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Old 03-02-2017, 12:57 PM   #11
RyanW
 
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Default Re: [Space, Spaceships] Mars Semi-Cycler?

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Originally Posted by DaltonS View Post
That's almost 8 weeks difference. Have I missed something?
The longer transfer allows a shorter return. A 130 day transfer would be a 4 year trip for free return abort (or for a cycler). A 180 day transfer would be only a 2 year trip for the return. See this Wikipedia article.
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Old 03-02-2017, 01:41 PM   #12
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Default Re: [Space, Spaceships] Mars Semi-Cycler?

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The longer transfer allows a shorter return. A 130 day transfer would be a 4 year trip for free return abort (or for a cycler). A 180 day transfer would be only a 2 year trip for the return. See this Wikipedia article.
Given that the period of my free-return orbit was 2 years, I calculated its semi-axis was 2^(2/3) or 1.5874 AU. Was that wrong? There are no equations in the Wikipedia article about how to calculate the time-of-flight. I used the ones here and the problem solution from here as my guide. Are they wrong? If so, what are the correct equations?

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Old 03-03-2017, 12:27 PM   #13
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Default Re: [Space, Spaceships] Mars Semi-Cycler?

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Given that the period of my free-return orbit was 2 years, I calculated its semi-axis was 2^(2/3) or 1.5874 AU. Was that wrong?
I'm getting similar numbers as you now that I'm running the math out. Either the formulas are wrong, Zubrin is wrong, or there are some different assumptions going on in the background.

In any event, you have shown me even more evidence that Elon Musk is pulling numbers out of his butt. He seems to believe that his ITS could be improved to 60 or even 30 day transfers. That would, I believe, require a solar escape trajectory. Shoot for Mars. If you miss, you'll die a slow, excruciating death by asphyxiation and your corpse will drift among the stars.

This on top of the fact that the geometry of the craft just doesn't work for the claimed 100 passengers and the delta-V. After subtracting the necessary volume for the methane-oxygen fuel, and you're not left with much space. Imagine being on an airliner for a flight that lasts several months.
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Old 03-03-2017, 03:33 PM   #14
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Default Re: [Space, Spaceships] Mars Semi-Cycler?

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I'm getting similar numbers as you now that I'm running the math out. Either the formulas are wrong, Zubrin is wrong, or there are some different assumptions going on in the background.

In any event, you have shown me even more evidence that Elon Musk is pulling numbers out of his butt. He seems to believe that his ITS could be improved to 60 or even 30 day transfers. That would, I believe, require a solar escape trajectory.
30 days requires solar escape velocity, 60 doesn't. It does, however, involve quite substantial delta-V (around 30 km/s).
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Old 03-03-2017, 04:19 PM   #15
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Default Re: [Space, Spaceships] Mars Semi-Cycler?

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If you miss, you'll die a slow, excruciating death by asphyxiation and your corpse will drift among the stars.
Good to note that this applies to any spaceflight beyond earth orbit these days. Apollo 13 didn't even miss, and they almost asphyxiated for other reasons. It's not hard to die in space. There's lots of info on that in Basic Set 2. :)
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Old 03-03-2017, 04:27 PM   #16
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Default Re: [Space, Spaceships] Mars Semi-Cycler?

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Good to note that this applies to any spaceflight beyond earth orbit these days. Apollo 13 didn't even miss, and they almost asphyxiated for other reasons. It's not hard to die in space. There's lots of info on that in Basic Set 2. :)
Apollo 13 did miss (i.e. did a free return). Zubrin's plan can do a no fuel abort with a total flight time of two years. Musk's 60 day scheme would be decades, 30 day scheme would be never.
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Old 03-04-2017, 12:21 PM   #17
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Default Re: [Space, Spaceships] Mars Semi-Cycler?

It's really hard to find the naked equations. (It's almost as if they are being hidden. FNORD!) So far this is the best source I could find. If someone finds a better one I'd really like to know. I'm beginning to be worried that the commonly quoted flight times were calculated incorrectly and accepted uncritically because of the author's credentials. Once a number gets lodged in the info-sphere, it gets hard to unseat (particularly if the author has a point to make). "After all, somebody must have checked his math."

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Old 03-04-2017, 12:37 PM   #18
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Default Re: [Space, Spaceships] Mars Semi-Cycler?

This seems to be a reasonably definitive book on the subject, and used copies are available cheap, but the maths is complicated. I gave up fairly quickly.
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Old 09-14-2020, 05:16 PM   #19
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Default Re: [Space, Spaceships] Mars Semi-Cycler?

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Here is an enlarged version of the Perdue-class Semi-Cycler.
Please note that:
  1. While this ship contains no superscience systems, it is listed as TL9^ because the Orbital Taxis it uses have a reconfigurable fission reactor/NTR ram-rocket to power its VASMIR drive and liftoff from Mars. (I'll post its design later.)
  2. The control room remains the same size (SM+8 system) and the crew habitats have been moved from the center and rear hulls to the front hull core.
  3. While the semi-cycler still launches in a 2-year free return trajectory for safety reasons, its doubled ∆V allows it to perform the Mars Orbit burn itself, so it can loiter in Mars orbit until the proper return window occurs.
Wow! I didn't mean to post the Orbital Taxi design this much later, but a promise is a promise.
Quote:
MARS TRANS-ORBITAL TAXI
PILOTING/TL9 (LOW-PERFORMANCE SPACECRAFT) or PILOTING/TL9 (AEROSPACE) Page:
Code:
TL            Name           dST/dHP  Hnd/SR  HT       Move       LWt.     Load    SM      Occ      dDR   Range   Cost   
9^  Mars Trans-Orbital Taxi     50     -4/5   12  0.0002G/10 mps   300  167.3 [2]  +7  3+20SV [3]  7/5/5    —    $11.8M
Length: 60 yd. (180 ft.) Crush Depth: 6.3 Atmospheres (208 ft.)
Power Points: +1 / -1
Space Performance #1: Hnd/SR: -4/5 sAccel: 0.0002Gs/10 mps
Space Performance #2: Hnd/SR: -4/5 sAccel: 0.002Gs/0.5 mps
Space Performance #3: Hnd/SR: -1/5 sAccel: 1.4Gs/2.025 mps
Air Performance: Hnd/SR: +3/6 aAccel: 1.4Gs (30.8 mph/s) aSpeed: 3,000 mph (1,500 yps / 0.83 mps)

SHIP SYSTEMS
Code:
   FRONT     
[1]          Advanced Metallic Laminate Armor
               dDR 7
[2]          Control Room
               Comp: C5 / Comm/Sensor: 5 / 3 Stations
[3]          Passenger Seating
               20 Seats
[4-6]        Cargo Hold
               45 Tons / SM+3 Bay Doors
Code:
   CENTER    
[1]          Metallic Laminate Armor
               dDR 5
[2-6, Core]  Cargo Hold
               90 Tons / SM+3 Bay Doors [6]
Code:
    REAR     
[1]          Metallic Laminate Armor
               dDR 5
[2!]         VASIMR Electric Rocket
               0.0002 Gs / 10 mps / 0.002 Gs / 0.5 mps / 95 days Fuel [5,5,8]
[3]          Reconfigurable System
  [a†]       Fission Reactor
               1 Power Point / 50 yr Fuel [7]
  [b]        Nuclear Thermal Rocket
               1.4 Gs / 2.025 mps / 2.83 min Fuel [4,5,9]
[4]          Fuel Tank
               15 Tons of Methane
[5-6]        Cargo Hold
               30 Tons / SM+2 Bay Doors [6]
[Core]       Fuel Tank
               15 Tons of Hydrogen
Design Switches, Features, & Notes: 3 Airlocks (Capacity: 3 people each), Fuel Cost: $97,500; Total Cost: $11,897,500
IR SIGNATURE: +12 (+6 Fission Reactor, +6 Nuclear Thermal Rocket)
CAMPAIGN OPTIONS: Exposed Radiators [1], EBS's Unofficial rules
SHIP OPTIONS: Streamlined, Winged
[1] Counts as Exposed System when deployed (see p. SS1:66), Targeted at SM+7 if central hull is facing attackers, SM+2 otherwise
[2] Load includes: 2.3 tons of Crew & Passengers, 165 tons in Cargo Hold
[3] Crew Requirement: 3 Control Stations (1 Pilot/Sensor Operator, 1 Co-Pilot/Navigator, 1 Payload Specialist)
PERFORMANCE PROFILES: [4] Air #1, [5] Space #1
[6] Collapsible Methane Tank
FUEL USED: [7] Fissionables, [8] Hydrogen (15 tons), [9] Methane (135 tons)
TROOP STRENGTH (TS): 0 CLASSES: Air, Space FEATURES: Night, Sealed

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.
This is my first Mars Trans-Orbital Taxi which had two hard tanks (LH2 and CH4) for VASMIR and NTR(RR) drives, and 8 collapsible methane tanks in the middle and rear cargo holds for Mars launch. My rationale was the Earth/Mars trade imbalance during the early years: 165 tons imports vs. 45 tons exports. (Also, reducing the ship's effective mass by 45% on takeoff extends the LH2 tank's ΔV by 81%. ;) ) After achieving maximum airspeed of the ram-rocket (0.83mps) the rocket mode of the NTR can reach Martian orbital velocity (2.48mps) with 0.35mps ΔV to spare. It will then change to reactor mode, powering the VASMIR drive to rendezvous with the semi-cycler. Even with 5.68mps of the VASMIR's ΔV used by that point, the remaining 4.32mps will extended by Taxi's lowered mass to 7.8192mps leaving 2.1392mps ΔV to spare.

While this model of Taxi would work well with the larger version of the Perdue that made the one-tangent burn itself and was orbiting Mars, it should have been originally conceived as a long range craft. I finally calculated how long the burn would take at 0.0002 Gs (a bit less than 54 days) and boy, was I embarrassed! My first mod involved making the Passenger Seating system modular and that just added $100k to the cost. I'm sure the bean counters would have insisted on making it permanent when the larger Perdue went into service to save money (well that's the excuse I'm using anyway). Next I replaced that system with a habitat module for the crew with 1 bunk room, 3 hibernation chambers and 1.25 tons of steerage cargo, 0.45 tons of which would be food. (This allows a supercargo to use the left over bunk without bringing their own rations.) This adds $500k to the original cost.

Dalton “just call me an equine necromancer … again! Spence

Last edited by DaltonS; 09-15-2020 at 09:15 AM. Reason: Quoted design and added comments about variants.
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Old 09-18-2020, 06:45 PM   #20
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Default Re: [Space, Spaceships] Mars Semi-Cycler?

To add to my embarrassment I decided to check the length of the burns required to enter and leave the 2-year free-return orbit. 3.16 mps * 0.0455/0.0002 Gs = 29 days, 22.9 hours. Ouch! This could make my scheduling idea a bit problematical. (See the beginning of this thread.)
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