
Magnetoplasma drive could make Mars transit take 39 days? - blach
https://orbitalindex.com/archive/2020-08-19-Issue-78/#magnetoplasma-to-mars-in-39-days
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sandworm101
[https://www.spaceflightinsider.com/conferences/vasimr-
plasma...](https://www.spaceflightinsider.com/conferences/vasimr-plasma-
engine-earth-mars-39-days/)

[http://spacenews.com/vasimr-hoax/](http://spacenews.com/vasimr-hoax/)

"Zubrin wrote in SpaceNews: “To achieve his much-repeated claim that VASIMR
could enable a 39-day one-way transit to Mars, Chang Diaz posits a nuclear
reactor system with a power of 200,000 kilowatts and a power-to-mass ratio of
1,000 watts per kilogram. In fact, the largest space nuclear reactor ever
built, the Soviet[-era] Topaz, had a power of 10 kilowatts and a power-to-mass
ratio of 10 watts per kilogram. There is thus no basis whatsoever for
believing in the feasibility of Chang Diaz’s fantasy power system.”

Note the word used by spacenews: Hoax.

~~~
sobellian
The idea isn't ludicrous, although the torch-ship speeds of 39-day transits
are definitely out of reach for now. The basic idea is, let's attach a nuclear
reactor to an ion engine. Further, let's make the propellant hydrogen so we
can refuel anywhere in the solar system.

If we want to get serious about exploiting the solar system, we'll eventually
have to give in and embrace nuclear technology, whether something like VASIMR
or a nuclear-thermal design like NERVA. We already routinely park 100MW+
nuclear reactors in port for our navy, why not consider civilian use for space
exploration? We already know many ways to mitigate risk for the launch of
nuclear material.

I think Musk, Zubrin, et al. analyze propulsion technologies from the
perspective of how to enable a journey to Mars _now_. In that light, something
like Raptor makes much more sense. You still need a chemical rocket engine to
lift off from Earth or Mars, so in the near term a nuclear ion thruster just
adds far too much complexity to justify its inclusion. Further, it's difficult
to imagine SpaceX obtaining the political backing to put nuclear tech into
space as a private company. This is also why Musk would rather power Martian
propellant plants with fields of solar arrays instead of the much more mass-
efficient space-rated nuclear reactors that NASA has been developing.

But imagine if we actually developed a Mars colony with millions of people.
The logistics of seeding a colony on Mars with chemical thrusters already
boggle the mind. Economics would basically forbid meaningful interplanetary
trade unless we develop new technology with much higher specific impulse. It
would be even more impactful than moving from air-freight to container ships.

~~~
giantrobot
Physics, logic, and economics preclude interplanetary trade.

There's no realistic space propulsion that would enable interplanetary trade
to be in any way economically feasible. It's economical to ship finished goods
and even raw material around the surface of Earth because it costs less than a
dollar per pound by sea, rail, or road. Air freight is more expensive at
around two dollars a pound. SpaceX's best price to LEO (Falcon Heavy) is $750
a pound. Just to LEO. Even if it was ten times cheaper it would still be
almost forty times more expensive than air freight.

Even with magic super efficient interplanetary transport the surface-LEO
portion of the trip makes it ridiculously expensive. To get a Martian colonist
to LEO would cost (at our magic $75 a pound) $11k just to get their body to
LEO. Assuming the water and air they need can be recycled with 100% efficiency
the food for the 40 day Mars trip would cost another $13k.

If every colonist needs a ton of material to support them on Mars (far too low
of a number) you're looking at $175b per million colonists. That's with a
bunch of magic hand waving and completely unrealistic pricing. What in the
shit are Martian colonists going to produce in any quantity that will pay down
the $175b capex?

~~~
sobellian
For what it's worth Musk believes that Starship _will_ bring a >10x
improvement for $/kg to LEO. Not throwing away your second stage will do
magical things. At any rate a Martian colony would be mostly self-sustaining
with trade being a relatively small fraction of gross domestic product (like
with international trade right now). On planetary scales, moving trade from
0.01% Gross Martian Product to 0.1% would more than justify the development
costs for such an engine.

I think we can safely say that we are very far from 1 Martian colonist, let
alone 1e6. I think we would outperform expectations to set up a very small
research station by the end of the century at great expense on the Martian
surface. So there's clearly a lot of magical thinking going on here. If we
allow for some amount of aspiration, there are a few ways to provide a return
on long time scales (much more than 1 century, with capex far exceeding $175B
just looking at the cost of staging the necessary propellant in LEO):

\- Such a base will have a propellant depot in a much shallower gravity well
than Earth and a far thinner atmosphere. This is a huge comparative advantage
for launches into deep space.

\- Necessity breeds ingenuity. A Mars colony would likely generate many
advances useful to Earth that do not make sense to pursue terrestrially. Think
hydroponics, insulation, radiation shielding, so on and so forth.

\- There are plenty of completely desolate locations on the Martian surface -
ideal locations for radio telescopes, neutrino observatories, and other hyper-
sensitive experiments.

\- ??? (We're trying to predict centuries ahead, after all!)

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d_silin
From what I heard from industry people, overheating is an unsolved problem for
VASIMR. It is 60-70% efficient, meaning that 60kW of thermal power has to go
somewhere.

Waste heat management for 200MW system in space is firmly outside the realm of
present-day technology.

~~~
RandallBrown
Could you use the waste heat to boil something and then spray it out the back
as extra reaction mass?

~~~
ryanmarsh
Yes, but what? You have to lift that mass to orbit. What mass boils off with
greater energy than, say, mass that combusts?

~~~
TheOtherHobbes
You don't necessarily need to lift that mass to orbit for interstellar flight.

[https://en.wikipedia.org/wiki/Bussard_ramjet](https://en.wikipedia.org/wiki/Bussard_ramjet)

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snake_plissken
Some quick math: 32 miles per second (see one of their linked articles,
[https://www.spaceflightinsider.com/conferences/vasimr-
plasma...](https://www.spaceflightinsider.com/conferences/vasimr-plasma-
engine-earth-mars-39-days/)) is 155,200 miles per hour. Assume you accelerate
at 21 MPH, which is about 1 g (100 KPH ~ 60 MPH, 100 KPH / 32.81 KPH/S^2 =
2.85 s, 60 MPH / 2.85 s). So at 1 g It takes you about 228 days to reach
cruising speed, 2 g takes 114 days, 3 g 57 days. So I don't think this would
not be useful to transport space crews? Still really cool tho.

~~~
sandworm101
I think you should check your math again.

155,200mph = 69,380 m/s

At 1g acceleration (10m/s/s) that is 6,938 seconds. 6,938 seconds is like 2
hours.

A ship that can sustain 1g acceleration continuously for periods measured in
hours, that would indeed be an interplanetary drive. Sustain that for
days/weeks/months and it will take us to other stars.

~~~
onlyrealcuzzo
How long do you need to accelerate at that pace to reach C? 299,792km/s /
.01km/s => 29 million seconds? ~335 days?

I'm assuming a lot of things would go wrong as you get closer to C...

~~~
sandworm101
Or go right. Time dilation would start working in your favor. You might get
there, and back, within your lifetime. Of course everyone on earth would be
long dead.

~~~
rimunroe
Micrometeors and radiation would be the major problem. Also, it's not just
time dilation that would help. At relativistic speeds, space contracts quite
noticeably in the direction of travel, so the actual distance you need to
cover diminishes as well.

~~~
rbanffy
That's because you won't be locally faster than light. You may cover 4 light
years in one, but only because those light years will be much smaller from
your point of view. You'll never "feel" faster than light.

~~~
rimunroe
I think you may have misinterpreted what I said. I didn't claim that you'd
feel like you were covering the distance faster than light. I was saying that
as you go faster (relative to your destination), space contracts ahead of you
so the distance becomes shorter than it would if you were moving slower.

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pinewurst
Seriously, I've been reading VASIMR press releases for the last 20 years or
so. Looking into it, I get the impression it's more a livelihood (from grants)
for the inventor than an ongoing technological trajectory.

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blakesterz
"A massively scaled up version running at 200 MW could make the one-way
transit to Mars in as little as 39 days, but generation of this amount of in-
space energy isn’t currently anywhere near possible and would require both an
advanced onboard nuclear reactor as well as super-efficient heat radiators. "

200 MW is a helluvalotta power! Disappointing to see it's so far off.

~~~
api
How much thin film PV is that? There's no wind and the solar constant is a lot
higher in space even further from the sun.

Also remember that space is not dark. Unless you are behind something you are
always in daylight if you are near a star.

~~~
a_crc
It's a lot of solar panel. A quick search gives us a figure of 220 watts of
power per square meter of space based solar panel. This would mean a 200MW
solar collector would need to be almost a million square meters in size. For
reference that would be a square the length of 155 NYC city blocks on each
side.

~~~
codeduck
A million square metres is one square kilometre. It could be built but it
would be heavy. A circular sail with a radius of just over 600 metres would do
it.

~~~
chasd00
wouldn't a million square metered be a megametre and not kilometre?

~~~
codeduck
no. 1000m * 1000m = 1000000m^2 or 1 km^2

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elihu
> The most recent tests of VASIMR have run at 200 kW and expelled ions at
> 180,000 km/h

Is this the power consumed by the device, or a measurement of the propulsion
power created? It would be interesting to know the energy efficiency; i.e.
what percentage of the input power is converted to thrust.

~~~
d_silin
At least 60%.

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peter_d_sherman
>"VASIMR (Variable Specific Impulse Magnetoplasma Rocket) is a helicon
magnetoplasma thruster designed to provide a variable thrust profile, from
low-specific-impulse / high-thrust to stupidly-high-specific-impulse / low-
thrust. Specific impulse could max out at ~12,000 seconds, drastically higher
than the roughly 2,000 s from current hall thrusters. VASIMR creates thrust
through a multi-step process. First, it bombards a neutral gas with RF energy
in helical waves to ionize the gas and create plasma (it can use multiple
gases: argon, hydrogen, or even CO2). Then, it uses magnetic fields and an
additional RF coupler to contain and energize the plasma to a superheated
state (in the neighborhood of the temperature of the Sun’s core). Finally, a
magnetic nozzle ejects the plasma at exceptionally high velocity. The most
recent tests of VASIMR have run at 200 kW and expelled ions at 180,000 km/h
(test fire video… or a blue party light being turned on, we’re not sure). A
massively scaled up version running at 200 MW could make the one-way transit
to Mars in as little as 39 days, but generation of this amount of in-space
energy isn’t currently anywhere near possible..."

Thoughts: _We need a functioning ITER -- in space..._

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natcombs
Using nuclear bombs for propulsion was estimated to make it to Mars and back
in 4 weeks. First time I heard of this, I thought it was a joke.

[https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion](https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion)

The project Orion study anticipated a one-way trip could reach Alpha Centauri
in as short as 133 years

~~~
jandrese
I would take the figures from Project Orion with a grain of salt. It was very
much a cocktail napkin exercise.

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Animats
You still need to carry reaction mass. It just gets pushed out the back
faster. It's another variation on the nuclear rocket idea.

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tectonic
If you're into this sort of thing, we write about nerdery like this every week
in The Orbital Index :)

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catmistake
Everyone is missing the fact that even if this technology was available 100
years ago, and worked just like we wanted, you could not get to Mars in 39
days. It would take about twice as long, because you have to turn the ship
completely around half way to the destination and slow down pretty much for
the same about of time it took to get to whatever velocity.

And Mars is a rock. I am sure there are astounding discoveries to be made, but
resources would be far better spent fixing ourselves and what we've done to
the planet before we cause our own extinction. Maybe we can do both, but let's
not ignore the fact that we have major problems, and it is unlikely we'll ever
find a better home than Earth.

