Does leaving Mars from a circling orbit effect departure any differently from the Martian surface or an elliptical orbit?
Quote from: Rei on 03/26/2017 07:08 pmThey were launched retracted, right? Nowadays, large solar power systems are generally deployed on retractable booms or fans. If you needed to, there's no reason you couldn't engineer the deployment to be reversible, so you can retract for aerobraking.We're taking about acres of something like cellophane here. Retracting and deploying them each trip would be a nightmare.
They were launched retracted, right? Nowadays, large solar power systems are generally deployed on retractable booms or fans. If you needed to, there's no reason you couldn't engineer the deployment to be reversible, so you can retract for aerobraking.
Quote from: Rei on 03/26/2017 07:08 pmOn the other hand, if your objective is to have a large heavy transfer stage, and a small but high dV ascent/descent stage , it raises the possibility question, why not go all the way and put the heavy craft on a cycler trajectory?I don't think an Aldrin cycler is plausible. All Mars cycler schemes I've seen rely on the notion that earth Mars synodic period is 2 1/7 years giving a cycler geometry that repeats every 15 years. The fact that it isn't quite 2 1/7 complicates things. Moreover Mars eccentricity and inclination also requires course corrections. And the big kicker is you need to rotate the Aldrin cycler line of apsides 50º every orbit. It would take a lot of propellent to maintain the cycler orbit, especially if the cycler is massive.Also the Aldrin cycler has an aphelion clear out in the Main Belt. During fly by the cycler's velocity vector is almost 70º from Mars' velocity vector. This makes for a big delta V budget for the taxis traveling between Mars and cycler. Around 9 km/s, if memory serves.The VISIT 1 and 2 cyclers are lower maintenance and the Taxis don't need whopping big delta V budgets. But these cyclers don't pass by each planet each synodic period. If I remember right, sometimes passengers would need to sit tight for as long as 7.5 years.
On the other hand, if your objective is to have a large heavy transfer stage, and a small but high dV ascent/descent stage , it raises the possibility question, why not go all the way and put the heavy craft on a cycler trajectory?
Quote from: redliox on 03/26/2017 07:12 pmDoes leaving Mars from a circling orbit effect departure any differently from the Martian surface or an elliptical orbit?It can make a huge difference. If departing from a capture orbit (periapsis just above atmosphere and apoapsis just within Sphere Of Influence), you're traveling a hair under escape velocity at periapsis. And escape velocity is 1.414 faster than circular orbit velocity.I talk about this at Inflated Delta VsThere's a problem, though. For departing from a capture orbit, the periapsis of the capture orbit needs to be at the right place and time during a launch window. There's no specific capture orbit that will serve general needs.However from a high circular orbit it's possible to time a braking burn so your elliptical orbit's periapsis is at the right place at the right time. I talk about this at What about Mr. Oberth?
Two performance values to consider:1) delta-v2) mars ascent vehicle availability for a return launch
Clearly launching to a circular orbit about 300km is easiest. But the rendezvous window could be a kicker (availability to launch may need to be very high w/no launch scrubs allowed). It seems we cannot get near 100% as it is on Earth with every resource needed provided within reach of the launch pad.Launching to a Phobos/Deimos station seems more practical for the ascent vehicle design, but it requires more weight to be landed. It seems that any surface mission on mars requires landing half of Hoboken, and the design concept doesn't close. There just isn't a practical way to land
If recent news accounts are accurate, Trump wants a plan in a few months around the fall. When the report shows no practical solution...
Retraction of deployed solar arrays, etc is a long studied problem. Eg:https://arc.aiaa.org/doi/abs/10.2514/3.26722?journalCode=jsrIt's seldom done because there's usually little need, but it certainly can be done. And given the dV advantage for aerobraking, it's hard to say it's not worth it.
At the 500 km periapsis (from a circular orbit naturally) it states the orbital velocity is 3.3158 km/s and that escape is 4.6893 km/s. With the elliptical-Phobos orbit I see ellipse periapsis velocity at 3.9 km/s with a burn of 1.45 km/s to hit hyperbolic velocity. With the elliptical-Deimos orbit the ellipse periapsis velocity is ~4.3 km/s requiring a burn of just above 1 km/s.So, under the conditions to setup for Oberth Effect, an escape from Mars would be reasonably possible from either moon if you had a reserve of 1.5 km/s to spare...assuming I'm interpreting this right. Beyond that I'd wish a double-check on the burns to adjust from Deimos and Phobos to reach the 500 km periapsis.
Quote from: Rei on 03/27/2017 12:41 amRetraction of deployed solar arrays, etc is a long studied problem. Eg:https://arc.aiaa.org/doi/abs/10.2514/3.26722?journalCode=jsrIt's seldom done because there's usually little need, but it certainly can be done. And given the dV advantage for aerobraking, it's hard to say it's not worth it.Economical, reusable SSTO spaceships are also worthwhile. As is warp drive. The question is, is it doable?Again, we're not talking about solar array wings like Hubble uses or even the I.S.S.. To get the needed power and alpha for an interplanetary ion propelled vehicle we need a very large and very light weight power source.And you're going to fold up and then redeploy this very large and fragile power source every trip? How many moving parts? What is the life expectancy or these acres of Seran Wrap solar arrays?
Quote from: redliox on 03/27/2017 07:40 amAt the 500 km periapsis (from a circular orbit naturally) it states the orbital velocity is 3.3158 km/s and that escape is 4.6893 km/s. With the elliptical-Phobos orbit I see ellipse periapsis velocity at 3.9 km/s with a burn of 1.45 km/s to hit hyperbolic velocity. With the elliptical-Deimos orbit the ellipse periapsis velocity is ~4.3 km/s requiring a burn of just above 1 km/s.So, under the conditions to setup for Oberth Effect, an escape from Mars would be reasonably possible from either moon if you had a reserve of 1.5 km/s to spare...assuming I'm interpreting this right. Beyond that I'd wish a double-check on the burns to adjust from Deimos and Phobos to reach the 500 km periapsis.I'm happy you're paying close attention to the Insertion burn from periapsis cell. However be careful that you don't regard this as the total delta V budget.Besides the periapsis burn you must also do a braking to drop from the moon to the 500 km periapsis. To get this braking burn look at the Apoapsis circulize burn. (Just noticed I misspelled "circularize" - d'oh!). Since orbits are time reversible, the circularize and decircularize braking burns are the same.
So. To get to earth from Phobos and using the Oberth benefit, you first do a .4997 braking burn. Then at periapsis you do a 1.4444 km/s burn to give you Trans Earth Insertion (TEI). Total dV is 1.9441 km/s. It's worth noting TEI directly from Phobos is 1.8818 km/s. So in this this case a braking burn to enjoy a periapsis Oberth kick isn't worth it.From Deimos the braking burn is .6301 to drop to a 500 km periapsis. Add that to the Insertion burn from periapsis and you get 1.6734 km/s. TEI directly from Deimos is 1.9150 km/s. So in this case a braking burn to enjoy a periapsis Oberth kick is worth it.
When I think about Mars, the one fixed point I keep returning to is the need for a light weight ascent vehicle. What I don't have an absolute handle on is whether the ascent vehicle should have the capability to go beyond low Mars orbit. If it does then the options are either a near-escape orbit and rendezvous with an Earth return vehicle, or something similar but focused around one of the moons. I probably lean towards having something more substantial (with good life support) being in a circular low Mars orbit. But I could be persuaded for that vehicle to sit at an L-point, probably near Phobos. The question is what would be the advantage in this to justify the extra fuel? It may require more time in flight but I'm not sure.
Quote from: Hop_David on 03/27/2017 01:56 pmEconomical, reusable SSTO spaceships are also worthwhile. As is warp drive. The question is, is it doable?Again, we're not talking about solar array wings like Hubble uses or even the I.S.S.. To get the needed power and alpha for an interplanetary ion propelled vehicle we need a very large and very light weight power source.And you're going to fold up and then redeploy this very large and fragile power source every trip? How many moving parts? What is the life expectancy or these acres of Seran Wrap solar arrays?Well, for one you could read the paper. Or if you'd rather a video, here's today's state of the art, ATK Megaflex (150W/kg):Note how several boom segments repeatedly go backwards, some almost to the point of being fully collapsed. When they're done with a test deployment, they just run the booms in reverse to repack it. And this is a system for which reversibility was not a design goal.Your notion that solar cells would be so fragile that they can't be repacked is unrealistic because then they'd be too fragile to deal with incidental impulses. And let's just say that you have to add some mass to allow for repacking - so what? Who cares if reversibility adds some mass when you're saving kilometers per second dV by doing so?
Economical, reusable SSTO spaceships are also worthwhile. As is warp drive. The question is, is it doable?Again, we're not talking about solar array wings like Hubble uses or even the I.S.S.. To get the needed power and alpha for an interplanetary ion propelled vehicle we need a very large and very light weight power source.And you're going to fold up and then redeploy this very large and fragile power source every trip? How many moving parts? What is the life expectancy or these acres of Seran Wrap solar arrays?
Weir gave Hermes a 2 mm/s^2 acceleration. That assumes an implausibly good alpha (in my opinion) but we'll go with it. An extra 2 km/s at 2 mm/s^2 acceleration would take a million seconds or about 12 days. I believe 1 mm/s^2 is perhaps plausible in which case it'd take an extra 24 days to spiral down to LMO.An interplanetary ship harbored in a planet's orbit must depart as well as arrive. Harboring at Deimos vs LMO would save 4 km/s of propellent and 24 to 48 days of time depending on what acceleration an ion craft could achieve.For a similar reason I advocate harboring something like the Hermes at EML2 rather than LEO. Parking at the edge of earth's gravity rather than LEO would save Hermes 14 km/s climbing in and out of earth's gravity well. And skipping the climb to near bottom and back would save 80 days if acceleration is 2 mm/s^2. 160 days if we use a more plausible 1 mm/s^2.
Quote from: Russel on 03/27/2017 12:41 pmWhen I think about Mars, the one fixed point I keep returning to is the need for a light weight ascent vehicle. What I don't have an absolute handle on is whether the ascent vehicle should have the capability to go beyond low Mars orbit. If it does then the options are either a near-escape orbit and rendezvous with an Earth return vehicle, or something similar but focused around one of the moons. I probably lean towards having something more substantial (with good life support) being in a circular low Mars orbit. But I could be persuaded for that vehicle to sit at an L-point, probably near Phobos. The question is what would be the advantage in this to justify the extra fuel? It may require more time in flight but I'm not sure. I think Hop_David's charts can help answer where best to apply your light MAV ideas. I believe your idea is possible, but the limit of it is keeping the astronauts comfortable during. In the case of this thread's subject, it would specifically be during the ascent from Mars and venturing out to Phobos or Deimos during the transfer orbit.Now the trick is time itself. A transfer orbit that links the surface to Phobos is roughly 5 hours, with Deimos just over 13 hours. The MAV would be in that orbit presumably just for the ascent, then perform a maneuver to circulize the orbit (or at the least raise it so there isn't a horrific return to the surface); but the main fact is your crew spends half of these respective orbits in the mini-MAV. For Phobos 2.5 hours is reasonable, for Deimos 6.5 which isn't as reasonable.So your MAV design would need to incorporate the following factors:1) Launch with enough delta-v to march the target moon - 4.3 km/s for Phobos and 4.7 km/s for Deimos2) Perform a second burn to raise periapsis - 0.6 km/s for Phobos and 0.7 km/s for Deimos3) Keep the crew alive and (somewhat) comfortable for at least 3 hours for Phobos and 7 hours for DeimosYour lighter MAV would be able to accommodate extra fuel for maneuvers like a periapsis raise readily, but the trick is how quickly can the MAV dock with the awaiting Hermes-eque orbiter. So I think your idea works best for lower orbits and even Phobos but not so well for Deimos.
The question I have is what is the equivalent length of time spent transferring from the surface to a circular low Mars orbit? And what are the issues with launch windows when comparing going to a specific low Mars orbit and going to Phobos?
Three basic philosophical points here. 1. I've grown less worried about having lots of fuel, at least in Mars orbit. Musk is right about one thing. Its about economy of scale and while its nice to be mindful of total mass, I don't think that mass of fuel is as big an issue as has been thought.2. While I'd like to put boots on the moons, its not essential. It actually makes more sense to teleoperate a robot on Phobos from the surface, than the other way around.3. If we're talking about putting more hardware on or near a moon (particularly Phobos) the main bonus for me is protection from radiation. However, is this still true at the L point? There is an argument for putting the small habitat (mentioned above) actually on the Phobos surface (but with rendezvous at the L point).
When you're talking saving mass with ion propulsion it can be a tricky question. What you're doing, to a degree, is trading the immense mass of fuel chemical rocketry burns and exchanging it for a power system; if your power supply is big and heavy it is going to ruin the fuel/mass-saving merits of electric propulsion you wish to achieve.
Scenario 3 Breakdown: Deimos with Oberth Effect maneuvering1: Launch from Mars to Deimos Transfer Orbit: 4.7 km/s2: Periapsis Raise: 0.7 km/s3: Deimos Ops: 0.16 km/s4: Periapsis Brake: 0.64 km/s5: Trans Earth Injection: 1.68 km/sTotal Delta-V: ~7.9 km/s
Quote from: redliox on 03/27/2017 05:19 pmScenario 3 Breakdown: Deimos with Oberth Effect maneuvering1: Launch from Mars to Deimos Transfer Orbit: 4.7 km/s2: Periapsis Raise: 0.7 km/s3: Deimos Ops: 0.16 km/s4: Periapsis Brake: 0.64 km/s5: Trans Earth Injection: 1.68 km/sTotal Delta-V: ~7.9 km/sYou're getting better! One criticism: since you already included the brake to drop to periapsis in step 4, I think you can call TEI 1.04 km/s.
I've eavesdropped conversations between astrogators and I get the impression they like to pad delta V budgets by 10% for safety reasons. Something to think about.
I'm tossing out another scenario. Departing directly from Mars. If you're going to include steps 1 and 2 (getting off Mars and rendezvous with Deimos) in your total delta V budget, why not leave directly from Mars?
Also there is the possibility of ISRU propellent at Deimos. If Deimos is a propellent source, you can start over with a new delta V budget.
Anyway, here's my way of getting delta V for TEI from Mars' surface. Set periapsis at 0 altitude (Mars surface). Since Mars surface is moving 0 km/s with regard to Mars, just use the hyperbolic periapsis velocity. 5.7 km/s. Well, actually Mars equator is moving about .25 km/s wrt to Mars center. Launching from Mars equator can confer a little help, sort of like French Guiana on earth.
JPL, NASA, etc is heavily focused on Mars itself so it is very unlikely we'll see a decent robotic visit to them (although JAXA is readying something up that alley it appears! ).
Jeff Foust @jeff_foustZelenyi: studying a mission called Boomerang to return samples from Phobos in 2020s. Also interested in Mars sample return.
QuoteJeff Foust @jeff_foustZelenyi: studying a mission called Boomerang to return samples from Phobos in 2020s. Also interested in Mars sample return.https://mobile.twitter.com/jeff_foust/status/846819996177182720
Quote from: redliox on 03/28/2017 06:18 pmJPL, NASA, etc is heavily focused on Mars itself so it is very unlikely we'll see a decent robotic visit to them (although JAXA is readying something up that alley it appears! ).That's one mission I'm really crossing my fingers for Mineralogical mapping with a focus on hydrated minerals (globally for Phobos, partially for Deimos), followed by two landing site selections and two sample return. If there's anything of interest exposed on the surface of interest, we'll have two samples (each from >2cm deep) of it in the lab to make simulants of. How cool is that? And even if there's nothing neat right near the surface, they'll be doing gravity mapping to look for possible subsurface ice deposits that a followup mission could check out.JAXA's budget isn't huge, but they do some neat stuff with it.