SPACE****P
Creating a multi-planetary space****p is creating a vehicle that is
capable of tremendous speed by Earthly standards. So, the first step
in figuring out how to make space****ps is to figure out what sort of
speed you're going to need. The next step is to figure out what sort
of propulsion system is needed to attain that speed. The last step is
to figure out the final weights you're going to handle to propel the
payload you want.
PAYLOAD
All ****ps - whether space ****ps, aeroplanes, or water borne craft,
have a payload they carry, and around that payload you build a
propulsion system that is capable of attaining the speed you want
Surviving in space isn't really an issue. People have been building
nuclear submarines for decades. These can carry crews of 100 or more
for 100 days or more without surfacing
http://www.fas.org/man/dod-101/sys/****p/nssn.htm
Submarine Specifications
Contractors General Dynamics Electric Boat Division [lead design
authority]
New****t News ****pbuilding
Lockheed Martin Federal Systems (Combat System)
Raytheon Electronics Systems (Combat System)
Power Plant One S9G pressurized water reactor
??,000 shp, one shaft with pumpjet propulsor
Improved Performance Machinery Program Phase III
one secondary propulsion submerged motor
Displacement 7,800 tons submerged
Length 377 feet
Draft 32 feet
Beam 34 feet
Speed 25+ knots submerged
Depth Greater than 800 feet
Horizontal Tubes Four 21" Torpedo Tubes
Vertical Tubes 12 Vertical Launch System Tubes
Weapons 38 weapons, including:
Vertical Launch System Tomahawk Cruise Missiles
Mk 48 ADCAP Heavyweight Torpedoes
Advanced Mobile Mines
Unmanned Undersea Vehicles
Special Warfare Dry Deck Shelter
Advanced SEAL Delivery System
Sonars Spherical active/passive arrays
Light Weight Wide Aperture Arrays
TB-16, TB-29, and future towed arrays
High-frequency chin and sail arrays
Countermeasures 1 internal launcher (reloadable 2-barrel)
14 external launchers
Crew 113 officers and men
Total Program 30 systems
Total program cost (TY$) $67034M
Average unit cost (TY$) $2110M
Status Full-rate production 1QFY07
So a 7,800 long-ton vessel (8,580 metric tons) carries 113 officers
and men, a nuclear power plant, lots of weaponry, and counter-
measures, and has a hull that can withstand more than 800 feet of
depth.
A more detailed analysis of the mass of the weapons systems, power
systems, undersea structural elements, and so forth, indicates that a
payload module that used the very same life sup****t system as this
vessel, with a lightweight aerospace structure - surrounded by high
density polymer radiation sheilding with tungsten coating on an
aluminum superstructure - would be less massive than the hull of a
submarine.
http://science.nasa.gov/headlines/y2005/25aug_plasticspace****ps.htm
The cost of this system, should be less than $750M to develop and less
than $250M per copy to build say 10 of them. These are just payload
modules to carry the crew reliably for 3 to five years.
This is off-the-shelf technology adapted for space use combined with
the best space radiation sheilding we can find for long term exposure
to interplanetary conditions - and lasting 5 years overall (USS
Enterprise five year mission!)
So, we're talking about 850 metric tons vehicle mass, with 550 metric
tons of consumables to sup****t 113 officers and men/women crew - for
up to 5 years in space - with a $1,000M price tag for the first copy,
and $250M per copy for additional units - for this crew module only.
This is our payload - 1,400 metric tons.
ASTRODYNAMICS OF MISSION
Now lets look at the speed we need to project it to. This will tell
us what sort of missions we can do. In order to answer this question
we need to understand a little about astronomy.
The surface of the Earth is at a pretty much constant spot in the
Earth's gravity field. That's why the Earth, and all the other
planets above a certain size, are ball shaped. Gravity pulls
everything down to its most compact form - into a ball.
To leave the Earth and stay in orbit requires a minimum speed of about
7 km/sec. To lift something above the Earth's atmosphere and do it
quickly means you have air drag loss and lifting loss. Those add to
the speed required to maintain orbit. Since power and what forces our
****p and crew can take are limited, these losses can be reduced to
about 2.2 km/sec. So, the first rung on the ladder into space is 9.2
km/sec. This is the minimum. This is what the Space Shuttle does.
We'll get into propulsion systems in a minute... lets just keep
building our speed / mission list.
The next point is the moon. The minimum speed you need to achieve to
fly around the moon is 10.82 km/sec. This takes about 4 days to get
there. Flying there at 11.0 km/sec gets you there in 3.5 days but if
you miss the moon, you'll go flying off into space, since 11.0 km/sec
is the Earth's escape velocity.
If you're orbiting at 7 km/sec - that means you have to ADD 3.82 km/
sec to loop around the moon.
To actually land on the moon requires about 2.8 km/sec. To take off
and head back to Earth another 2.8 km/sec. Since the Earth has air,
you can use that to glide to a landing when you get back to Earth
without any additional use of propellant.
So the total is 2.82 + 2.80 + 2.80 = 8.42 - this in addition to the
9.2 km/sec needed to get to orbit. So, this is the speed it takes to
get to the mon and back - 17.62 km/sec - with course corrections and
allowing for guidance errors and so forth - 18.0 km/sec
A similar analysis for Mars indicates that for a minimum energy
transfer orbit - called a hohmann orbit - we can get to mars and back
- using Mars' atmosphere to help us glide to a landing on that planet
http://en.wikipedia.org/wiki/Hyperbolic_trajectory
http://en.wikipedia.org/wiki/Orbital_equation
http://en.wikipedia.org/wiki/Hohmann_transfer_orbit
http://en.wikipedia.org/wiki/Delta-v_budget
In Low-Earth Orbit it takes 11.6 km/sec to transfer to Mars - to
that's 4.6 km/sec. It'll take 10 months to get there, and at this
speed, we can only do it when the planets are in the right position -
which happens about once every 2.1 years..
Using the Martian atmosphere its possible to glide to a landing on
Mars when you get there- the same way we are planning to glide to a
landing on Earth when we return. Mars is bigger than the moon. Mars
surface gravity is 1/3 of Earth. Moon surface gravity is 1/6 that of
Earth. Mars is also farther away.
To blast from Mars surface to a transfer orbit back to Earth taks 6.4
km/sec. So, we have;
4.6 km/sec + 6.4 km/sec = 12.0 km/sec
from Earth orbit. Add this to the 9.2 km/sec to get to orbit in the
first place, and we have 21.2 km/sec - say 21.5 km/sec
These are minimum speeds that can only be travelled when the planets
are aligned properly - and that occurs every 2.1 years.
Earth Orbit - 9.2 km/sec - 20 minutes
Moon 18.0 km/sec - 8 days
Mars 21.2 km/sec - 2.5 years
Its interesting to see that going to Mars takes nearly as much speed
as going to the moon. Its farther away, and takes longer to get
there, and more things can go wrong, and not much can be done when
it does, but in terms of propulsion, if you can go to the moon, you
can also go to Mars.
Which explains why vonBraun and all the other rocket guys were very
interested in Mars as well as the moon.
PROPULSION
Well, we're now ready to figure out our propulsion system.
There are a few to choose from.
I'll look at two varieties
Chemical rockets
Nuclear rockets
And I'll limit my consideration to rockets that can lift themselves
off the Earth and the planets we'll be travelling to.
The im****tant things to know about rockets are the rocket equation and
the specific impulse of a rocket. Knowing these and the speeds we
need, we can figure out the propulsion we need.
http://en.wikipedia.org/wiki/Rocket_equation
http://en.wikipedia.org/wiki/Specific_impulse
Vf = Ve*LN(m0/m1)
The final velocity of a rocket propelled projectile is equal to the
exhaust velocity of the rocket engine times the natural logarithm of
the mass ratio. This gives you the mass ratio you need in order to
achieve the speed you want.
Chemical bi-propellant rocket Ve= 4.5 km/sec
Nuclear thermal solid core Ve=10.0 km/sec
Nuclear liquid core Ve=15.0 km/sec
Nuclear pulse Ve=20.0 km/sec
Rewriting the rocket equation to solve for mass ratio
m0 = m1*EXP(Vf/Ve)
where m1 is the payload we computed earlier, plus the structure of the
propulsion system itself, and m0 is the full up weight with all of
that and the rocket propellant that gets ejected
So, here's the m0/m1 ratio (mass ratio) for each mission and each
propulsion type
Mission Mass Ratio Requirements
Vf Chemical Nuc solid
Nuc liq Nuc gas
Ve= 4.5 kps 10.0 kps
15.0 kps 20.0 kps
Earth Orbit 9.2 km/sec 7.72 2.51
1.85 1.58
Moon 18.0 km/sec 54.60 6.05
3.32 2.46
Mars 21.2 km/sec 111.18 8.33
4.11 2.89
Now, the next thing we need to figure out is m1 - we figured out the
payload, but now we need to figure out the size of the rocket engine
and tanks and so forth. This can be represented as a fraction of the
propellant that's handled. Generally speaking, a kg of vehicle
without propellant or payload can carry about 6 kg of propellant. So,
that lets us figure out the structural mass.
If we size the vehicle so the structure is the same as the payload,
then you'll only have a mass ratio of 3.0 - which says chemical
propellants are out for any mission.
But we use chemical propellants exclusively. How does that work?
Well, that works because we do things in stages. For example, say we
are limited for structural reasons to a 3:1 mass ratio. this means
that for a chemical rocket we can only go;
Vf = 4.5 * LN(3) = 4.94 km/sec
This might be stretched to 3-1/3 or 3.33 - but that's about it.
Which is about half of the speed we need. So, we take this rocket,
and treat it as a payload for a bigger rocket - it too with a 3:1 mass
ratio - and we can achieve nearly 10 km/sec. Do this a second time,
and we can achieve nearly 15 km/sec and so on.
We get really big rockets that way!!
Here are the number of stages we need to accomplish our mission with
chemical rockets
Staging for Chemical Rocket Missions
Earth Orbit stages: 2 at 2.78 overall mass ratio 7.73
Moon stages 4 at 2.72 overall mass ratio 54.7
Mars stages 4 at 3.25 overall mass ratio 111.6
So we can see that if we start out with a 1400 metric ton payload
we'll have rockets the following sizes;
Earth Orbit: 10,822 metric tons
Moon 76,580 metric tons
Mars 156,240 metric tons
This are big rockets, but they're smaller than big ****ps like aircraft
carriers.
One of the problems of using stages like this is retrieving all the
stages to reuse again. Especially if they're dropped off on Mars.
If we use chemical rockets to launch a nuclear upper stage, we can do
some very interesting missions; First we revise our Vf downward once
we're operating in Earth orbit;
Nuclear Upper Stage from Earth Orbit Mass Ratio Requirements
Vf Nuc solid Nuc liq
Nuc gas
10.0 kps 15.0 kps
20.0 kps
Moon 8.8 km/sec 2.41 1.80 1,56
Mars 12.0 km/sec 3.32 2.23 1.83
All of these are easily achievable within our 3 to 1 overall mass
ratio.. We can see that big improvements in engine performance don't
translate to big improvements in mass ratio - when the exhaust speed
is equal to or bigger than the final speed. So, by not using nuclear
rockets inside Earth's biosphere - we really don't need to go much
beyond solid core nuclear rockets like the kind we built in the 1950s
and 60s - and still build today in nuclear subs.
So, we can compute the size of the booster rockets and the nuclear
upper stage for the sorts of systems we're talking about here.
Nuclear Solid Core m***** for 1,400 metric ton payload
Moon 3,374 metric tons on orbit
26,081 metric tons at lift off
Mars 4,648 metric tons on orbit
35,929 metric tons at lift off
So a 4 stage chemical rocket that could go to the moon would have its
two upper stages replaced by a solid core nuclear thermal rocket and
that 3 stage combination wold fly nearly triple the payload to the
moon and double the payload to mars.
Which is what the big argument over lunar orbit rendezvous was all
about, and the basis for NERVA upper stages for the Saturn/Nova
rockets;
http://en.wikipedia.org/wiki/Direct_ascent
http://en.wikipedia.org/wiki/Saturn_C-8
http://en.wikipedia.org/wiki/NERVA
http://en.wikipedia.org/wiki/Nova_rocket
http://www.astronautix.com/lvfam/nova.htm
http://www.astronautix.com/lvs/nova8l.htm
We were nearly there!!!
But we built too small and too limited.
The direct ascent saturn could only put 181 metric tons into Earth
orbit. We are using m***** on orbit 25.6 times larger. So, the
payload for this Nova/C8 rocket to Mars would have been 54.5 metric
tons. The crew would have been 3 to 5 people.
But it would have been a single upper stage that flew to the moon and
mars and returned to Earth - all for what about Apollo cost us - and
glided to a landing on Earth to be reused..
Of course had we been SERIOUS about space travel, and built on the
scale of our nuclear sub programs or on the scale of our nuclear
weapons programs, we would have built rockets that were 3x taller, 3x
wider and 3x deeper - 27 times heavier - and carried 120 crew members
- on the same mission.
Can we really build ****ps on that scale?
http://www.cruise-reviews.com/cruiselineinfo/****p_detail.asp?f****pID=34
Here's a ****p, like many ****ps that travel the ocean these days. It
costs about $200 million - weighs about 45,000 tons - carries about
1200 people - and is 757 feet long. Twice the length and 10x the mass
of the Saturn V
If we started with a 45,000 ton ****p (49,500,000 kg) - we'd end up
with 5,821,000 kg on orbit - in a nuclear stage. This would allow us
to propel 1,753,000 kg to Mars and back, and 2,419,000 kg to the moon
and back - all in one stage, which means we reuse it.
So, subtracting the 1,400,000 kg payload module with 113 crew on board
from this total we have a freight capacity of 353,000 kg to Mars AND
BACK, and 1,019,000 kg to the Moon and back.
The ****p at launch would stand approximately 1,200 feet tall, and have
a 150 ft diameter first stage. The winged payload module - with
nuclear stage - would be 45 ft in diameter and 300 ft long. All
stages would be recovered and reused.
Going one way with the payload, and only bringing back the 1,400,000
kg payload module, we nearly double these cargo values.
So, we easily establish a 5 year base on mars or the moon by reusing
the life sup****t systems built for the mission module. Merely produce
a scaled back version of the payload module - 1/3 the size for Mars -
a 30 person base for Mars (per trip) and and 3/4 size mission module
for the Moon - an 82 person base for the Moon (per trip) all in the
same hardware- carried on board the cargo hold of the nuclear stage -
and the base crew use the scaled back base module to live in during
the trip.
A fleet of 18 vessels - with 6 dedicated to lunar operations and 12
dedicated to martian operations - would send 6 payloads a month to the
moon - and 4 payloads a year to mars (on average) - with a five to six
year duty cycle - the Mars base sup****ted by this operation would
sup****t a base of 360 people - and the planetary surface hardware
would be plugged in and unplugged every few years - using a tower
crane and mobile platform specially built at each site.
The moon base sup****ted would be 24,600 people - a small town.
Permanent fixtures would be built over time and the base module
approach would ****ft to specialized personnel carriers for people and
cargo carriers for cargo. This permits larger populations to be
sustained. Its like the difference between a caravan of people in
cars versus people in a bus followed by a tractor trailer carrying
supplies - you get more per engine. .
It takes about 1 ton per year to keep a person alive. Reducing this
to half a ton a year with recycling and use of local resources,would
be possible with a dedicated research effort. But, we're focusing on
off the shelf stuff we could build today without too much trouble.
4 ****ps each carrying 353 tons per year to Mars could grow populations
to 1,400 to 2,800 people using the cargo approach. A source of water
once found, is used to reconstitute freeze dried food, is broken down
for oxygen in the air and the hydrogen is used for rocket propellant -
on the nuclear rocket. The oxygen gets breathed and converted to CO2
- which replaces the water- but adds a biological component and makes
plant life possible.
This would increase the Mars base to 28,000 - or more.
The moon base with 72 payloads per year of 1,019 - could grow to
72,000 to 144,000 people. A water supply found on the moon to supply
a ****tion of the air, and reconstitute freeze dried foods, as well as
refuel ****ps - increasing their useful payload - increases moon base
to 1.4 million or more.
All without building anything using technology beyond what we had in
1959. Using only 18 ****ps - smaller than a cruise liner.
Somewhere between 10,000 people and 1,000,000 people, we reach
criticality - even with 1950s technology - where the moon base and
mars base become independent of resupply from Earth and become their
own independent planetary Republics.
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