c takes not far from double the energy as 3/4c. (not exactly)
  15. Jul 10, 2017 #14
    Yeah sorry I made a typo there. Not sure where that came from and I don't have my notes in front of me atm.

    But I know that no acceleration can reach 1c. Hence why I said "nearly" but my probe is traveling basically at half that speed. Which is why it made sense that it would use aproximatly half the amount of gees.
  16. Jul 11, 2017 #15


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    There are many reasons beyond acceleration that make it difficult to send a manned space craft to another solar system. Time is one of them; keeping astronauts alive, healthy and fed for potentially decades with no chance of help if anything goes wrong (from a micrometeorite smashing through the carbon dioxide filter systems to an unexpected tumour killing a key member of the crew). You have to build in so much redundancy that the size and cost of the mission explodes.

    I don't know what you're trying to say given the typo of a velocity 4x the speed of light. One thing that would make it easier to justify from a hard science perspective (which I assume you want given that you're asking for help making the setting realistic) would be to make the probe a starwisp.

    Essentially this would be a light probe that is mostly a light- or magsail that is accelerated by a beam station in the solar system. To slow down the probe can detach its sail and use it as a reflector for the main body. Given that a probe weighing a few hundred kilograms is a lot easier to build and launch than an interstellar manned space craft massing hundreds of tonnes it would also solve your problem of explaining why the mission is unmanned.
  17. Jul 11, 2017 #16
    If you have a nuclear powered spaceship, why do you need to do all of the acceleration in such a short period of time? The whole point of nuclear power is to be able to run on continuous power for long periods of time. The engine on a nuclear submarine can run continuously for something like 25 years. Most ideas about how to get to other worlds involve long accelerations, perhaps even accelerating half until they get halfway there, then decelerating the rest of the way. A small acceleration over 4 years can pick up a lot of speed.
  18. Jul 11, 2017 #17
    I meant to say that the probe is moving at .55c for an average velocity, it isn't a manned spacecraft though. I think I accidentally quoted the distance of 4.37 lightyears instead of the average velocity so that's my bad.

    I'll look into the probe being a starwisp though! That sounds interesting. Thank you for the tip!
  19. Jul 11, 2017 #18
    Well I never said that the probe was nuclear powered, simply that it needs to accelerate at a g-force that is not capable of human survivability. The character is trying to explain to another character the many reasons for why what is working for sending a probe to another solar system would not work for a manned spacecraft, and the acceleration being one of the many reasons why a manned spacecraft *wouldn't* work the same way. I'm aware of all of the many complications of sending a manned space craft to another solar system. So that's where I'm trying to go with this.
  20. Jul 15, 2017 #19
    That means that the propulsion must be based on nuclear power or something even worse (e.g. matter/antimatter). You could use laser propulsion or linear accelerators for the start but you can't avoid rocket propulsion to finally brake down to zero and no conventional rocked would do the job.

    Is it your intention that the argumentation of the character is wrong?
  21. Jul 15, 2017 #20

    stefan r

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    You can brake using interstellar gas. Might be better not to mention the interstellar gas and dust. A snowflake at 0.05c is very lethal and can penetrate armor plating. Single atoms of hydrogen or helium have more impact energy and penetration than nuclear radiation. 0.55 would be much worse.
  22. Jul 16, 2017 #21
    The density of the interstellar medium is far too low. It is not even high enough for a Bussard ramjet.
  23. Jul 16, 2017 #22

    stefan r

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    We could say the density is too high fora buzzard ramjet. The energy lost scooping a proton subtracts from the energy gained by fusing it. Here is a paper on combined electric and magnetic braking. The magnetic brake gets more efficient per unit mass with higher velocity. You would need to brake for most of the trip and also have some way of slowing down after the mag-sail becomes inefficient.
  24. Jul 19, 2017 #23


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    I agree with Ryan_m_b. There are plenty of logistical reasons that a probe would have a far easier time then humans. manned crafted would have to be many times more massive just to house all the air, food and water needed for the crew, not to mention radiation shielding

    If you are going with the star shot or starwhisp probe concept then deceleration is a moot point. Such probes are accelerated, sent on their way and just keep going. Such a probe would fly through the Alpha Centurie Star System at .5c (or whatever speed you want) and pass through within a few hours and keep going until it leaves the system. In those hours, the probe would scan the system and transmit the data back to the ship.

    Realistically the probe should be sent years ahead of the ship. It makes sense to get as much data as you can before you plan a manned mission. There would be no point in launching the probe then launching the ship the year after because what if your probe gets to the system and finds out that the planet you thought was habitable is no longer so. or if there is something else in the system that is a deal breaker, If the manned ship is already in route....this is a problem.
  25. Jul 19, 2017 #24


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    If you bump up the sophistication of the technology then you can justify a close manned mission, and the probe stopping in system. Give the probe a payload of machinery that the probe can unpack and use to build things. By the time the manned mission arrives the probe may have built a sustainable base complete with basic ecosystem.. This saves you from carrying a sustainable biosphere and instead can just pack the crew in suspended animation or something.

    Obviously this would push the story further into the future into a more advanced world, but it would be believable and interesting.
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