The ship would spend about 54 minutes in the neighborhood portrayed.Is there enough time to accelerate .6 km/s, disengage, turn 180º and then decelerate .6 km/s?
But I was looking at a near parabolic orbit with a 400 km perigee. Precious little time is spent in the neighborhood of perigee. In the attached graphic a region of the ellipse is divided into 16 time increments, each increment about 3.4 minutes. The ship would spend about 54 minutes in the neighborhood portrayed.Is there enough time to accelerate .6 km/s, disengage, turn 180º and then decelerate .6 km/s?
Quote from: Hop_David on 05/26/2014 05:12 pmBut I was looking at a near parabolic orbit with a 400 km perigee. Precious little time is spent in the neighborhood of perigee. In the attached graphic a region of the ellipse is divided into 16 time increments, each increment about 3.4 minutes. The ship would spend about 54 minutes in the neighborhood portrayed.Is there enough time to accelerate .6 km/s, disengage, turn 180º and then decelerate .6 km/s?As others have noted, I would think so. It'll depend strongly on your system's T/W ratio though. The T/W ratio of the stage after separation (during the breaking burn) should be much higher. For instance, with a Dual Engine Centaur stage and a 60-ish tonne payload being slung on say a TMI trajectory, your T/W ratio is down around 0.25 for the departure burn (taking roughly 4min for the burn). But once you've staged, your T/W ratio is probably >1 (my super quick BOTE is saying you'd have a burnout mass on the Centaur of <9tonnes, which would give a T/W of ~2.5 for the Centaur, meaning it would only take it 24 seconds for the retro burn.~Jon
Thanks! Do you know the dry mass and propellent mass a Dual Engine Centaur? Newtons? If I knew those things I believe I could do my own BOTEs.
A few things still unknown:Time it'd take to flip 180 for the braking burn.
How much delta V it would take to return to EML2. I believe it's doable to time the third apogee to be in the moon's neighborhood. But the route from an apogee in the moon's neighborhood to a halo around EML2 still hasn't coalesced in my imagination.
Well, if you reuse the EDS you can have it accelerate several payloads into Mars transfer orbit in one window. You can use it for sending stuff to the moon or to near earth asteroids as well.
Where they discuss resonance orbits is in relation to the OBO to minimize the energy and braking needed by the Skylon's to the departure window to meet up with the OBO. The braking needed for the EDS to meet up and refuel with the OBO would be minimal, nothing that could not be achieved with simple RCS systems. Then the OBO maneuvers around the returning EDS to allow it to overtake the OBO upon return, which optimally doesn't need any braking. Still it is a somewhat risky maneuver.
After TMI, the EDS disengages, turns 180º and then does a braking burn. Slowing down to just below escape velocity would put the EDS on an ~ 9 day orbit. The 3rd perigee would be 27 days later and thus back in the moon's neighborhood. Then it could return to EML2 to get ready to send another spacecraft on its way.
My inner manufacturing engineer is a big fan of getting more "inventory turns" on your expensive hardware than once every two years. It would be interesting to see if you could find a way to enable multiple Mars departures in a single launch window with a single reusable EDS...~Jon
MCT: 100 tons payload to Mars, has a 75 tons dry mass, and contains 775 tons of propellant when fully loaded. Has an Isp of 340 and a Delta-V of ~5.6 km/s with the full payload, 6.5 km/s with a 50 ton payload, 7.2 km/s with a 25 ton payload, and 8 km/s with no payload.[...]Mars toss transfer missions: gets fully refueled at LEO, places a 100 ton payload in a Mars transfer orbit with a 4 km/s burn, separates from the payload with ~200 tons of propellant(so ~4km/s delta-v) left. Slows down and lands at the launch site or docks with the depot within a day or two for a second mission.
Quote from: jongoff on 05/28/2014 10:02 pmMy inner manufacturing engineer is a big fan of getting more "inventory turns" on your expensive hardware than once every two years. It would be interesting to see if you could find a way to enable multiple Mars departures in a single launch window with a single reusable EDS...~JonI worked on some numbers on the "SpaceX FX/FXX/BFR Speculation Thread" for Mars transfers for the case where the MCT was an upper stage of an FXX-class rocket. I came up with some numbers for what I called a "Mars toss" mission:Quote from: Nilof on 05/13/2014 01:57 pmMCT: 100 tons payload to Mars, has a 75 tons dry mass, and contains 775 tons of propellant when fully loaded. Has an Isp of 340 and a Delta-V of ~5.6 km/s with the full payload, 6.5 km/s with a 50 ton payload, 7.2 km/s with a 25 ton payload, and 8 km/s with no payload.[...]Mars toss transfer missions: gets fully refueled at LEO, places a 100 ton payload in a Mars transfer orbit with a 4 km/s burn, separates from the payload with ~200 tons of propellant(so ~4km/s delta-v) left. Slows down and lands at the launch site or docks with the depot within a day or two for a second mission.The basic idea is that you can get back to the depot rather quickly if it's in LEO, using a high T/W transfer stage that can place a payload in a hyperbolic orbit and then quickly brake into an orbit that will put it back at the depot within a small integer multiple of the depot's orbital period.With that said, Mars transfers do chug through propellant, as in roughly 7-8 tons of propellant per ton of payload for Kerolox or Hypergolics. However, stage reuse doesn't change that much if your stage has a decent mass ratio. For multiple launches, the large depots needed would be the bottleneck either way.
One problem would be that the EDS's engines would be pointed exactly at the departing payload for the braking burn. So you would need to wait long enough for the stage to move away from the vicinity of the payload so it would be safe to start its engines. That might take a few minutes depending on the strength of your RCS thrusters.
Quote from: metaphor on 05/27/2014 03:04 pmOne problem would be that the EDS's engines would be pointed exactly at the departing payload for the braking burn. So you would need to wait long enough for the stage to move away from the vicinity of the payload so it would be safe to start its engines. That might take a few minutes depending on the strength of your RCS thrusters.I hadn't thought of that.Of course the EDS needs to be separated from it's payload. Is there a separation method that would push the payload forward as well as pushing the EDS backward?Such a push might give a little distance between payload and EDS in the time it takes for the EDS to turn 180º. The push might also help with the delta V, both for accelerating the payload and decelerating the EDS.
Total trip time from first to last perilune is 54 days. Perilune to EML2 is about 3 days -- so add 6 days for a total of 60 days. Is two months too long for oxygen/hydrogen?
Quote from: Hop_David on 05/30/2014 04:25 pmQuote from: metaphor on 05/27/2014 03:04 pmOne problem would be that the EDS's engines would be pointed exactly at the departing payload for the braking burn. So you would need to wait long enough for the stage to move away from the vicinity of the payload so it would be safe to start its engines. That might take a few minutes depending on the strength of your RCS thrusters.I hadn't thought of that.Of course the EDS needs to be separated from it's payload. Is there a separation method that would push the payload forward as well as pushing the EDS backward?Such a push might give a little distance between payload and EDS in the time it takes for the EDS to turn 180º. The push might also help with the delta V, both for accelerating the payload and decelerating the EDS.Will the return trajectory start at exactly 180 degrees reversal? After all, the Earth has moved a little and firing just 20 degrees off would both slow and move the vehicle laterally. Separation would happen very quickly once the engines fired after which orientation wouldn't be a concern.
[quote author=Hop_David link=topic=34822.msg1206963#msg1206963 date=14014671Some possibilities:1- Use retrorockets on the EDS to push back quickly.2- It might also be possible for the EDS to fire a little off of the velocity vector (to avoid plume impingement), and then cancel out that off-vector component as soon as there's sufficient space with the payload.3- Or if the EDS has 2+ engines, you might be able to splay them outward and just take some cosine losses.4- Or if Magnetoshell Aerocapture works and scales right, maybe you could just turn on the "deflector shield" once the payload has a little distance.There are probably other options, but those are the first three that come to mind.~Jon
The thing is that reusable chemical EDS do not make any sense so long as it would have to be refueled by a expendable rocket.
Quote from: aero on 05/30/2014 05:01 pmQuote from: Hop_David on 05/30/2014 04:25 pmQuote from: metaphor on 05/27/2014 03:04 pmOne problem would be that the EDS's engines would be pointed exactly at the departing payload for the braking burn. So you would need to wait long enough for the stage to move away from the vicinity of the payload so it would be safe to start its engines. That might take a few minutes depending on the strength of your RCS thrusters.I hadn't thought of that.Of course the EDS needs to be separated from it's payload. Is there a separation method that would push the payload forward as well as pushing the EDS backward?Such a push might give a little distance between payload and EDS in the time it takes for the EDS to turn 180º. The push might also help with the delta V, both for accelerating the payload and decelerating the EDS.Will the return trajectory start at exactly 180 degrees reversal? After all, the Earth has moved a little and firing just 20 degrees off would both slow and move the vehicle laterally. Separation would happen very quickly once the engines fired after which orientation wouldn't be a concern.Good point. Jon Goff mentioned Centaurs and I've found dry mass, propellent mass and newtons thrust of a Centaur. Knowing the newtons gives me a handle on how long accelerations would take for different masses. However elsewhere in this thread someone mentioned a long duration round trip might have hydrogen boil off problems and the shortest round trip I've found so far is 60 days.If hydrogen is out I'll have to plug in a different exhaust velocity to the rocket equation although my thrust might be improved. When I get a better handle on burn times, I'll have a better idea at what longitude the EDS starts its braking burn. I believe you're right, the needed rotation might be less than 180.
Good point. Jon Goff mentioned Centaurs and I've found dry mass, propellent mass and newtons thrust of a Centaur. Knowing the newtons gives me a handle on how long accelerations would take for different masses. However elsewhere in this thread someone mentioned a long duration round trip might have hydrogen boil off problems and the shortest round trip I've found so far is 60 days.If hydrogen is out I'll have to plug in a different exhaust velocity to the rocket equation although my thrust might be improved. When I get a better handle on burn times, I'll have a better idea at what longitude the EDS starts its braking burn. I believe you're right, the needed rotation might be less than 180.
The ULA depot adds a conical sunshield to the transfer stage, which brings these rates down an order of magnitude, perhaps 0.1%/day for LH2 away from LEO....
Quote from: muomega0 on 05/30/2014 07:41 pmThe ULA depot adds a conical sunshield to the transfer stage, which brings these rates down an order of magnitude, perhaps 0.1%/day for LH2 away from LEO....Its not just about boil off, long duration loiter for Centaur involves multiple other adjustments that need to be made, including things like batteries.
The thing is that reusable chemical EDS do not make any sense so long as it would have to be refueled by a expendable rocket. When you compare an architecture that utilizes a reusable chemical EDS that is refueled by an expendable rocket to an architecture that utilizes an expendable chemical EDS that is launched by an expendable rocket the expendable system comes out superior. Chemical reusable EDS stages would only make sense if the cost of fuel in LEO is rather low. There are a number of systems that could theoretically make that happen. One would be gun launch. Another would be some form of space based ISRU.Now for EDS that use high ISP propulsion systems like nuclear thermal, nuclear electric, solar thermal, and solar electric it would make no sense for them not to be reusable. The cost of refueling them even with an expendable launch system would be lower than the cost of replacing them.
Quote from: Nilof on 05/27/2014 10:30 pmWell, if you reuse the EDS you can have it accelerate several payloads into Mars transfer orbit in one window. You can use it for sending stuff to the moon or to near earth asteroids as well.My inner manufacturing engineer is a big fan of getting more "inventory turns" on your expensive hardware than once every two years. It would be interesting to see if you could find a way to enable multiple Mars departures in a single launch window with a single reusable EDS...~Jon
If you are not using hydrogen due to boiloff problems then the EDS can use the same engines as the lander. Same propellant and a common pool of replacement parts will simplify the logistics. Possibilities include Super Draco (NTO/MMH, Isp 235) and Morpheus HD5 (methane/LOX, Isp 321).
Quote from: A_M_Swallow on 05/30/2014 08:00 pmIf you are not using hydrogen due to boiloff problems then the EDS can use the same engines as the lander. Same propellant and a common pool of replacement parts will simplify the logistics. Possibilities include Super Draco (NTO/MMH, Isp 235) and Morpheus HD5 (methane/LOX, Isp 321).You might like to revise that Isp for NTO/MMH for Super Draco as Musk said the the chamber pressure is about 1000psi.
Am reposting the ~50 day route back to the moon but as attached image.
Quote from: Hop_David on 06/01/2014 09:01 pmAm reposting the ~50 day route back to the moon but as attached image.I love images like these; thanks for reposting them!Does your software have the ability to show the pellet trajectories in a rotating frame of reference where both the Earth and Moon appear fixed? I understand a rotating frame is strange, and I won't try here to justify its use, but note it is used in e.g. the Farquhar route shown in your previous post....
Quote from: savuporo on 05/30/2014 08:12 pmQuote from: muomega0 on 05/30/2014 07:41 pmThe ULA depot adds a conical sunshield to the transfer stage, which brings these rates down an order of magnitude, perhaps 0.1%/day for LH2 away from LEO....Its not just about boil off, long duration loiter for Centaur involves multiple other adjustments that need to be made, including things like batteries.That's part of what the whole Integrated Vehicle Fluids project is about. It replaces the batteries, the hydrazine thrusters (used for settling and ACS), and the helium pressurization (for repressurizing the tanks prior to a burn) with their IVF system. It taps boiled-off GOX/GH2 from the tanks to run a small internal combustion engine, which recharges the batteries, and runs a compressor for boosting the pressure of the GOX/GH2 prior to warming it for autogenous pressurization. With IVF you can run the stage as long as there is LOX and LH2 left in the tank. They had a lot of the prototype hardware for it at the Space Symposium. It's looking like they were going to fly part of it in 2015, and the rest of it in I think 2016. Once it's there, the duration of Centaur goes way up, the dry weight goes down quite a bit, and refuelability becomes easier since you're just dealing with two fluids.Combine that with the improved insulation (MLI or a sun-shield), and using the rotational settling, and there's no reason you couldn't handle months-long missions.~Jon
Then I read Jon Goff's comments that boil of could be mitigated -- he seemed to be talking about the same techniques that Goff, Zegler, Kutter and Bar have written about in their propellent depot papers.