Re: Building Real Spaceships
- From: Willie.Mookie@xxxxxxxxx
- Date: Sat, 9 Feb 2008 18:07:38 -0800 (PST)
On Feb 9, 1:23 pm, swyck <a...@xxxxxxxxx> wrote:
Interesting topic.
I agree that we can build large interplanetary spaceships with the
techonology we have. I also agree with those that say we won't do it
until there is a strong economic incentive.
The economic incentive is there already. Teledesic and Iridium saw
opportunities, bt missed them because of delays in getting to market
and over-regulation. This was with 40 year old space technology -
comsats.
We will not have economic incentives for interplanetary flight until
we have been on the planets for 20 to 30 years. Then, economics will
develop. Comsats, wouldn't exist without the commitment to orbit a
satellite. Same for interplanetary development.
The central question, is there a national incentive to take the lead
in interplanetary development? I believe there is. As a nation we
spent $4 trillion on the cold war, $2 trillion on smaller conflicts,
and only $400 billion on civilian space.since its inception. We've
spent more in Iraq than our entire Civilian space budget since 1958.
All I am saying is that annual expenditures five to ten times larger
than our current budget (2.5% to 5.0% of the federal budget) -
directed toward building a small fleet of modest reusable spaceships
is doable,affordable, and gives us the ability to establish outposts
on the moon and mars from which commercial activities will arise.
IMO those incentives will
eventually materialize
Only if the infrastructure is in place.
, or techonological changes may make those
incestives cheaper,
They're cheap enough already, we just need to spend the money to use
them.
but unfortunately I dont expect that any time
soon.
We do not lack technology. We lack the will to put money into these
things. 2.5% to 5.0% of the Federal Budget to build up the sort of
capacity I'm talking about is highly affordable and pays huge
dividends geopolitically.
Until then I think any interplanetary journeys, including to
the Moon, will be on a small scale.
Then they will fail to inspire the sort of geopolitical leadership we
need, fail to fire the imagination of the world, and fail to attract
the necessary capital from the market to build upon the basic
infrastructure. In short, they're at a scale that gaurantees failure,
and leaves the door wide open for another nation to take these steps
and steal the lead from us.
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
ship 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.
snip
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.
Is building a large nuke submarine sized interplanetary vessel that
can land on and take off from earth really the most efficient design?
Yes - this is the small end recall. Ulam and company imagines 8
million ton spacecraft the size of cities.
I had envisioned building such a craft in orbit,
Its easier and cheaper to build things on Earth.
and transferring
materials
Its easier to load things on Earth.
and personnel via rockets or shuttles.
You can reduce capital investment by reducing the size of the
vehicles. No doubt about it. But in doing so you increase the
recurring operating costs and reduce the mass flow rate you can
sustain to the moon and planets, thus reducing the size of the
facility you can support. Going from cities to pup tents, going from
permanent installations to overnight stays, obviously reduces capital
costs. It also makes it impossible for a self-sustaining political
entity 9which trades with Earth) to arise.
Not only do you raise recurring costs, but you also increase mission
complexity and reduce reliability.
As for Mars and the
Moon, landing craft launched from the spaceship would land resources.
Why? You get improvements no doubt. This is what lunar orbit
rendezvous was all about. Do you want to build a general purpose
spaceship capable of multiple missions and get really good at flying
it? Or do you want to build optimal ships for each stage of the
flight, learn how to fly each through that specialized mission, and
transfer things as you say? One system costs more to buy per ship but
the entire fleet costs less and is operated at a low lower cost,while
the other system is optimal for each stage of the journey,
demonstrably the most efficient, but actually costs more overall to
complete an overall journey (more vehicle types, each one has its own
non-recurring costs to engineer test and build) - and reliability of
the entire system is considerably less than a single ship.
That way you'd only need the energy to land the materials needed,
This optimizes each step, but creates a nightmare of different vehicle
types that are flown diffferently in each phase of the flight, each
vehicle might be optimal and least expensive in terms of a general
purpose ship - but taken together - the entire fleet costs more than a
modest fleet of larger ships that have multiple roles.
the Nova and Nerva combinations suggested in 1959 to Eisenhower, and
again to Kennedy, achieved what Kennedy stated in his Rice University
speech - namely, by going to the moon, the US would develop the core
technologies to give us mastery over the moon and mars.
We'd build a 4 stage direct ascent spacecraft that would take 3 to 6
men to the moon and back with all chemical stages before 1970. Then,
replace the upper two stages with a single reusable nuclear third
stage that would allow 9 to 18 ment to journey to the moon and 6 to 12
men to journey to mars.
I propose that Nova was at the low end for size.
I have suggested a 45,000 ton spacecraft which requires 63,000 tons at
lift off.
The engine is the major cost component. I submit that it is not as
difficult as it may appear at first.
The J2 engine has 100 metric tons of thrust with a specific impulse of
420 seconds.
http://en.wikipedia.org/wiki/Image:J-2_rocket_engine.jpg
The J2 masses 1.7 metric tons each.
These were designed for upper stage use, but they have been proposed
for the Ares rocket for launcher use. This entails a slight change of
expansion ratio. Increasing throat area reduces Isp slightly and
increases thrust a lot.
Hydrogen Oxygen engines similar in performance to the J2 have been
built, the RS-68 for example, and others larger still - approaching 1
milion pounds of thrust, and up to 3 milion pounds of thrust.(500 to
1,500 tons)
http://www.aiaa.org/tc/gt/resources/newslettersum2001.pdf
Annular aerospike engines are possible using a large number of
combustion chambers attached to a ringlike expansion nozzle throat
areas feeding into a common variable expansion area behind the
vehicle. This allows the expansion ratio and thrust to automatically
compensate for changing aiir pressure.
http://en.wikipedia.org/wiki/Image:Annular-Aerospike.jpg
A ring of 42 hydrogen-oxygen engines around an annular aerospike body
forming a ring 150 ft in diameter, each emgine with 11 ft throats,
generate the needed thrust. 7 engines operating at 50% pump rates,
allow the vehicle to land vertically on a mobile platform.
A ring of 14 engines similar to the first stage, only fewer, form a
ring 50 ft in diameter create an aerospike nozzle for the second
stage. .
The third stage is 33 ft in diameter and propelled by a cluster of
five nuclear thermal engines with conventional nozzles each
generating . 670 tons each, 3,400 tons total for teh five. This is
34 times the thrust proposed for Nerva - and hence 34 times the power
- for the cluster of five. A 7,000 ton module will mass
approximately 4,000 tons at landing - and so, 3,400 tons - in 1/3
gravity of mars, or 1/6 gravity of the moon - will be more than
adequate for maneuvering the vehicle.
The empty chemical powered vehicles are designed for re-entry, and
then once they slow to subsonic speeds, small winglets are deployed in
a manner very similar to a cruise missile, to increase L/D to about
12:1. The vehicle masses 16% its lift off weight - 7500 tons for the
first stage, and 2,500 tons for the second stage. meaning 625 tons
and 157 tons of thrust are needed to pull it through the air at
subsonic speeds to maintain flight. Tow planes snag each stage upon
re-entry- downrange, and the stage is towed back to the launch center
and released. It is then restarted at 20% thrust levels - and
executes a tail sitting maneuver similar to early VTOL aircraft of the
1950s.
http://en.wikipedia.org/wiki/Tailsitter
These aircraft could be landed quite accurately. With modern GPS and
computer guidance, converting from horizontal glided flight to
vertical - and then reducing thrust would be rather simple.
This allows a reduction of about 30% in structural mass, and reducs
the area needed for landing these large vehicles. The stage
approaches the touchdown point, lights its engine, pulls into a
vertical position, hovers over the landing site, and reduces thrust to
land - making use of the same thrust structures and support structures
that supported the vehicle at launch.
On mars the nuclear stage would execute the same sort of manuver to
land in a small space. On the moon, without air, the vehicle would
come in for a vertical landing similar to the way the LEM and surveyor
spacecraft landed on the moon. They would use rocket power to kill
all orbital velocity, and then pitch up to vertical as the vehicle
fell, and then at the last second, pour on full thrust to kill the
downward velocity to arrive at zero velocity at zero altitude.
The nuclear stage is imagined to be a 33 ft diameter vehicle some 180
ft tall, with tail fins similar to the tail sitter shown, that spread
out across 60 ft to stabilize the vehicle - Since the nuclear reactors
are the heaviest part of the ship,the CG is low and the vehicle can
land and take off and remain stable at angles up to 45 degrees..
Three of these nuclear reactors are bimodal, that is, in transit mode
they have a thermophotovoltaic energy cycle that generates several
megawatts of power to operate the mission module throughout the
flight..
instead of having to land the whole spaceship, which would eventually
have to take off again.
Yes, this optimizes each ship for each stage, but increases the
overall budget, the overall complexity and overall risk for the
mission. A single large ship capable of all phases of the mission
while slightly less efficient is vastly superior in terms of total
capital expenditures and in terms of creating the neessary skills
needed for general purpose space navigation.
It seems to be that would be more efficient,
For that specific step yes - but overall, it reduces flexibility,
reliability, throughput, while increasing overall mission cost and
complexity.
but I haven't done any real numerical analysis.
NASA - analyzed three approaches to the moon landing. Direct Ascent,
Earth orbit rendezvous, and lunar orbit rendezvous. Direct Ascent had
the largest vehicle,at launch, but set the stage for landing large
lunar base elements on the moon. EOR provided for the smallest launch
vehicles, but increased overall cost dramatically as the types and
number of vehicles proliferated. LOR provided for a single launch -
but required the development of the LEM - and procedures for docking
and transfer and all of that - which took most of the budget from
64-68 - while making the Apollo hardware a single mission system with
limited ability to do lunar basing or mars flights - which was in the
original proposals as indicated in Kennedy's Rice speech.
Of course you'd have
to carry along the extra craft so you may lose efficiency with
redundancy.
And increase cost developing different vehicles, and increase
complexity and risk as pilots have to familiarize themselves with
different vehicles - redundancy can help if there is a failure of one
of the vehicles though, provided the failure is survivable.
I guess Its also possible that if you leave an interplanetary stage in
orbit, then landing the rest of the ship may make sense, especially if
the whole purpose of the ship was to deliver/remove material.
The mass ratio calculations tell the story. You can leave the
propellant you need to get back to Earth in orbit and not carry it
down. This can be accomplished by a tank with its own RCS. It does
entail some risk however. Also, as the specific impulse of the engine
increases, the savings in doing this are diminished.
Finally, in the case of Mars, if we make use of the atmosphere to slow
the vehicle down, its a greater savings to have everything done by the
atmosphere, rather than by rocket in entering the Mars system.
The simplest way to improve things is to find a source of hydrogen at
your destinatino and top off your tanks when you get there. This lets
you start with a lower mass system, and then move your propellant
bulkhead increasing cargo volume and decreasing propellant volume -
once you can reliably refuel at your destination.
Here's the relevant numbers for teh nuclear stage proposed
1400 mission module
1000 cargo
2400 structure
900 Isp
8838 Ve
2800 Vf- landing
2800 Vf - take off
4800 vehicle mass (tons)
0.316813759 Vf/Ve
1.372746886 nass ratio
1789.185053 take off propellant
6589.185053 mass+ take off propellant
2456.09821 landing propellant
So, by refueling on the mon - we need only 1789.2 tons of propellant
to land instead of 2456 tons of propellant to land because we don't
have to take the extra propellant down. So we save 667 tons on the
landing, plus 1789.2 tons to take off again. That means we could
carry 2456 tons one way leaving Earth orbit with the same mass as
before - Of course we could carry 2456 tons of propellant to land, and
refuel with 2456 tons of propellant to take off - and carry the extra
1789.2 tons as payload - allowing us to carry 2789 tons round trip -
by refueling on the moon.
So we can spend a lot designing specialty ships to do minimal missions
in tiny steps - or we can build the vehicles big and capable, and then
spend our money prospecting for fuel at our destination (water which
we break down into hydrogen and oxygen using our nuclear reactors -
using the hydrogen for propellant and the oxygen to breathe) - and
increase the capacity of our ships.
Rather than moving a bulkhead, we could design the ship with removable
tanks with these sorts of things in mind. In fact the removable tanks
could be the backbone of habitation modules designed to operate on
Mars and moon surface. Which was the idea behind Skylab
http://en.wikipedia.org/wiki/Wet_Workshop
That
would be a bit different then carrying a separate landing craft along.
Still, landing and taking off on Earth seems like a lot of energy to
spend.
Mass ratio is the key - and we computed that at the outset. Two
chemical stages boosting a 6,000 ton nuclear stage to orbit achieves
everything we need in order to settle the solar system.
A three stage vehicle sufficiently large - to loft a 5,000 ton nuclear
stage into LEO - is sufficient to do rather amazing things on the moon
and mars. While dropping a 5,000 ton spacecraft straight down out of
the sky to soft land on the lunar surface may seem risky, its no more
risky than dropping a 5 tons spacecraft out of the sky, and all you
have to do is watch a 45,000 ton ship come in and dock to realize that
with skill and planning and repreated experience - people can do it
routinely.
.
- References:
- Building Real Spaceships
- From: Willie . Mookie
- Re: Building Real Spaceships
- From: swyck
- Building Real Spaceships
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