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RCS of nuclear spaceship is an interesting topic: even a high-Isp nuclear spaceship needs secondary rocket engines for attitude control, course correction and spin-despin maneuvers (in case of artificial gravity): during thrusting she can make an efficient TVC by gimbaling the main engines, but during coasting she has to rely on smaller rocket, usually NTO-MMH, which have an Isp up to 335 s.
Stanley Borowsky put 9.1 tons of NTO-MMH in his more than 336.5 tons NTR-Copernicus, giving a total chemical deltaV of 90 m/s.
If the ship uses artificial gravity - like GW's rigid baton - she needs more RCS propellant, because she has to be de-spinned and spinned again every times she needs a course correction or an orbital plane change: I calculate almost 150 m/s of chemical deltaV.
https://ntrs.nasa.gov/archive/nasa/casi … 017461.pdf
I think the best kind of RCS for a NTR spaceship would be a system that uses the same propellant either for the main engine or the RCS rocket: a bimodal-NTR spaceship, with two NTR, which also give 200 KW of electric power, may use low thrust rhenium resistor-jet rockets for fine attitude control and spin and de-spin operation, which gives almost 80 N of thrust with an exhaust velocity of 8 km/s, employing almost 5 h to reach a full-gravity spin rate.
https://archive.org/details/DTIC_ADA064236/page/n15
For higher course corrections (20-30 m/s) she can use something like hot-gas thruster, where hydrogen is not heated electrically, but by passing through the heath-exchanger of the bimodal rocket (which gives almost 600 KWth to produce 200 KW of electric power). There is an interesting work about this kind of "indirect-NTR":
https://ntrs.nasa.gov/archive/nasa/casi … 066527.pdf
With 600 thermal KW of power it gives almost 280 N of thrust with an exhaust velocity near 6 km/s.
For ullage and quick emergency maneuver, like dodging a small meteor, she can use cold H2 jets: which give an exhaust velocity of 2.9 km/s, better than any kind of mono-propellant rocket.
Would it be a nice idea?
Last edited by Quaoar (2018-11-13 09:41:39)
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You could spin-up and -down using reaction flywheels. That does use electric motors (generally heavy moving-iron devices) and a source of significant electric power. The flywheels themselves inherently have to be heavy. I don't honestly know whether that's fewer tons than tons of MMH-NTO. But the electrical could be solar, with a big battery for surge power during spin operations.
GW
Last edited by GW Johnson (2018-11-13 11:42:00)
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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You could spin-up and -down using reaction flywheels. That does use electric motors (generally heavy moving-iron devices) and a source of significant electric power. The flywheels themselves inherently have to be heavy. I don't honestly know whether that's fewer tons than tons of MMH-NTO. But the electrical could be solar, with a big battery for surge power during spin operations.
GW
Thanks GW,
I have to spin a 175 meter long, 12 meter diameter, 864000 kg GW-like modular spaceship: using 200 KW, 80 N, 820s Isp. rhenium resistor-jets it takes almost 840 kg of LH2 for a spin-despin cycle at full load (to simplify calculation I considered the ship an homogeneous cylinder).
I don't know how big must be reaction wheels to spin such a monster-ship.
Using the same propellant for RCS and main engines makes things simpler.
The main engines are two gas-core NTR 2000 kN and 2000 m/s of exhaust velocity: I think I cannot use them for small course correction i.e. 5-30 m/s deltaV, so I used the indirect-nuclear passing hydrogen through the heat-exchanger of the solid core start-up rings (the engines comes from the Russian Glushko RD-600). It may work?
http://www.astronautix.com/r/rd-600.html
Last edited by Quaoar (2018-11-13 16:00:52)
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Not having done the calculations, electric thrusters may be a good praxis for spin-up and spin-down. Their fuel consumption is very small, and their consistent low thrust is fine (even desirable?) for adaptation to rotation and for minimizing the oscillation of the spaceship-tether-counterweight system.
-Josh
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Not having done the calculations, electric thrusters may be a good praxis for spin-up and spin-down. Their fuel consumption is very small, and their consistent low thrust is fine (even desirable?) for adaptation to rotation and for minimizing the oscillation of the spaceship-tether-counterweight system.
Hi Josh,
I have approximate the inertial moment of the ship, considering her as an homogeneous cylinder, using the formula J=m*l^2/12, where m is the mass of the ship in kilos and l is the length in meters: having a 175 meters ship with a diameter of 12 m and a mass of 864000 kg, it gives an inertial moment of 2205000000 kg*m2.
To have one Gee of artificial gravity at the aft deck she has to spin at 3.12 RMP (0.335 Rad/s)
Considering an iron reaction wheel of 3 meters of outer radius, 2.5 meters of inner radius, 0.5 meters of height and 34000 kg of mass: using the formula J=m*(Ro^2+Ri^2)/2 it has an inertial moment of 46750 kg*m2 (46750)
to spin the ship at 3.12 RMP, the wheel has to spin at 147157 RPM. I don't know if is possible to build devices that spin so fast. Usually reaction wheels have a maximum speed of 7000 RMP. So we need an order of magnitude bigger and massive reaction wheel.
But using 200 KW, 105 N rhenium resistor-jets with 826 s of Isp. , I need only 521 kg of H2 to spin the ship (the complete maneuver takes almost 11 h, but this may be OK, because the crew need time to learn how to cope with Coriolis force). Even if I spin and despin my ship for six times during my mission, the propellant needed is only 6510 kg.
So I've a system simpler, lighter and less prone to failure.
Last edited by Quaoar (2018-11-14 16:16:46)
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Quaoar,
I don't see a reaction wheel as a practical means to impart spin in this case. The bearings would have to be damn near perfect and the machining of the reaction mass would also have to be nearly perfect. You're talking about spinning the mass of a light tank at 147,000 RPM!
If the bearings ever fail, then you can kiss that ship goodbye.
You're only need to spin up and spin down 4 times per mission:
Spin up after TMI
Spin down before Mars EDL
Spin up after TEI
Spin down before Earth EDL
There could be one or two contingency spins in the propellant mass budget allocation, but if you have too many problems that require spin down, then maybe you're better off dealing with the consequences of micro-G. You'll never approach the tonnage of the flywheel system using electric thrusters. Loading propellant for 8 spin operations per mission is just 4,168t. Using thin film solar arrays, the mass allocation with deployment structures shouldn't weigh more than half a ton. The PMAD would weigh substantially more, but we're probably looking at 6t on the high side.
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Spin up after TMI
Spin down before Mars EDL
Spin up after TEI
Spin down before Earth EDL
But there are also mid course corrections and orbital plane changes, so during the Earth-Mars trip, I have to stop spinning when my Hohmann orbit intersect the orbit of Mars, make the plane change and course correction then spin again.
So there are:
1) a spin up after TMI
2) a spin down before plane change
3) a spin up after plane change
4) a spin down before EDL
5) a spin up after TEI
6) a spin down before plane change
7) a spin up after plane change
8) a spin down before Earth EDL
8 spin maneuver unless there is a way to perform course correction while spinning: I don't know if it's possible
Last edited by Quaoar (2018-11-19 06:19:08)
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For Quaoar regarding Post #7 in particular and the topic overall:
I've been following the discussion between yourself and kbd512 with interest.
kbd512 recently pointed out the challenge of using small reaction wheels to change the orientation of a large space vehicle.
For readers of this Forum who may not be acquainted with his work, Dr. Gerard K. O'Neill imagined a variety of space habitats, among which were large rotating cylinders to provide artificial gravity for human and other living residents. Pertinent to the present discussion (I am hoping) is Dr. O'Neill's concept for pointing solar arrays at the Sun, as the proposed habitats traveled around the Sun in an orbit separate from the Earth.
Dr. O'Neill's solution was to arrange two rotating habitats side by side, joined by struts with appropriate bearings. By changing the angle of rotation of the cylinders with respect to each other, it was expected the habitat pair could rotate as a unit so it was always pointed at the Sun.
https://en.wikipedia.org/wiki/Gerard_K._O%27Neill
The next person I'd like to bring into the discussion is Buzz Aldrin.
https://en.wikipedia.org/wiki/Buzz_Aldrin
Dr. Aldrin proposed the "Aldrin Cycler" for regular travel between the Earth and Mars.
It seems to me (without knowing details of how the Cycler was planned to orient itself) that Dr. O'Neill's idea of using the entire habitat as a set of reaction wheels would lend itself well to Dr. Aldrin's concept, by insuring that the Cycler could orient itself in space without consuming mass for propulsion.
SearchTerm:AldrinCycler
SearchTerm:GerardKO'Neill
(th)
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Do mid course corrections have to use high thrust propulsion? I'm wondering if an electric propulsion system could be used that would thrust through the centre of rotation, which for a baton shaped spacecraft would be at a different axis to the main engines. But given how little thrust we're talking, the structure should be able to handle it.
Use what is abundant and build to last
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It means that you need to know the precise location of the spacecraft better, in order to use such a gentle force over such a long period of time to put it on the right course given the ongoing effect of various gravitational fields. It's a heavier computational load too.
I don't know exactly where our capabilities lie here but it should be doable.
-Josh
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Rotation is in two forms; spiral and tumble to which surface area that each can see the effects of artificial gravity are different..Each will depend on the building blocks used in there respective design shape as its built most likely in orbit.
The tumble or baton is an end over end that has a shifting center of balance that is mass dependent and not length with a shape of a straight line.
Spiral can take on both straight lines and ring along the main axis and does not suffer from the wobble of rotation.
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Terraformer,
The Deep Space 1, Hayabusa, and Hayabusa 2 missions use Hydrazine-based thrusters for gross attitude adjustment, but also use their ion engines for course changes. Another way of looking at the problem is that while more compute power is required to determine how the use of artificial gravity affects trajectory, the course correction burns can be more precise than what a Hydrazine thruster could produce, limiting the requirement for subsequent corrections from over-correction associated with less precise changes imparted by higher thrust chemical engines.
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Josh,
The gravitational fields of the Sun and other planetary bodies always have to be accounted for. 15 million miles from Earth and Mars, I would expect the gravitational effects of the Sun to dominate, but I'd have to do the math to actually know. In the case of a rotating baton vehicle using ion thrusters on gimbals to perform the mid-course correction burn over a day or two, the computer that controls what direction the thrusters are pointed in must have more precise estimates of the spacecraft's mass during the maneuver, center of mass, and moment of inertia.
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This topic (RCS propulsion) seemed like a reasonable match for this story, about a possible replacement for hydrazine:
https://www.yahoo.com/news/nasa-test-sa … 06085.html
The new fuel is apparently safer and more efficient.
(th)
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I did see an article on green fuels to be tried but did not get the chance to read through it.
Post 7/8 is talking about the same system that the ISS uses to control it orientation without using fuel but power and special gyro's..
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