
The Tyranny of the Rocket Equation (2012) - bane
http://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html
======
Animats
Right. This has been known for a long time. It's why rockets aren't much
better than they were 40 years ago. Chemical fuels are as good as they can
get. Space travel with chemical fuels is just barely feasible.

In the 1960s, it was assumed that nuclear power would be necessary for space
flight. Everybody involved knew the rocket equation. The original plan for
Apollo included a nuclear upper stage. The engine (NERVA) was built and
tested. A Nuclear Assembly Building at Canaveral was planned. But, because the
goal was so narrow ("man, moon, decade"), the solution chosen was a disposable
rocket the size of a 50-story building to send an RV-sized payload to the
moon.

The crash of a nuclear rocket would produce a radioactive mess. Not Chernobyl
or Fukishima sized, but at least small-town sized. Launching from an isolated
island would help.

Various schemes have been tried or proposed to beat the rocket equation.
Launching from a balloon was tried early. Launching from an aircraft is still
used by Virgin Aerospace. It helps a little.

A space vehicle that's an air-breather while it's in the atmosphere and
transitions to rocket mode once out has been proposed many times, but making
something that's both a rocket and an airplane is hard and adds a lot of
weight. As an airplane, it has to go hypersonic to get up enough speed that
it's worth doing this. Building a hypersonic aircraft is very hard; so far,
only a few small demo craft have done it. The National Space Plane (hypersonic
single-stage-to-orbit) was proposed in the 1980s. Ben Rich, head of the
Lockheed Skunk Works and the designer of the SR-71's propulsion system,
declined to let Lockheed bid on it. (His comment: "We used titanium (on the
SR-71). You know anything stronger?") Remember, it has to be strong at a few
thousand degrees.

The same problems apply to launch track systems. Going hypersonic near the
ground is possible; the Holloman AFB test track, 50,000 feet of very straight
railroad track, has been used to reach Mach 8.6. The required acceleration is
about 14g. Far too much for humans.

The "space elevator" requires not only unreasonable strong materials but the
ability to put so much mass in space that you wouldn't need a space elevator
if you had that kind of launch capacity.

~~~
mikeash
Everyone Knows™ that putting stuff into space is expensive. Then Everyone
Assumes™ that it's because of all the fuel. But no, fuel is cheap, hardware is
what's expensive. If you look at the costs involved in putting something into
orbit, the cost of fuel is a trivial detail, on the order of 1% of the total
costs. Compare that to an airliner, where fuel is around 1/3rd of the total
costs, or a car, where fuel can easily be over 50% of the total costs.

I happened to be reading up on rocket efficiency and I was surprised to learn
that rockets are fairly efficient for launching stuff into orbit. Wikipedia
uses the example of the Space Shuttle, where 16% of the energy in the
propellants ends up in the kinetic and potential energy of the orbiter. That's
pretty good!

The problem is that you use a rocket once and then throw it away. Imagine if
your car was one-time-use. How often would you drive somewhere? How often
would _anyone_ drive _anywhere_? It wouldn't matter how efficient they are and
it wouldn't matter if they didn't even require fuel at all.

Now, the rocket equation still comes into play here, because it means you need
a lot of rocket for a little bit of payload. But the main problem is the one-
time-use thing. A nuclear disposable rocket wouldn't improve things much. A
reusable nuclear rocket would be great, but then so would a reusable chemical
rocket.

This is the genius of SpaceX. For decades, rocket designers have looked at the
rocket equation and tried their hardest to save fuel. SpaceX looked at the
economics of rocketry and realized that fuel costs more or less don't matter,
and instead concentrated on building their machines cheaply, and on making
them reusable. We'll see how it works out, but if they succeed in making
reusable rockets then they'll cut the cost of launches by an order of
magnitude or more.

~~~
marcosdumay
People have been trying to make reusable rockets since the beginning. The
problem is that a reusable rocket is more complex, what means it's heavier,
and that it needs much more fuel to launch, thus a bigger rocket, and the
rocket equation makes everything astronomical.

Like everybody else, I cheering for SpaceX to solve this problem, but the
challenge is not making a reusable rocket - it's making a light enough
reusable rocket.

~~~
mikeash
I'm not so sure. The Space Shuttle had a pretty big payload capacity. It had a
lot of problems as well, but those were mostly due to being underfunded and
hit with weird requirements beyond simple reusability. And that's really the
only serious attempt at reusability that got beyond the early stages. It's
still a pretty unknown area at this point, but I don't think building stuff
sufficiently lightweight is necessarily the challenge. SpaceX doesn't seem to
think it is, anyway. Their engines are not particularly high performance
(meaning they need more fuel for the same job) and their rockets are
engineered more for cost effectiveness than light weight. They seem to be
making great strides precisely because they're _not_ concentrating on weight.
For example, their first stage reuse system involves carrying a bunch of extra
fuel, where virtually every other attempt at reusability involved some sort of
unpowered or nearly-unpowered landing after expending all fuel on the launch
phase.

------
lordnacho
"If our planet was 50% larger in diameter, we would not be able to venture
into space, at least using rockets for transport."

I thought this was quite though-provoking from a Drake Equation standpoint.
From what I've gleaned, the earth-like planets we know of seem to be a big
bigger than Earth. Perhaps if there's civilization out there, they are
hampered by the misfortune of being on a planet that's practically impossible
to escape from. If they find it hard to put up a Hubble Telescope, perhaps
they'll just not be as likely to bother.

My other remark is to the engineering. There's a quip that you have to be an
engineer to make something that only just satisfies the requirements. Plenty
of people build houses without much in the way of calculation. Even cars can
be built by enthusiasts without degrees. This is what makes the space stuff
such amazing engineering.

~~~
chton
We're still very early in the development of spaceflight. If you compare with
the car industry, we're at the early industrialized phase, where large
companies carefully start to develop the necessary technology. Now, 100 years
later, a lot is standardized and the knowledge is so common that everybody has
the basics, and people can build cars in their back yards. It's the same
development that happened with airplanes.

At its core, a rocket isn't more complicated than a car. The challenges are
just different. My dearest hope is that eventually rocket components will be
as commoditized as car parts, so people can build and maintain their own
spacecraft. I want to see rockets held together with duck tape and spit,
because that's the point where spaceflight is available to everybody and
gravity stops becoming a hurdle.

A quick disclaimer: I have the greatest respect for rocket engineers. They are
taking the first steps, the most difficult ones, and I don't believe for a
second that they their work is easy. I just believe that eventually, they'll
become obsolete to the majority of spacetravel :)

~~~
iliis
You might be interested in Copenhagen Suborbitals (copsub.com): A danish group
trying to reach space in a DIY fashion financed by donations. It's quite
impressive what they've already achieved with not that much more than duct
tape!

~~~
baq
as the article says, space != orbit. i don't see how you can get 8 km/s
delta-v in a garage.

edit: doesn't mean it's not interesting, but orbit is so much more challenging
than just space it's not funny.

~~~
chton
It's true that orbit is more challenging, but you have no hope of getting
there if you can't do a suborbital jump first. Aiming for suborbital first
might see them get a lot more funding if they succeed. You need to learn to
crawl before you can run.

------
edraferi
_The common soda can, a marvel of mass production, is 94% soda and 6% can by
mass. Compare that to the external tank for the Space Shuttle at 96%
propellant and thus, 4% structure. The external tank, big enough inside to
hold a barn dance, contains cryogenic fluids at 20 degrees above absolute zero
(0 Kelvin), pressurized to 60 pounds per square inch, (for a tank this size,
such pressure represents a huge amount of stored energy) and can withstand 3gs
while pumping out propellant at 1.5 metric tons per second. The level of
engineering knowledge behind such a device in our time is every bit as amazing
and cutting-edge as the construction of the pyramids was for their time._

That's awesome, I had no idea.

------
tempestn
> In the 1970’s, an experimental nuclear thermal rocket engine gave an energy
> equivalent of 8.3 km/s. This engine used a nuclear reactor as the source of
> energy and hydrogen as the propellant.

That was intriguing, but didn't go into detail on why a nuclear thermal rocket
hasn't been tried since. The obvious explanation is that there could be
serious consequences if such a rocket exploded, spreading radioactive
material, etc. And rockets tend to explode sometimes. However, it sounds like
while that is certainly a concern, it is not as great of one as it might seem:

[http://en.wikipedia.org/wiki/Nuclear_thermal_rocket#Risks](http://en.wikipedia.org/wiki/Nuclear_thermal_rocket#Risks)

And indeed there is still work ongoing on such designs.

~~~
daeken
> And rockets tend to explode sometimes.

Chemical rockets -- the only ones we've ever actually used -- explode because
that's what they're intended to do. The only difference between a successful
rocket firing and a catastrophic rocket failure is the speed at which the
explosion happens. A nuclear rocket engine has basically no risk of explosion;
tearing itself apart at speed maybe, if the aerodynamics aren't done properly
or there's a structural weakness. But that's about it.

~~~
pjc50
Normal rocket operation is a burn, not a detonation, and there's a clear
difference between those that's not just about speed.

------
jwilliams
People that have played different versions of Civilization will remember the
Triremes.

They couldn't end a term in the ocean, only the coast. However, you could take
a punt and travel over the ocean - with only the hope there was land on the
other side.

Horribly expensive, but discovering new land or another civilization early
could be transformative for the same.

------
bainsfather
This rocket equation is not shown, just it's consequences.

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

for the equation (ignoring gravity),

and: [https://en.wikipedia.org/wiki/Delta-
v_budget](https://en.wikipedia.org/wiki/Delta-v_budget)

for a diagram of the consequences.

------
ars
Lest you despair of ever making space flight routine, a rocket is not the only
way to get into orbit.

Virtually all of the needed velocity is tangent to the surface, not away from
it. So you can accelerate the vehicle along the ground at least part of the
way, and only then turn heavenward and burn fuel to get into orbit. With this
boost you _significantly_ reduce the amount of fuel needed.

There are many way of doing this. You can have magnetic propulsion (very
futuristic and powerful), you can have a simple motor on the vehicle, powered
by contact with rails on the ground (but motors have a limit of how fast they
can work). You could have fuel "guns" fired into the back of the vehicle as it
passes them, which would then capture the container and burn them as a normal
rocket would. This has the benefit of not requiring a large rail, just fuel
stations that the rocket would pass over.

These are just some basic ideas, there are many more.

~~~
has2k1
Given that earth is a ball, aren't you always pointed heavenward if you are
between 0-180 degrees. So you might as well start of by pointing directly
heavenward, i.e 90 degrees. Plus, turning requires acceleration you cannot
just capitalise on the velocity you have built up otherwise you would violate
the 1st law of thermodynamics.

~~~
ars
You don't want to turn. To orbit you need to have velocity tangent to the
earth (i.e. along the ground), not directly up.

You only turn ever so slightly heavenward, mainly to avoid air resistance.

~~~
has2k1
I'm still lost. Isn't being tangential along the equator less advantageous
than being perpendicular at the poles. When you start tangential, as soon as
you cover a distance equivalent to the radius of the earth, you are then
perpendicular (maybe not directly so but there is no practical difference as
far as the gravity well is concerned).

~~~
SamReidHughes
When going tangentially, you'll maintain tangentiality until you're going so
fast that gravity isn't pulling you down fast enough to keep you pressed
against the track you're on (because the Earth's surface curves downwards like
the widdle spheroid it is). At that point, you're in low earth orbit (though
you might want to fire the rockets a little bit to give yourself an orbit that
doesn't intersect the planet). The whole point is that you gain your velocity
while you're pushing off the earth, instead of your own propellant. That way,
you don't have to carry so much propellant.

> When you start tangential, as soon as you cover a distance equivalent to the
> radius of the earth, you are then perpendicular (maybe not directly so but
> there is no practical difference as far as the gravity well is concerned).

If you were assuming the tangential take-off would continue going a straight
line instead of curving in an orbit, I still don't get what you mean -- after
covering one Earth radius, you'd be traveling at a 45-degree inclination
relative to the surface of the planet, not perpendicularly.

~~~
has2k1
I see it this way. You have to accelerate the mass to the escape velocity. You
also have to achieve a net effect of being in orbit (some distance x above
surface of earth). The most direct vector to that distance is perpendicular.
The two combined should give you the minimal energy requirement. Any
engineering (and aerodynamics) creativity cannot give you anything better.

~~~
lmm
It's escape velocity, not escape speed - and it's orbital velocity that
matters here, we're not escaping entirely. If you're in the same place,
travelling at the same speed, but pointed down, you're not in orbit, right?
It's the same if you're pointed straight up. You need to be at an orbital
altitude and travelling at the right speed _in the right direction_.

(well, any combination of speed, direction, and position will be an 'orbit' in
some sense, in that there's a conic section you're on that you would follow if
you were in freefall. But if you're sitting still over a planet then it's the
degenerate ellipse that's a straight line down to the planet's core).

So in the absence of atmosphere the ideal ascent trajectory would look pretty
much like a Hohmann transfer orbit: you'd accelerate horizontally until you
were in orbit at surface level, and then you'd do the minimum energy transfer
from that orbit to a higher orbit. In reality it's worth getting to altitude
where the air is thinner before turning horizontal, but even so, the vast
majority of a rocket's acceleration is horizontal, not vertical.

You can do the maths, but if you want to really understand these things, play
Kerbal Space Program. Seriously.

~~~
baq
escape speed is technically correct. you need to go fast enough, direction
doesn't really matter.

~~~
ubernostrum
Some directions may require more speed than others.

Or perhaps you are having a bad problem and will not go to space today.

------
ChuckMcM
I really enjoyed this. It lays out in some very accessible ways, the
challenges of getting into space. The recoverability of the Falcon9 will cut
its costs dramatically as it reduces the cost of the launch by several tens of
millions of $. I'll mention on-orbit refueling as well since you don't need
recoverability per-se if you can refuel in orbit. Then your Mars lander / Crew
Module can launch with enough fuel to get to Low Earth orbit, refuel, and then
head out to Mars.

Air launched (Skylon, Pegasus, Et alia) are also interesting, laser boosting
(using lasers to add energy during the initial launch) would also help. As
Elon points out though, a multi-gigawatt laser for boost to orbit is
impractical both from a construction standpoint and a diplomatic stability
standpoint.

If the quantity of water on the moon is accurate, then it should be possible
to create a 'refinery' on the Moon which could more easily get material into
Earth orbit. We'll see though if we can get a group established there.

I had hoped to visit the Moon at some point (I was assured by NASA in my youth
that would be able to :-)) but I don't expect that to come to pass unless
something amazing changes.

~~~
nickff
If you do the math, air-launch does not save much in fuel costs, while it adds
a great deal of complexity to the launch system, and you do not save much in
terms of reduced energy requirements. Most of the rocket fuel and oxidizer are
used to increase velocity; comparatively little is used to gain (the first
30k') altitude or lost to atmospheric drag. The main benefit to air-launch is
reduced range safety costs, and ease of scheduling, as the launch vehicle can
be taken out over an unoccupied stretch of ocean, and range rental/lease costs
are basically eliminated.

~~~
ChuckMcM
An interesting paper that is doing that math :
[http://mae.engr.ucdavis.edu/faculty/sarigul/aiaa2001-4619.pd...](http://mae.engr.ucdavis.edu/faculty/sarigul/aiaa2001-4619.pdf)

------
AndrewDucker
Why not assemble some rockets in space?

Get the pieces up there as efficiently as possible, and then assemble a more
efficient rocket that's not designed to leave the gravity well.

~~~
danielweber
If you skip the middle step, that's called staging.

It's really expensive to try and keep a permanent base in space.

------
clumsysmurf
If anyone is interested, these researchers from Russia have proposed a way to
accelerate a spaceship while in flight – firing a ground-based laser up its
backside

[http://www.osa.org/en-
us/about_osa/newsroom/news_releases/20...](http://www.osa.org/en-
us/about_osa/newsroom/news_releases/2014/supersonic_laser-propelled_rockets/)

~~~
neurobro
Well that seems less preposterous than my own idea of using lasers to
accelerate a ring of air, generating a plasma circuit that would pull the
craft up magnetically. (Version 2 would have a series of plasma rings that the
craft would fly through like a coil gun.)

------
Pxtl
The soda can analogy is horrifyingly informative. I wonder what kind of
efficiencies could be obtained with semi-science-fiction fuels like the Orion
nuclear-explosion propulsion system (which is different from a nuclear rocket)
or even straight-up antimatter.

en.wikipedia.org/wiki/Project_Orion_a(nuclear_propulsion)

------
Lambdanaut
If you've got a poor mind for Math but want to understand rocket science on a
more intuitive level, check out Kerbal Space Program. It's the most
educational game I've ever played that still retains being fun.

I usually avoid video games because I feel like I'm wasting time, but I make
an exception for KSP.

------
agopinath
Fascinating read. Are there more blog-type posts like this from NASA? I'd be
interested in reading more but I can't seem to find them from the main page
and can't find links to these posts...

------
ytturbed
>Currently, all our human rated rocket engines use chemical reactions
(combustion of a fuel and oxidizer) to produce the energy.

Yes, however, for completeness: an explanation of why we must limit designs to
chemical rockets ought to include an explanation of why the dozen or so fusion
projects underway around the world will _all_ fail, i.e. let's inject some
rational optimism. Note that the Apollo programme began before its tech was
ready.

~~~
pjc50
The fusion projects have been running for a long time with little success.
Apollo was built on scaling tech that already worked (1940s rocketry could
reach space although not achieve orbit)

~~~
DennisP
If you consider exponential progress since 1970 or so to be "little success,"
you're right. Fusion has a very high threshold before it becomes useful, but
we've come a long way, and we're not that far from the breakeven point now.

NASA is currently working with John Slough's company on a fusion rocket for
interplanetary travel.

[http://www.nasa.gov/directorates/spacetech/niac/2012_phaseII...](http://www.nasa.gov/directorates/spacetech/niac/2012_phaseII_fellows_slough.html)

[http://www.washington.edu/news/2013/04/04/rocket-powered-
by-...](http://www.washington.edu/news/2013/04/04/rocket-powered-by-nuclear-
fusion-could-send-humans-to-mars/)

------
im2w1l
What about firing the rocket from Jules Verne's space gun[0]? Now, I don't
mean an actual gun what with the high g-forces and so, but only some machinery
that gives the rocket high initial speed. Maybe a super sonic evacuated tube
maglev that ends with a ramp? Put it on Mt. Everest for less air resistance.

[0]
[http://en.wikipedia.org/wiki/Space_gun](http://en.wikipedia.org/wiki/Space_gun)

~~~
danielweber
You still get a massive hit from the atmosphere when you exit the tunnel if
you've built up any significant speed. Air pressure up there is still a third
of air pressure at sea level.

------
mrfusion
So this is something I've always wondered about.

Can anyone explain the equation for getting to an earth sun Lagrange point?

Since were already on earth orbiting the sun it would seem we already have the
correct orbital velocity to hang out at a Lagrange point. So we could really
approach these points at any speed no? Is there potential to use less fuel
than you'd need to reach an earth orbit?

~~~
gizmo686
By "orbital velocity", I assume you mean orbital velocity relative to the sun.
The need for this clarification should be the first indication that we do not
already have the necessary orbital velocity (as there is no particular reason
to use the sun as the reference other than the fact that it is the more
massive body). This diagram [0] shows the Earth-Sun Lagrange points. As you
can see, they all have a slightly different orbital radius around the sun, and
therefore will have different orbital velocities (larger radius=slow
velocity). Additionally, (most) of these points have a different direction,
which would means a different velocity. (However, we can change direction for
free simply by orbiting, and as long as our orbit is different from Earth's we
will change our direction relative to Earth as well).

The bigger issue is 'escaping' Earth's gravity well[1]. The most efficient way
to do this is through an orbit. To see this, consider the effect of
gravitational drag. That is to say, the speed lost due to the force of
gravity. If you are in orbit, the net gravitational drag is 0 [2]. Suppose you
are at point P of an orbit, travelling at speed V. If you accelerate with X
delta-V at this point in the orbit, your new orbit will still contain point P,
and you will pass through P with velocity (V+X), indicating that there was 0
gravitational drag. Intuitively, this is because you are accelerating
perpendicular to the force of gravity.

Another way to look at this is to remember that there is no particular reason
to prefer using the Sun as a reference point, in which case we can see that
Lagrange points are also in Earth orbit.

However, your idea does point to a (theoretical) way to reduce fuel usage.
When calculating the fuel needed to establish an orbit, we assume that Earth
is the only massive body. However, we can (in theory) use the gravitational
force of the sun to reduce the needed fuel to reach Earth orbit (in a way that
has nothing to do with Lagrange points). However, the effect is not
significant enough to be worth talking about.

[0]
[http://upload.wikimedia.org/wikipedia/commons/e/ee/Lagrange_...](http://upload.wikimedia.org/wikipedia/commons/e/ee/Lagrange_points2.svg)
[1] Of course, by definition of Lagrange points, we are not actually escaping
[2] Unless the orbit is circular, gravity will change your speed, but the
acceleration/deceleration will balance out.

------
robertfw
I found the Slingatron to be a particularly novel approach for launching small
amounts of unmanned mass to LEO - I am curious to know what came of their
concept.

[https://www.kickstarter.com/projects/391496725/the-
slingatro...](https://www.kickstarter.com/projects/391496725/the-slingatron-
building-a-railroad-to-space)

------
mcguire
" _If a vehicle is less than 10% propellant, [c]hanges to its structure are
readily done without engineering analysis; you simple weld on another hunk of
steel to reinforce the frame according to what your intuition might say._ "

This is why you don't let cabinet makers build ships.

~~~
Mvandenbergh
The point is that they can actually. A cabinet maker could build a fairly
large boat at least (maybe not a ship...) entirely by hand and mostly using
very simple rules of thumb. It probably wouldn't perform very well but it
would float and be able to sail around.

Amateurs build small boats that cross the Atlantic and even the Pacific all
the time, precisely because ship building is easier than building rockets.

When I say it's easier, that doesn't mean that naval architects aren't as
smart as rocket engineers, but the ratio of engineering effort to performance
is much more favourable.

~~~
mcguire
I read this story[1] once, although I don't remember where:

Someone somewhere needed a large number of cargo vessels built quickly (think
WWII liberty ships, but those weren't made of wood), so they brought in a
bunch of cabinet makers to bolster their shipwrights. It worked great, until
the ships built by normal woodworkers saw significant wave action, at which
time they broke up and sank. The punchline being that, on land, rigidity is
the primary constraint; if you build it not to be floppy, it will be plenty
strong. At sea (and especially in aerospace), strength is primary; if you
build it to be rigid, it will be too heavy and if you build it to be not-
heavy, it will probably fall apart.

An amateur can build a fairly large boat (although usually to plans by an
actual naval architect), and a small boat can make it across an ocean, but if
you're serious about schlepping things around, the design constraints for
ships aren't much looser than those in aerospace.

[1] Maybe this, although that's not the cover I remember:
[http://www.amazon.com/Structures-Things-Dont-Fall-
Down/dp/03...](http://www.amazon.com/Structures-Things-Dont-Fall-
Down/dp/0306812835/ref=sr_1_1?s=books&ie=UTF8&qid=1414781193&sr=1-1&keywords=structures)

------
morearguments
This might be interesting for those that haven't come across it yet
[http://en.wikipedia.org/wiki/Space_elevator](http://en.wikipedia.org/wiki/Space_elevator)

------
001sky
"If our planet was 50% larger in diameter, we would not be able to venture
into space, at least using rockets for transport."

Don't tell hollywood.

------
shahar2k
what about the feasibility of injecting energy on the ground (say a maglev
rail based launch) such input wouldnt be contributing to the rocket's weight
and therefore would reduce the needed fuel consumption quite a bit I would
imagine.

------
nraynaud
just a noob question: why transport all the fuel, and not start from a
cannon/catapult, something that make you start with some speed?

edit: it's not clear, I was talking about partially using a cannon, not about
a complete ballistic launch.

~~~
MPSimmons
If you were in a vacuum, that could be a wonderfully efficient way of
converting mechanical movement into kinetic energy. We have to deal with
atmospheric friction, though, and the places where the cannon would launch
from (the surface) has the thickest part of the atmosphere, so the most
friction.

~~~
nraynaud
what about vacuuming the cannon? like those ping pong ball cannons?

~~~
SideburnsOfDoom
What happens when you leave the cannon? Either the cannon exit is near ground
level, in which case you still have the air resistance problems; or it's not,
in which case you have the problems of building very a tall cannon.

~~~
nraynaud
you don't have to exit the cannon at high speed, the v^2 drag will kill you.
You're just trying to put some kinetic energy in the rocket in a way that
doesn't make you carry it. you don't have to put all the energy in it that
way, the current system works, it's just a tentative improvement.

But your answer makes me feel you've not seen the ping pong ball cannons,
because they have very good results with a very low mass.

------
scotty79
How hard would be building small size space fouintain model?

------
trhway
the rocket equation with fusion engine expelling alpha-particles at 0.05c
doesn't look that bad, and at least it is good enough for Solar system
colonization and for sending a probe to the closest star.

