# Reaching Orbit With an Assist

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#### jimfromnsf

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"17470 mph is the orbital velocity for 100 mile altitude. If the tug were to go to 17380 mph, it would be at 145 miles. A spacecraft can not be in a circular orbit at 100 miles and have a velocity of 17380 mph. The only way it could be is if it was in an elliptical orbit and was passing through 100 miles "<br /><br />Here's the problem. An unpowered spacecraft can't stay at 17,470 mph in a circular orbit at an altitude of 100 miles. A powered spacecraft can stay wherever it wants, as long as it spends propellant. <br /><br />Someone who works with electrodynamic tethers will understand that two equal ballast weights on the end of a long tether will circle the earth at the same number of orbits per year, despite the fact that the weight at one end of the tether is a bit closer to the earth. The ballast at the bottom end appears to be going slower than orbital velocity, and the ballast at the top end appears to be going faster than orbital velocity. <br /><br />I'm going to extrapolate from your given orbital velocities that a 50 mile long tether whose center of gravity is at 125 miles altitude, in a circular orbit, will be orbiting at 17470 mph. The top ballast weight will be "orbiting" 150 miles up at roughly 17470 mph and the lower ballast weight will be "orbiting" 100 miles up at roughly 17370 mph, each 100 mph off. Both ballast weights stay in a circular motion around the earth because the tether provides a tiny amount of lift to the bottom weight and an equal downward pull to the top weight. The lift or downward pull is less than 1 percent of earth-normal gravity. <br /><br />Now, imagine replacing the tether with a small thruster and a big tank of propellant. The bottom ballast weight can be kept in a circular "orbit" until the propellant runs out. 1 kg of thrust can keep 100 kg of ballast in this circular path, which a purist would not call an orbit.<br /><br />How long could such a path be maintained? Hours? <br /><br />For a really slow docking maneuver we

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<i><br />4. A. Firing a cannon or basically expulsing mass (the net) is going to result in a reaction in the opposite direction.<br />B. the impact and spin of the net is going to impart forces on the target making it move and spin. This might causing the target to wind up the tether like a yoyo and hit the tug<br />C. The tug canâ€™t just fire its engine and go into another orbit. Since the engines on the tug are not through the center of mass of the system (tug, tether and launcher), it is going to spin up everything.<br />D. Reeling in the tether is going to send the launcher moving towards the tug without away of stopping it.<br /></i><br /><br />A. Let's assume that the full tug has at least 20 times the mass of the net and tether. The tug won't go very far.<br />B. Let's assume the payload has at least 20 times the mass of the net alone. The payload won't go very far backwards before the tug imparts a gentle pull on the tether.<br />C. This is a problem. I would suggest that as soon as the payload is snagged and the loose tether cable recovered, the tug would accelerate for a few seconds, swing 50% of the way around the tether, then accelerate in the other direction, swing 50% of the way around the tether again, and so on until both tug and payload had regained their missing 100 mph and achieved a stable orbit. <br /><br />D. At the point of orbit, the tug could take hours to dock with the payload, pull the net off the payload, and store the tether for another day.

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#### jimfromnsf

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"Now, imagine replacing the tether with a small thruster and a big tank of propellant. The bottom ballast weight can be kept in a circular "orbit" until the propellant runs out. 1 kg of thrust can keep 100 kg of ballast in this circular path, which a purist would not call an orbit. "<br /><br />This won't work. You have introduce a whole lot of mass and it changes the initial conditions. Also the thruster does not replace the tension in the tether. Remember, F=ma, there is going to be an acceleration from the thruster.

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#### jimfromnsf

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"B. Let's assume the payload has at least 20 times the mass of the net alone. The payload won't go very far backwards before the tug imparts a gentle pull on the tether.<br />C. This is a problem. I would suggest that as soon as the payload is snagged and the loose tether cable recovered, the tug would accelerate for a few seconds, swing 50% of the way around the tether, then accelerate in the other direction, swing 50% of the way around the tether again, and so on until both tug and payload had regained their missing 100 mph and achieved a stable orbit.<br /><br />D. At the point of orbit, the tug could take hours to dock with the payload, pull the net off the payload, and store the tether for another day."<br /><br />B. It still will cause a rotate<br /><br />C. This is nearly zero-g, there is no swinging, they are going to spin around the tether<br /><br />D. "pull the net off"? Why even bother with the net? It is not the docking part that takes the time, it is getting to the position (station keeping) where you can say the have rendevzous.

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<i>"Now, imagine replacing the tether with a small thruster and a big tank of propellant. The bottom ballast weight can be kept in a circular "orbit" until the propellant runs out. 1 kg of thrust can keep 100 kg of ballast in this circular path, which a purist would not call an orbit. "<br /><br />This won't work. You have introduce a whole lot of mass and it changes the initial conditions. </i><br /><br />Just because initial conditions are changed is not a good reason why something won't work. <br /><br />You haven't quantified "a whole lot" of mass. What if the little thruster is only on for two minutes of thrust and only imparts two pounds of force for a 100 kg rocket?<br /><br /><i>Also the thruster does not replace the tension in the tether. Remember, F=ma, there is going to be an acceleration from the thruster.</i><br /><br />The point of the thrust is to replace the tension in the tether. Simply denying that the thrust replaces the tension in the tether is not specific. Could you explain more? <br /><br />Are you worried that the thruster could not make the 100% perfect imitation of a circular path that a tethered object would make, because thrusters are imperfect? Having a perfect imitation of a circular path is not a criterion here. A completely rough approximation will suffice for the netting task. Periodic burns of the thruster will suffice.

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#### jimfromnsf

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"Just because initial conditions are changed is not a good reason why something won't work."<br /><br />It one of the best reasons or something not to work. But anyways.<br /><br />"You haven't quantified "a whole lot" of mass. What if the little thruster is only on for two minutes of thrust and only imparts two pounds of force for a 100 kg rocket? "<br /><br />on and off operation of the thruster is still going to change the orbit<br /><br />"The point of the thrust is to replace the tension in the tether. Simply denying that the thrust replaces the tension in the tether is not specific. Could you explain more?"<br /><br />Basic physics. The tether with the 2 weights is a closed system. replacing the tether force with a thruster opens up the system (mass is expelled) and therefore not in equilibrum and will accelerate.<br /><br /><br /> It is not doable. The launcher has to get into orbit on its own

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<i>Basic physics. The tether with the 2 weights is a closed system. replacing the tether force with a thruster opens up the system (mass is expelled) and therefore not in equilibrum and will accelerate.</i><br /><br />I'm still trying to get a handle on your logic. The explanation above would also argue that a lunar lander won't ever hover above the moon's surface for a few seconds, because every rocket thruster expels mass and opens up the system.<br /><br />"Basic physics" is starting to sound like, "If it's not basic mathematics, it's too hard for mechanical engineers to handle." I mean, this stuff isn't rocket science. Let me think about that last statement. Mmmm, actually it is rocket science, and maybe that's our problem. Maybe the command and control, the equations, the changing thrust requirements due to the tug's steady loss of mass, are too hard for many engineers and mathematicians to handle. I'm just trying to get to the bottom of your argument. <br /><br />There appears to be nothing fundamentally wrong with the Newtonian physics of my example. In theory any regulated acceleration (from a thruster) can replace any other equal acceleration (from a tether) on an object to give an equal result (a circular orbit).

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#### jimfromnsf

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So what is the point of using a thruster to maintain an orbit? It still doesn't change my diagram. The idea of "grabbing" a suborbital rocket is still not going to work. Even though your tug is in a "slower" circular orbit that might have the same velocity as the launcher when it is at apogee. It will still be in a different orbit. (as my diagram shows).

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#### j05h

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Your thruster-on-tether will raise the orbit (if attached at the top mass) or cause the tether to collapse upward (if attached to the lower mass). <br /><br />Josh <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

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<i>"Your thruster-on-tether will raise the orbit (if attached at the top mass) or cause the tether to collapse upward (if attached to the lower mass"</i><br /><br />After netting the payload our next task is to quickly raise the orbit, to get out of any drag. We are done with the lowest possible earth orbit. The payload usually has to go to geostationary orbit or into some particular orbit. <br /><br />I don't plan on using a thruster in a direction that collapses the tether.

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