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NASA proposes new propulsion to cut Mars voyage to 3 days

wired.co.uk

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Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#91

Post by gherkin0 »

chiph wrote: post #90
gherkin0 wrote: post #84

Earlier quoted context omitted.

> Not my idea - it's been around for decades. > It works because you can treat the two kites as one system, because they're both connected to the ship. Care to supply some citations?

Physicist Robert L. Forward mentioned it in his book Rocheworld (later republished as The Flight of the Dragonfly )in the early 1980's, with the concept work being done in the 1970s. https://en.wikipedia.org/wiki/Solar_sail#Interstellar_flight

That's a science-fiction book, though. I'm still not getting how the ship could basically sail straight into the laser that's pushing it (e.g. decelerate). The main sail is still getting pushed forward by the laser, even while the second sail is pushing backward (with the reflected light from the main sail).

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#92

Post by Someone »

rubidium wrote: post #9

back-of-envelope: conservation of momentum: 2 (num photons) h/lambda = (100 kg)*(c/3), so 5.9×10^36 photons needed to propel a 100 kg object to 1/3 speed of light. 1.5×10^18 joules of energy, which is 11% of the energy output of the US in 2001 (according to wolfram alpha). That's a 5.8 Tera Watt laser running for 3 days straight. I haven't even gotten into dispersion of a laser beam. The ludicrousness of this proposa…

I think it is good that people are thinking very far out of the box for this. Reason? What we think we can do now isn’t enough, so if we want to go to Mars, some ridiculous idea must eventually turn out to be practical.

For that reason, I would not worry too much about that 11%. _If_ we go into space, we will have to build enormous solar arrays in space, and they could provide order of magnitude more power than we produce now.

I would not aim for Mars, though. Instead, send it out to overtake Voyager. To do that, you do not need to decelerate, and you do not need to goto 0.3c, either.

(Actually, some billionaire should put a huge price on the first one to overtake Voyager and send a video of it back to earth. That could become a fun race, with later, faster, spaceships overtaking early entries.)

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#93

Post by 05 »

nabla9 wrote: post #86
mbrubeck wrote: post #55

Earlier quoted context omitted.

Nit: 0.00001% of 5.8 TW is 580 kW. (I assume you just misplaced a decimal point.)

5,800,000,000,000 W × 0.00001 = 58,000,000 W = 58 MW I misplaced decimal point, but it's for different direction.

0.00001% != 0.00001

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#94

Post by 05 »

api_or_ipa wrote: post #76
kemiller wrote: post #63

Earlier quoted context omitted.

How would it slow down at the other end?

Aerobreaking could net you some gain. Obviously, this requires a more engineeered vehicle, including possibly an ablative shield. Even at that, however, you'll be skimming very close to Mars' surface and incurring very high temperatures. Perhaps (and this is a big perhaps) we could engineer lifting body surfaces to increase the time-in-atmosphere and dissipate heat better. This is why I love playing Kerbal Space Prog…

Aerobraking from .3c would require materials many orders of magnitude more heat-resistant than known to humanity.

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#95

Post by Joof »

kemiller wrote: post #63
rubidium wrote: post #9

back-of-envelope: conservation of momentum: 2 (num photons) h/lambda = (100 kg)*(c/3), so 5.9×10^36 photons needed to propel a 100 kg object to 1/3 speed of light. 1.5×10^18 joules of energy, which is 11% of the energy output of the US in 2001 (according to wolfram alpha). That's a 5.8 Tera Watt laser running for 3 days straight. I haven't even gotten into dispersion of a laser beam. The ludicrousness of this proposa…

How would it slow down at the other end?

We just build a second megalaser on mars. Super easy.

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#96

Post by vannevar »

rubidium wrote: post #9

back-of-envelope: conservation of momentum: 2 (num photons) h/lambda = (100 kg)*(c/3), so 5.9×10^36 photons needed to propel a 100 kg object to 1/3 speed of light. 1.5×10^18 joules of energy, which is 11% of the energy output of the US in 2001 (according to wolfram alpha). That's a 5.8 Tera Watt laser running for 3 days straight. I haven't even gotten into dispersion of a laser beam. The ludicrousness of this proposa…

Not to mention the ~350 G's of acceleration/deceleration you'd experience over those three days.

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#97

Post by vannevar »

MaysonL wrote: post #74
rubidium wrote: post #9

back-of-envelope: conservation of momentum: 2 (num photons) h/lambda = (100 kg)*(c/3), so 5.9×10^36 photons needed to propel a 100 kg object to 1/3 speed of light. 1.5×10^18 joules of energy, which is 11% of the energy output of the US in 2001 (according to wolfram alpha). That's a 5.8 Tera Watt laser running for 3 days straight. I haven't even gotten into dispersion of a laser beam. The ludicrousness of this proposa…

The .3c estimate was a best theoretical case , not proposed for Earth to Mars voyage. Note 3 days to Mars is ~50M miles/day ~2M miles / hour ~500 miles/sec or .3%c - 1/100 of his best case estimate.

Good point. Cuts the acceleration/deceleration down to about 1 G, which would actually be useful.

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#98

Post by chiph »

gherkin0 wrote: post #91
chiph wrote: post #90

Earlier quoted context omitted.

Physicist Robert L. Forward mentioned it in his book Rocheworld (later republished as The Flight of the Dragonfly )in the early 1980's, with the concept work being done in the 1970s. https://en.wikipedia.org/wiki/Solar_sail#Interstellar_flight

That's a science-fiction book, though. I'm still not getting how the ship could basically sail straight into the laser that's pushing it (e.g. decelerate). The main sail is still getting pushed forward by the laser, even while the second sail is pushing backward (with the reflected light from the main sail).

His credentials in the field are pretty good, and his books are always hard-science (no elves or wizards, and very little hand-wavy stuff).

https://en.wikipedia.org/wiki/Robert_L._Forward

If I recall the design correctly (forgive me - it's been over 15 years since I read it), there's a significant size difference in the sails. The larger sail collects photons from the distant laser and focuses them on the smaller "backwards facing" deceleration sail.

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#99

Post by pmontra »

zwetan wrote: post #77

"if it works" between parenthesis, in the title ... so I'm not a scientific but even if it works at the tech level, even if the deceleration problem is solved, even if not colliding with something along the way is solved what about the impact of travelling at 1/3rd the speed of light on the human body ?

Nothing, you'll feel only the acceleration which is is equivalent to a gravitational pull. If it's 1 G you won't feel any difference from being in a room on Earth.

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#100

Post by kpil »

vannevar wrote: post #97
MaysonL wrote: post #74

Earlier quoted context omitted.

The .3c estimate was a best theoretical case , not proposed for Earth to Mars voyage. Note 3 days to Mars is ~50M miles/day ~2M miles / hour ~500 miles/sec or .3%c - 1/100 of his best case estimate.

Good point. Cuts the acceleration/deceleration down to about 1 G, which would actually be useful.

Exactly how do the deceleration work... ? Hoping someone put a really big haystack on mars?
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