Gemini: We can rebuild it, we have the technology

[mrmorris]

<font color="yellow">"It may very well be that an impractical combination of weight, size, and motor strength is required. "</font><br /><br />After thinking about this for a bit -- I realized that mass and diameter can be optimized for the spacecraft -- but what determines whether it will have enough *power* to provide the roll control will be the electric engine(s) actually turning the wheel. Depending on the size/mass of the wheel -- the engines may have to turn it slower or faster (a matter of gearing), but in the end -- it all comes down to the engine itself.<br /><br />Turning this into a definitive answer still isn't really possible -- but I can approximate. I looked up the RCS thrusters on the original Gemini. It had 4 25-pound thrusters (actually it had two rings of four -- but this was for redundancy -- not power). I'll assume that all four were fired simultaneously to provide 100 lbs/sec of force. I'm going to ignore distance from the centerline of the craft because this is just a WAG anyway, as G-X3 won't have the same specs. Our electric engine has to be able to supply ~100 lbs/sec of turning power to the craft. <br /><br />100 lbs/sec equals 0.181745 horsepower<br /><br />Converting horsepower to Watts so we can look into electric engines, we have:<br /><br />kW = hp * .7457<br />kW = 0.181745 * .7457<br />kW = 0.13552<br />~135 Watt motor<br /><br />That's easily accomplished. A very quick Google search brought up scooter motors at this site beat that number easily. It's certain that we can find a smaller/lighter and more efficient motor for G-X3's RWCS. I'm confident then that the RWCS can be made powerful enough for re-entry control purposes.

 

[scottb50]

I like the idea of two fairly small reaction wheels mounted on the center of gravity, the farther apart they are the smaller they could be. Spun at identical speeds they would allow stability, applying a braking force at different locations around the perimeter of the wheels would cause a reaction in a specific direction. Apply the braking using a liniar generator and you could recover the energy. Once the proper orientation is reached speeds would be matched again to hold the desired attitude. An RCS system would still be needed during manuevering but could be scaled back considerably. <br /><br />If I remember correctly Hubble uses a similar system to change orientation and maintain precise aiming for very long exposures. <div class="Discussion_UserSignature"> </div>

 

[mrmorris]

<font color="yellow">"Apply the braking using a liniar generator and you could recover the energy."</font><br /><br />Dunno what you mean by a liniar generator -- but an electric motor is just a generator in reverse. If you manually spin the shaft of an electric motor -- it will generate power. So yes -- it's <i>conceivable</i> that there could be some power recovery inherent in the system.

 

[scottb50]

http://www.memagazine.org/backissues/march98/features/linear/linear.html<br /><br />Sorry I misspelled it.<br /><br />The idea would be a segmented ring around the rotor, apply current to certain segments and they act as generators applying a braking force where needed. Reverse the current and it becomes a motor. <div class="Discussion_UserSignature"> </div>

 

[tap_sa]

I tried to run some number again to estimate the size and rpm of the reaction wheels. I assumed the system works by conservation of angular momentum ie. if we make the spacecraft spin then it's angular momentum must be equal but opposite that of reaction wheel's, L_craft = -L_wheel<br /><br />The big unknown is spacecraft's moment of inertia, to calculate it properly would require exact position and mass of each bolt, nut and whatnot and the axis of rotation. I made a very rough generalization that a thin spherical shell diameter of three meters represents that craft, maybe no cigar but close enough. Some equations:<br /><br />Moment of intertia for reaction wheel (all mass on the rim) I_wheel = m_wheel *(r_wheel)² <br />Moment of intertia for the craft I_craft = 2 / 3 * m_craft *(r_craft)²<br />Angular momentum L = I * w (w = omega = speed of rotation)<br /><br />Now w_wheel = (I_craft * w_craft) / I_wheel (yes lost the minus sign but we are interested in the absolute value...)<br /><br />Another unknown was what's the desired rate of turning for the craft, w_craft. Punched in following guestimates; <br /><br />m_craft 5000kg<br />r_craft 1.5m<br />w_craft 6 degrees / sec (1 rpm)<br />m_wheel 100kg<br />r_wheel 1m<br /><br />and got w_wheel of 1.25 rotations per second, 75rpm.<br /><br />To spin it up with 135W motor seems to take about 23 seconds; reaction wheel kinetic energy / motor power, ( 1/2 * I_wheel * (w_wheel)² ) / 135W <br /><br />Unless a gross mistake lurks somewhere the system seams feasible. More data would be needed such as exact requirements for maneuver rotation rates and possible outside torques, latter during descent especially. When reaction wheel fights against continuous outside torque it spins up more and more until the

 

[najab]

><i>Btw there's a very lowtech backup reaction wheel; the astronauts! Loosen your seat belt, make some room for yourself (OK a bit tricky inside a spacecraft), push yourself into rotation....</i><p>And they laughed at me when I suggested (remember the funny French accent) "Fart in your general direction!" <img src="/images/icons/laugh.gif" /></p>

 

[mrmorris]

<font color="yellow">"The idea would be a segmented ring around the rotor..."</font><br /><br />I'd never heard of linear motors before. Read the article. Interesting. <br /><br />However -- the point of LMs seems to be to use them when you need a linear movement (i.e. a straight line). Electric motors normally generate a rotating motion, but when the magnets are laid out in a line rather than a ring -- you can get a straight line motion (albeit for a short distance rather than a continuous cyclical movement as with a conventional motor). However -- since the reaction wheels by definition *need* a rotational impetus... why would you do this? A standard electric motor with a rotating shaft will provide the required force.

 

[mrmorris]

<font color="yellow">"To spin it up with 135W motor seems to take about 23 seconds"</font><br /><br />23 seconds is likely a bit long to wait. By your calcs then -- 135 Watts is probably a bit underpowered of a motor for what we want. That's not suprising -- the smallest wattage of the scooter motors on the site I referenced earlier was 300W. If we use a 1hp electric motor (750W), the spin up time will drop to about 4 seconds. Using a 2hp motor (1500W) will drop it to 2.2 sec. That would make for a fairly reactive reaction system. <img src="/images/icons/smile.gif" /> The actual energy used by the system (i.e. battery drain) will remain constant -- as the amount of energy being imparted to the RWCS remains the same whether you use a 135 Watt motor for ~23 seconds or a 1500W motor for ~2 seconds.

 

[tap_sa]

Linear electric motor in the reaction wheel's rim might be better than conventional one spinning the shaft for at least couple of reasons; motor's mass would contribute to the rw's weight and I imagine it's simpler to create the tangential force on the rim than some pretty heavy torque at the shaft. You would need a heavy duty wormgear or something to make the torque. Instead of that you could have relatively simple coils on the rw's rim so it would be like giant rotor and the circular box in which the wheel resides forms the stator.<br /><br />

 

[nacnud]

Why does the motor have to be at the centre of the wheel, in fact why does the wheel have to be a wheel at all? <br /><br />Use a hoop instead.<br /><br />Take your scooter motor plus tire push the tire against the hoop to spin up/slow down the hoop.<br /><br />Also would regenerative braking be worth it? It would produce less heating, and require less power drain for the orientation change as a whole.<br />

 

[mrmorris]

<font color="yellow">"Why does the motor have to be at the centre of the wheel"</font><br /><br />Yep. I never viosualized the motor at the center. I see one of two things:<br /><br />1. Motor at edge. Edge of wheel has 'teeth' -- shaft of motor has a gear which fits those teeth.<br /><br />2. Motor somewhere between edge and center (picked to make best use of motor's power curve). A toothed gear of the proper radius on the wheel has 'teeth' -- shaft of motor has a gear which fits those teeth.

 

[gladiator1332]

I agree with all of you here. I am the author of the following article which appeared on Project Constellation.us awhile back:<br />www.projectconstellation.us/articles/advanced-gemini.html<br /><br />I never knew the idea of a Gemini revival was getting so much attention. I kinda fell out of the loop the last few months. However, after seeing LM's proposal yesterday, I wanted to look into the Gemini idea again, that's where I found this thread. <br />There are a few things in the article about LM's proposal that I can't really agree with. The first of which is:<br /><br />"McKenzie said the lifting body design was preferred over a capsule for several reasons.<br /><br /> <br /><br />For one, that approach allows more cross-range maneuverability, thus the craft can touchdown on land versus water."<br /><br />I can't stand it when people think capsules HAVE TO land on water. This is not the case, and Gemini showed that there are many other options to allow a capsule to match the landing capabiities of LM's design. <br />I really think their design, as usual (remember the X-33), is way too complex, and a simple capsule like Gemini would be the best way to go. <br />We have yet to see Boeing's plans, let's just hope we see a miracle Gemini rebirth.<br /><br />KEEP THIS THREAD ALIVE! <br /><br />

 

[gladiator1332]

Oh yeah, to the guys who are drafting up the Gemini X-3 plans, would guys be interesting in actually simulating this things flights?<br /><br />You might want to look into the Free Orbiter Space Flight Simulator ( http://www.orbitersim.com )<br />A few others and me with some help from you guys can probably model this thing and get it working in Orbiter. <br /><br />Cantact me at gladiator1332 -at- yahoo.com if you are interested.<br /><br />

 

[mrmorris]

Welcome to SDC, gladiator. I love it that my thread is pulling in so many knowledgable new posters. I have no intentions of letting the thread die anytime soon. I have a lot of fun researching the possibility of an advanced capsule and posting innovative (sometimes wildly so) means of making it more efficient.<br /><br />This already takes up too much of my time right this second though to consider trying to model it in that fashion -- especially as the design changes from week to week as I find better COTS equipment or determine a new way of doing something.

 

[gladiator1332]

Yeah, this is truly a great thread. If you ever do want to work on something for Orbiter, just contact me.<br /><br />I'm really surprised that this idea has not been re-looked over the past few years. Gemini was and still is our best spacecraft.

 

[nacnud]

mrmorris<br /><br />Have you seen this before?<br />

 

[spacefire]

all I have to say about this thread is eeek!<br />Mrmorris knows why, we expressed our difference of opinions in another thread <img src="/images/icons/smile.gif" /><br /><br /><br /><font color="orange"> Lifting bodies rock/capsules suck </font>/safety_wrapper> <div class="Discussion_UserSignature"> <p>http://asteroid-invasion.blogspot.com</p><p>http://www.solvengineer.com/asteroid-invasion.html </p><p> </p> </div>

 

[danwoodard]

Will the Lockheed CEV crew be sitting upright for landing, as the drawing appears to show? This creates many concerns about G-z stress in the Shuttle. Since there's no windshield and the controls are autonomous, it's hard to see why this would be ncessary.

 

[mrmorris]

<font color="yellow">"Will the Lockheed CEV crew be ...</font><br /><br />There are several threads in M&L devoted to the Lockheed CEV proposal. Please talk about it there. I'd like to minimize the side-tracking on this thread. It's about G-X3. Thanks.

 

[mrmorris]

<font color="yellow">"Have you seen..."</font> <br /><br />If you'd asked me that question two days ago -- I'd have had to say no. As it stands -- I actually ran across that yesterday in another link. GE developed that as part of their Apollo D2 Proposal. It has *lots* of similarities to Soyuz, and there's even speculation that the russians stole the design.<br /><br />The Soyuz-shape capsule is extremely efficient in terms of volume (it closely approximates a sphere -- the optimum volumetric shape). The D2 design expands on this efficiency and is <b>particularly</b> low mass because the re-entry vehicle is little more than a shell. All of the goodies are in the front module which then burns up on re-entry (much like Gemini and Soyuz). Of course this bites for reusability. <br /><br />It's conceivable to size up the Soyuz/D2 'shape' and put more of the equipment on-board. However, I don't care as much for this shape because it has a very low l/d ratio: between .2 and .3 (as opposed to Gemini's .4-something, and Apollo's .52). While I don't believe in paying for the mass of wings to increase l/d -- a slightly less efficient shape is a much lower price to pay for extra maneuverability.

 

[spacefire]

Russia is moving away from the Soyuz design. Their Kliper (proposed Soyuz replacement) is a lifting body. <div class="Discussion_UserSignature"> <p>http://asteroid-invasion.blogspot.com</p><p>http://www.solvengineer.com/asteroid-invasion.html </p><p> </p> </div>

 

[mrmorris]

Spacefire:<br /><br />This thread exists for a specific purpose -- namely that of discussing the possibility of designing a capsule in a Gemini/Apollo style that would be possible for a commercial firm to build for the Americas Space Prize. <br /><br />You have no interest in capsules. That's perfectly fine and is your right. However, because of that belief, you have no real interest in the discussion being held here. That is not true for a lot of other readers of the thread. Please don't post in here unless you wish to contribute to the discussion at hand. Thank you very much.

 

[nacnud]

The GX3 is sized to fit on a falcon V and take five people to orbit. The kliper could also do this but the kliper weighs about twice that of the GX3.

 

[taralon]

Mr Morris,<br /><br />I don't know for certain, but that poster on the X-Prize board may have been me. I'd been running numbers around the time of the SS1 flights, and looking over prior manned capsule designs in order to get a feel for what would be needed in later prize attempts, namely a fully orbital vehicle. <br /><br />SpaceX and the never produced 'Big Gemini' capsule was a combination that seemed tailor made for the purpose. After all, the booster was low cost, or promised to be, and capsule technology is mature and well understood. <br /><br />If you would like to contact me, same username at hotmail works. <br />

 

[mrmorris]

<font color="yellow">"I don't know for certain, but that poster on the X-Prize board may have been me."</font><br /><br />I'm not really certain which poster you're referring to. If you mean the person who first suggested the Gemini on a Falcon V in the X-Prize Bigelow forum -- I already have communicated with him early on -- even directed him to this thread some time back. At this time -- I'm simply playing around with ideas and researching various options for the subsystems. <br /><br />If you'd like to contribute to the thread -- you're more than welcome to. If you have something to discuss off-thread -- as an SDC member you can send a private message to me.