Author Topic: Length of proposed antimatter drive (beamed core)  (Read 3598 times)

Offline bombnumber20

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I’ve got a question regarding the proposed antimatter beamed core drive (http://trs-new.jpl.nasa.gov/dspace/b.../1/00-0212.pdf).

This drive will utilise the following reaction:
Protons + antiprotons -> pions -> muons -> gamma rays
Apparently the idea is to electromagnetically funnel the charged versions of the reaction intermediaries to generate thrust.
Although the reference above doesn’t mention this, according to (many) other websites “The spacecraft will have to be designed to be very long as the annihilation products travel close to the speed of light.”

Unless the spacecraft is quite long the latter portion of the reaction would occur beyond the EM ‘nozzle’ - although the reaction takes very little time, the speed of light is very fast :)

I don’t see why the spacecraft needs to be long (unless the later pion -> muon -> gamma ray transitions generate further thrust).

Do they? If so, Why? (It's for an SF story).
Any help will be much appreciated...

Offline Stormbringer

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Re: Length of proposed antimatter drive (beamed core)
« Reply #1 on: 10/20/2013 01:45 am »
sorry for the necro.

the reason is that the pions leaving the initial reaction are unusable until they decay into other particles that can be harnessed for thrust. and unfortunately that is most of the products from the first reaction. unless you actually harness the pion decay products an antimatter rocket wastes most of the released power. so the pions travel at relativistic speed. this slows down their decay considerably.

Ordinarily slow moving pions decay in a tiny tiny fraction of a second before traveling more than a meter or so. but when traveling fast time dilation prevents them from decaying until they have traveled 1.6 KM. so in order to harness them the engine part of a beamed core AM rocket must be at least slightly longer than that. and that does not count the radiation shadow shield, cargo, life support, crew habitat, landers, probe hangar and so forth.

and if you do not use the pions; if you waste all that power, you would be better off using a fusion reactor because the primary antimatter reaction does not produce an amount of thrust that justifies the expense of antimatter, risk of going poof in a spectacular way, and the development of safe handling technology, and materials and so forth when compared to fusion. yeah it's faster but not spectacularly faster until you harness the secondary decay products.

Fusion rockets will be available within 5 to ten years if funding is made available. all the pieces are in place. a fusion rocket does not have to break even to be of use as a rocket. thus no further breakthrough at all is needed.

antimatter on the other hand requires so many things to happen for it to be of use as the primary propulsion technology that it isn't even funny. but...Antimatter hybrids certainly would be of use and we could do those in 20 years or so. Antimatter can be used to kickstart and maintain a fusion reaction and a fission fusion reaction. those (Hybrid systems such as AIMSTAR etc) require a miniscule amount of antimatter to work. The current state of the art for AM production and storage is just 1 order of magnitude below the threshold we need for those to work.

EDIT:  Incidentally; a beamed core AM rocket has a theoretical top end of just about .69 light speed. a tethered AM rocket has a theoretical top end of about.91 light speed. Fusion depends on who you ask and what maturity level you ask about. early fusion rockets vary from not being relativistic to being 1 to 3 percent light speed; whereas fully matured fusion can get over .30 percent light speed with an optimized magnetic nozzle design.
« Last Edit: 10/20/2013 01:57 am by Stormbringer »
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Offline Stormbringer

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Re: Length of proposed antimatter drive (beamed core)
« Reply #2 on: 10/20/2013 02:07 am »
for a science fiction story you could posit some sort of unforseen technology that allows the harnessing of the secondary antimatter decay chain far sooner than our current understanding allows.

or you could posit a new type of matter composed of exotic particles that are smaller than protons and neutrons like you see over at the Orion's arm fictional universe. If the particles are smaller than regular particles but electronically analogous to ordinary matter a chemical reaction would be thousands of times more powerful than ordinary chemical reactions and nuclear reactions would be 100s of thousands of times more powerful to million or billions of times more powerful.
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Offline FreeThinker

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Re: Length of proposed antimatter drive (beamed core)
« Reply #3 on: 04/24/2018 02:26 pm »
@Stormbringer (sorry to necro but this appeared to be the best location to ask)

You gave an explanation why an efficient beamed core antimatter engine had to be long enough to allow the charged pions to decay into charged muons which could be effectively used for propulsion. However, there has been a new publication (BEAMED CORE ANTIMATTER PROPULSION: ENGINE DESIGN AND OPTIMIZATION: https://arxiv.org/ftp/arxiv/papers/1205/1205.2281.pdf) which bring a new perspective to the table.  It conclusion include that it is much easier to create magnetic nozzles (than previously imagined)
 which effective enough to use the charged pions directly for propulsion without requiring very long engine shape to achieve overall efficiencies of 75%. As a result allowing effective exhaust velocities of 0.69c to be achievable, which is much better than the old design for beam core.

So In this light, do you still think the very long engine design is required or an obsolete concept?

The reason I ask is that I work on a game that tries to simulate interstellar engines

« Last Edit: 04/24/2018 02:29 pm by FreeThinker »

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