[Space, Spaceships] Mars Semi-Cycler?

[AlexanderHowl]

Why not just use the Aldrin Cycler Orbits?

[DaltonS]

Originally Posted by AlexanderHowl

Why not just use the Aldrin Cycler Orbits?

Firstly, as I said in the beginning "the idea of traveling on an interplanetary "Flying Dutchman" is a bit daunting." Secondly, I'm a big fan of free-return orbits. Thirdly, I really wanted to make use of the difference between the period of a two-year free-return orbit (730.5 days) and the Earth-Mars synodic period (780 days). I can half the orbital burn time by adding another engine (and reactor to power it) but now I'm wondering if I should have included the boost to escape velocity from LEO in my ΔV calculations.

Dalton “who should have known it was too easy” Spence

[AlexanderHowl]

The problem with having a LEO component to the cycler orbit is you also have to deal with a rather large amount of atmospheric and gravitational drag (large for the cycler I mean). Even the best LEO path will likely cost you 100 m/s per cycle, which you need to deal with on the way out (including a towing spacecraft with a magsail would probably be sufficient though). The LMO part is less problematic, though you may lose 30 m/s per cycle.

[DaltonS]

Okay, I may not have been clear about my concerns. Do I have to break Earth orbit before I start boosting to the two year free return orbit? The Perdue's VASMIR engine only provides 0.0002 Gs thrust (0.0004 if I double it) so the Oberth effect doesn't apply. I'm not sure about the timing you see; the calculations I've seen seem to assume an instantaneous boost which is not what my drive provides. Slow boost trajectories are a topic I'm not sure I understand at all. My plan already involves some tight scheduling; I need to know if my ship will need outside assistance to make it work.

Dalton “who thought he was so clever” Spence

[AlexanderHowl]

A VASMIR is unworkable for a cycler that dips into LEO and LMO, the losses due to atmospheric and gravitational drag are just too great (especially since you would need to double the effective delta-v needed to overcome them with such a weak drive). Once again, a smaller towing spacecraft using magsails would be your best option.

[DaltonS]

Okay, so I park the ship at GEO, HEO or L5 when it's at home. What I'm more worried about is the total boost times required to achieve/end a 2-year free-return orbit from/to Earth orbit. A mag-sail tug sounds like a great idea but a mag-sail's accel isn't that great either and how well would it work inside Earth's magnetosphere? (It works on solar wind and the magnetosphere protects Earth from much of that.) I might swap my VASMIR for a mag-sail but similar issues apply. BTW, how long does a mag-sail need 1 power point to "activate"? (The drive is the Perdue's only high-energy system so if I could get by with a small power system and an Energy Bank...)

Dalton “who should have thought of mag-sails before” Spence

[AlexanderHowl]

Originally Posted by DaltonS

Okay, so I park the ship at GEO, HEO or L5 when it's at home. What I'm more worried about is the total boost times required to achieve/end a 2-year free-return orbit from/to Earth orbit. A mag-sail tug sounds like a great idea but a mag-sail's accel isn't that great either and how well would it work inside Earth's magnetosphere? (It works on solar wind and the magnetosphere protects Earth from much of that.) I might swap my VASMIR for a mag-sail but similar issues apply. BTW, how long does a mag-sail need 1 power point to "activate"? (The drive is the Perdue's only high-energy system so if I could get by with a small power system and an Energy Bank...)

Dalton “who should have thought of mag-sails before” Spence

Well, that problem is that the Mars semi-cycler works best if it actually goes to lunar orbit rather than any Earth, as GEO would require an additional 1.6 km/s of delta-v coming and going (for an additional 3.2 km/s). An additional bonus to using lunar orbit is that you could use a lunar cycler to make the Earth-Moon.

As for the Mars portion, I would suggest using Deimos orbit as the Martian point for the semi-cycler. While it would only saving 0.2 km/s each way (saving 0.4 km/s), Deimos could serve as a propellant depot either due to a) its own ice or b) ice shipped from the Main Belt.

So, by doing a semi-cycler from lunar orbit to Deimos orbit, you would end up saving 3.6 km/s of delta-v for the entire trip (from the semi-cycler's point of view). That would probably mean that you would need to generate 3.3 km/s of velocity to maintain the semi-cycle on each leg (6.6 km/s if you are using a low thrust system). In the case of a magsail, this would take around 8 days of thrust, though this could be divided between accelerating away from Earth and decelerating towards Mars.

As for the power point, I would keep it and add a tertiary battery for meteor defense. The magsail would cost $10M for the SM+8 ($30M for the SM+9), so it would be worth defending. By the way, you do not really want a habitat in the same section as a cargo hold on a SM+8/SM+9 spacecraft, as someone could vent the section by opening the cargo hold. A semi-cycler should also have an engine room.

A possible alternative version would be the magsail tow truck and just toss aside the idea of a semi-cycler. The SM+9 version would have three steel armor, four external clamps, five magsails, one fission reactor, one tertiary battery, one control room, one engine room, two hanger bays, and two habitats. It would also have spin gravity and total automation. It would cost $236.9M before any other options and could tow 12,000 tons from Earth to Mars and Mars to Earth. Since it could accelerate continuously at 0.001g, it would be able to make the Earth-Mars run in 10 to 12 weeks (depending on orbit positioning), which would mean two round trips per year.

The magsail tow truck could also be easily diverted to other runs, so you would see them being used throughout the Sol System. For example, it would take it only 10 months to reach Saturn or 23 months to reach the Kuiper Belt. While a two year turn around for Saturn would be annoyingly slow, it is a lot better than any other realistic system at TL9. Of course, this assumes constant thrust by expanding and contracting the magsails. If magsails function more like solar sails though, then your trip is a lot slower.

[DaltonS]

Originally Posted by DaltonS

Firstly, as I said in the beginning "the idea of traveling on an interplanetary "Flying Dutchman" is a bit daunting." Secondly, I'm a big fan of free-return orbits. Thirdly, I really wanted to make use of the difference between the period of a two-year free-return orbit (730.5 days) and the Earth-Mars synodic period (780 days).

Yes, with a mag-sail one can pretty much ignore ΔV limitations. I really didn't want to do that because it added drama to the setting, but the boost times issue forced my hand. I wanted the return of the semi-cycler to be a big bi-annual event that was on a fixed schedule, with people rushing around transferring cargo and personnel, refurbishing, restocking and repairing, all trying to be ready before the next launch window. A mag-sail makes the whole scheduling issue rather … mushy. I'd retract the thing when not in use if possible so it would be behind armor. (Really?!? That's why you want to arm my peaceful ship?)
Also the point of a free-return orbit is that if the brakes fail there is still a chance of rescue. Leaving that orbit to rendezvous with Mars or its moons sort of defeats that purpose. I suppose I could redesign my Mars Trans-Orbital Taxi to use mag-sails instead of VASMIR too but that would mean the main reason for my Martian “Gas” Station to produce hydrogen would be as feedstock for the methane refinery. <sigh> Okay, here is my latest design.

PERDUE REDUX (PERDUE-CLASS MEDIUM INTERPLANETARY TRANSPORT)

TL      Name      dHP  H/SR  HT         Move          LWt.      Load     SM      Occ      dDR  Range   Cost   
9^  Perdue Redux  100  -5/4  13  0.001G/375 mps [1]  3,000  1,554.4 [3]  +9  36ASV [4,5]   15    —    $60.5M

Length: 70 yd. (210 ft.) Crush Depth: 32.5 Atmospheres (1,071 ft.)
Power Points: +1 / -0
Space Performance: Hnd/SR: -5/4 sAccel: 0.001Gs/375 mps

SHIP SYSTEMS

 FRONT   
[1]      Metallic Laminate Armor
           dDR 15
[2-4]    Hangar Bay
           Cap.: 300 Tons / SM+6 / Launch: 100 Tons/min
[5]      Magsail Sail
           0.001 Gs / 375 mps [1,2,7]
[6]      External Clamp
           3,000 Tons
[Core]   Smaller SM Systems
           SM+8
  [a]    Control Room
           Comp: C6 / Comm/Sensor: 6 / 4 Stations
  [b-c]  Habitat
           three bunkrooms, one cabin, one rec room
           one fabricator minifac, eight hibernation chambers, one lab, one sickbay

 CENTER  
[1]      Metallic Laminate Armor
           dDR 15
[2-3]    Vehicle Dock
           300 Ton Capacity / SM+7 [6,8]
[4]      Engine Room
           1 Control Station / 2 Workspaces
[5-6]    Vehicle Dock
           300 Ton Capacity / SM+7 [6,8]
[Core]   Habitat
           one bunkroom, nine passenger cabins, 50 tons of steerage cargo

  REAR   
[1]      Metallic Laminate Armor
           dDR 15
[2†]     Fission Reactor
           1 Power Point / 50 yr Fuel [9]
[3-6]    Cargo Hold
           600 Tons / SM+5 Bay Doors

User Notes: The modular Vehicle Docks can hold two SM+7 (300-ton) auxiliary craft, usually atmospheric craft used as orbital taxis. (Modularity allows the customized docks to be swapped in and out as required.) If an auxiliary craft has habitat modules installed, the crew usually stays aboard their ship to stretch life support; otherwise they travel as passengers or in hibernation chambers.
Design Switches, Features, & Notes: 5 Airlocks (Capacity: 5 people each), Auxiliary Craft Cost: $37,308,120; Standard Food Cost: $50,000; Total Cost: $97,858,120
IR SIGNATURE: +13 (+6 Fission Reactor, +7 Magsail)
CAMPAIGN OPTIONS: Exposed Radiators [2], EBS's Unofficial rules
[1] Accel assumes Luminosity 1.0 Star at 1.0 AU
[2] Counts as Exposed System when deployed (see p. SS1:66), Targeted at SM+12, Radiators are targeted at SM+9 if central hull is facing attackers, SM+4 otherwise
[3] Load includes: 4.4 tons of Crew & Passengers, 50 tons of Standard Food, (25,000 man-days
694 days), 600 tons in Cargo Hold, 600 tons in Vehicle Dock capacity, 300 tons in Hangar Bay
[4] Plus 8 in Hibernation Chambers, [5] Crew Requirement (18 total): 4 Control Stations (1 Captain/Pilot, 1 Operations Officer, 1 Chief Engineer,
1 Navigator/Comm/Sensor Operator), 1 Technician, 7 Auxiliary Craft Crew, 2 Passenger Care, 1 Entertainment, 1 Medical, 2 Scientists
SYSTEM NOTES: [6] Modular, [7] Uses 1 Power Point to activate, then consumes no power, [8] Mars Trans-Orbital Taxi (L)
FUEL USED: [9] Fissionables
TROOP STRENGTH (TS): 0 (+583 TS for Auxiliary Craft / 583 TS Total) CLASSES: Space FEATURES: Night, Sealed

AUXILIARY CRAFT

Qty.                 Ship                 Location     Cost    Mass  Crew  
   1  Aurora-Class OTV                    Front     $811.6k    30    1     
   1  Condor Spaceplane                   Front     $8.89M     100   2     
   1  Kobold Work Bug                     Front     $1.12662M  30    2     
   1  Midnight Sun-Class Orbital Shuttle  Front     $1.468M    100   2     
   1  Sling Liner Passenger Barge         Front     $216k      30    1     
   2  Mars Trans-Orbital Taxi (L)         Center    $24.7959M  600   8

HABITATS

Qty.   Location           Type          SM         Notes         
   3  Front [Cb]  Bunkrooms             +0            12 person
   1  Front [Cb]  Cabin                 +0             2 person
   1  Front [Cb]  Rec Room              +0  20 patrons, 2 staff
   1  Front [Cc]  Fabricator Minifac    +0         $500 per Hr.
   8  Front [Cc]  Hibernation Chambers  +0           8 chambers
   1  Front [Cc]  Lab                   +0   2 person, +1 Skill
   1  Front [Cc]  Sickbay               +0      1 bed; +2 Skill
   1  Center [C]  Bunkroom              +0             4 person
   9  Center [C]  Passenger Cabins      +0            18 person
  10  Center [C]  Steerage Cargo        +0              50 tons

Created using the GURPS Spaceships Design Spreadsheet Version 2.0 RC 29 copyright © 2009-2017 Eric B. Smith. This spreadsheet is based on information contained in the GURPS Spaceships series of books.

Yes, I could do more but I'm mentally tired contemplating this major concept change.

Dalton “Has anyone got an aspirin?” Spence

[AlexanderHowl]

Now, one way to keep the idea of a Martian semi-cycler is to go with a larger spaceship. A SM+12 spaceship would require SM+9 dedicated towing spacecraft to move into position, and they would likely not travel with the semi-cycler. They would instead tow cargo to the semi-cycler station, give the semi-cycler station a delta-v boost, and then return to either Luna orbit or Deimos orbit for local operations. By the by, magsails do not necessarily operate by the inverse square law, as you would open the coil away from the star and close them close to the star, creating constant thrust (this is why they can work in interstellar space as a breaking system).

In that case, the design becomes quite simple for the semi-cycler. You have an SM+12 spacecraft with six external clamps, six steel armor, three habitats, three open spaces, one control room, one hanger bay, and spin gravity. The cost would be $706M. It would be able to carrying 600,000 tons of cargo (while not as fast as a magsails design, it would be quite cheap to operate, as it would only need a tow for a week at each end rather than paying for the magsails all of the time). With magsails available to connect to them, you could actually have a proper cycling station, which would need almost no added delta-v.

[DaltonS]

I'd prefer to have a ship with a crew of gameable size. Too large and you have to have a list of generic NPCs to use in random encounters. Even if most of the Perdue's crew would be NPCs, they would likely have names, ranks, jobs and histories that the PCs (or at least the GM) would be familiar with. I also am not fond of ships big enough to get lost in; one of my favorite parts of the movie "Serenity" was the walking tour that took us to every part of the ship. (Perdue isn't quite that small, but neither is it so large it contains terra incognita.) There can even fiscal reasons for this; a really large ship is a major investment and the investors may not want to take that much risk. A big ship might be more impressive, but is a good place for a roleplaying adventure?

Dalton “Remember why we're here?” Spence