[Spaceships] Designers' Championship I: ground to LEO

[thtraveller]

Originally Posted by SuperGamera

If you don't mind the extra G's in atmosphere, what about an additional high-thrust ram-rocket, only used in atmosphere? That should save you ~0.22 mps worth of fuel per trip.

I am not sure there is one? There isn't an HT option listed under Fission rockets. There is one under AM NTR rockets, but I don't think I can use that.

I am looking for a way to use water as reaction mass, because then I can go down to 1 passenger per trip and it is still more economical. As long as I can fit the drives and fuel in 17 spaces it is doable.

Hmm, 15 fuel tanks (water) plus 2 water NTR ram-rockets is tantalisingly close to doable. ... I have an idea about burning some fuel to get air speed rather than orbit, but I need to do the math.

How long does it take to get air speed from 2500 mph to 4300 mph at 3G? I think it is 30 seconds, which is almost 4 tanks of fuel.

[thtraveller]

OK I can't make it work with just water, yet... But I can make it work with one water NTR and one hydrogen NTR, a quarter of the fuel is hydrogen.

Edit: Not sure if that helps the ticket price much.

[thtraveller]

Using two different drives: half water, half hydrogen I reduced the return ticket price to low-earth-orbit to $2,400.

If we can work out how to do an all water drive the ticket price should drop to about half this.

TL9+ Orbital Lighter (Spaceplane) v5 "Alida"

Alida class space planes are cheap reusable single stage orbital lighters that can take off and land at a normal airport anywhere in the world. The relatively low 2G drive allows for a reasonably comfortable passenger experience. It can easily make the return journey to and from orbit once for every two orbits of the space station or every 180 minutes.

Using Nuclear Thermal Rockets in fuel-less air-ram mode to take off it clears local population centres before going supersonic and accelerating to maximum air speed in an easterly direction. It then boosts into orbit in less than seven minutes. Having docked and exchanged passengers it re-enters atmosphere and then makes a rapid descent back to Earth. Again using its air-ram engines it lands at an airport.

A novelty of the design is it uses two different fuels to reach orbit in order to reduce both the craft price and running costs. Half of its fuel delta-V comes from Hydrogen and half from Water.

Front
1 Armor (Advanced Laminate)
2-3 Passenger Seating (6 seats)
5-6 Fuel Tank (Water)
0 Control Room

Central
1-0 Fuel Tank (Water)

Rear
1 Fuel Tank (0.75 Water, 0.25 Hydrogen)
2-4 Fuel Tank (Hydrogen)
5 Nuclear Thermal Ram-Rocket (Hydrogen 0.5G)
6 Nuclear Thermal Ram-Rocket (Water 1.5G)

dST/dHP 20, Hnd 4 atmo/0 space, SR 4/5, HT 12, Move Flight 1G/2500 mph/0.69 mps, Move Boost 2G/4.68 mps, LWt 30, Load 0.7, SM +5, Occ 1+6 SV, dDR 3, Cost M$ 1.93, TL9, Streamlined, Winged, Length 15 yards

DeltaV 5.37 mps (0.17 mps reserve)
-Ram Rocket 2500mph = 0.69 mps
-9.75 Water Fuel Tanks 9.75*.15*1.6 (13 tank adjustment) = 2.34 mps
-3.25 Hydrogen Fuel Tanks = 3.25*0.45*1.6 (13 tank adjustment) = 2.34 mps

Note losses due to gravity drag of a 2G drive means that the boost phase is only 87% efficient in fuel usage, and so requires a total of 5.2 mps delta-V to reach orbit ([4.6-0.69]/0.87 + 0.69).

Economics:

1 flight every 3 hours operating for 7200 hours pa= 2400 flights per year
6 passenger seats at 75% loading = 4.5 passengers per trip
= 10,800 passengers per year

Variable cost per trip:
Insurance = M$1.93 * 0.001% = $19
Servicing = M$1.78 * 0.01% = $178
Fuel: 4.875 tons Hydrogen = $9,750
Fuel: 10.75 tons Water = $293

Total Variable Cost = $ 10,240

Annual Costs to Amortise:
Requires 4 pilots at $7200 pm TL9 Comfortable salary each = K$228 pa
Other economics = Insurance + Depreciation + RoI = 10% = K$ 193

Total Fixed Cost = K$ 538.6

Divided over 2400 trips = $224 per trip

Return Ticket price = $10,465 / 4.5 passengers = $2,325 (less than 3 weeks income for an average TL9 job)

There is no significant difference between TLs. The pilots wages increase per TL, which adds only about $30 per TL to the ticket price.

On a backwoods world with only 10% of the demand you only employ one pilot part time so annual costs are K$34.56 + K$193 = K$376 / 240 trips = $950 per trip. So ticket price per passenger increases by $210 to $2,535 on backwoods worlds

The ticket price should stay approximately the same as ship size increases.

[Agemegos]

Originally Posted by thtraveller

If we can work out how to do an all water drive the ticket price should drop to about half this.

This is a bit of a cheat, but note that using water increased thrust and therefore increases airspeed, and that this works even when you are in air-ram mode, and not using your fuel--so the lost of delta-v doesn't matter much.

Actually, that isn't terribly unrealistic: air has an average molecular weight of about 29, and therefore really ought to give you 3.8 times the thrust of hydogen. It isn't RAW or groundrules, but realistically a winged design with four NTR ram-rockets can get up to about 2 miles per second without using any fuel at all. (Thrust is nearly 8 gee, but balanced by air resistance: it won't squash the passengers). Six tanks of hydrogen will get you up to 4.7 miles per second, plus 0.25 from rotation gets you into orbit. The craft is a bit pricey, but allows 14 spaces of passenger seating.

[elustran]

I like the progressive refinement that's been done to the Alida craft. Maybe we'll meet in orbit 20 years hence after riding a very similar spacecraft. However, I've had a few thoughts regarding the design. If you want to save the eye-strain, the first three are annoying commentary. The last one is potentially useful.

1)
From what I've read, it seems that the biggest issue doesn't come from radioactive exhaust (unless you directly pass air into the reactor instead of using exchangers) but directly from the reactor itself. Reactors would tend to require a lot of shielding to protect pilots, which is why some historical nuclear aircraft (ram or otherwise) were unmanned concepts. All that being said, it's possible that a TL9 reactor could be build efficiently enough that radioactivity and required shielding is minimized.

2)
Now, another issue that needs to be contended with in a nuclear aircraft is the potential for failure. Would the aircraft have failure modes to ensure that the nuclear reactors wouldn't pose a threat of contamination? It's possible to launch the aircraft over oceans, but there is always the possibility of the aircraft failing over land and the possibility that failure over an ocean could still lead to radioactivity being carried to a population center through ocean currents or causing problems with marine life. Again, here's a situation we could handwave away by saying that a TL9 reactor would be using safer power generation methods, maybe some variant of a pebble-bed, which would also help with refueling, but failure is still a potential problem. If a nuclear lighter such as this design is ever proposed, expect anti-nuke protests.

3)
Nuclear issues aside, I've had a couple of other thoughts specifically about the design. One issue is that the water rocket will burn out faster than the hydrogen rocket - they both provide the same amount of dV, but the water rocket produces 3x the thrust and will therefore burn out 3x as fast. You could assume that the pilot varies the level of water being fired in order to maximize ascent characteristics, or you could even assume that both rockets are capable of firing hydrogen or water and able to vary the mix since they are already capable of variable fluid-flow due to being ram-rockets. Of course, I'm not sure what the overall effect would be on your dV requirements, making this paragraph more of a nuisance than anything else...

4)
Now the only thought of mine that you can actually easily apply to the craft is to assume that the craft doesn't have a physical pilot. Alida could potentially be flown by computer, especially since the flight path would be set before-hand. This would allow for one more passenger seat where the controls used to be and eliminate the expense of a pilot, though it would add the expense of better computing and AI flight software.

[thtraveller]

Originally Posted by Agemegos

This is a bit of a cheat, but note that using water increased thrust and therefore increases airspeed, and that this works even when you are in air-ram mode, and not using your fuel--so the lost of delta-v doesn't matter much.

I thought about it and decided it was being cheesy. However if being cheesy actually makes it more realistic then perhaps I will rethink.

If I could get the average per engine air-thrust to 1G for each of two engines then an all water design is doable.

[thtraveller]

Originally Posted by elustran

I like the progressive refinement that's been done to the Alida craft. Maybe we'll meet in orbit 20 years hence after riding a very similar spacecraft.

See you in LEO, 12th January 2028 ;-) Unfortunately I don't think the Alida is early TL9. Maybe 40 years?

1) From what I've read, it seems that the biggest issue doesn't come from radioactive exhaust (unless you directly pass air into the reactor instead of using exchangers) but directly from the reactor itself. Reactors would tend to require a lot of shielding to protect pilots, which is why some historical nuclear aircraft (ram or otherwise) were unmanned concepts. All that being said, it's possible that a TL9 reactor could be build efficiently enough that radioactivity and required shielding is minimized.

Check. Anti rad nano are Transhuman Space tech. Though not sure about the core Gurps TL for that.

2) Now, another issue that needs to be contended with in a nuclear aircraft is the potential for failure. Would the aircraft have failure modes to ensure that the nuclear reactors wouldn't pose a threat of contamination? It's possible to launch the aircraft over oceans, but there is always the possibility of the aircraft failing over land and the possibility that failure over an ocean could still lead to radioactivity being carried to a population center through ocean currents or causing problems with marine life. Again, here's a situation we could handwave away by saying that a TL9 reactor would be using safer power generation methods, maybe some variant of a pebble-bed, which would also help with refueling, but failure is still a potential problem. If a nuclear lighter such as this design is ever proposed, expect anti-nuke protests.

Check. But it's either nuclear drive or global warming drive - take your pick ;-)

Fission air-rams are a no-global-warming alternative for jet engines too. So if we can make them relatively safe then we can save all our flight carbon emissions.

3) Nuclear issues aside, I've had a couple of other thoughts specifically about the design. One issue is that the water rocket will burn out faster than the hydrogen rocket - they both provide the same amount of dV, but the water rocket produces 3x the thrust and will therefore burn out 3x as fast.

I thought I had that covered by having 3 times the fuel tank? Edit: The water rocket takes the same amount of time to burn the same deltaV.

4) Now the only thought of mine that you can actually easily apply to the craft is to assume that the craft doesn't have a physical pilot. Alida could potentially be flown by computer, especially since the flight path would be set before-hand. This would allow for one more passenger seat where the controls used to be and eliminate the expense of a pilot, though it would add the expense of better computing and AI flight software.

And AI is the same Gurps TL as the craft (TL9).

Or AI pilot with remote living pilot on standby (remotely piloted space shuttle).

Actually I can make an all water craft work (with 0.66 mps excess) if I just have one passenger in the pilot seat (16 tank, 2 drive, armor, controls). The return ticket price for that one passenger (actually 0.75 passenger by the ground rules) would be only $1200 assuming I have to pay the AI pilot (or $1000 if I don't).

[thtraveller]

Edit: updated for two orbiters, associated economics, and extra color text.

I found a way to use an all water design and get the ticket price below 1 weeks wages...

TL9+ Orbital Lighter (Lifter and Spaceplane) v6 "Alida"

Alida class space planes are cheap reusable two part orbital lighters that can take off and land at a normal landing strip anywhere in the world that has a clean water supply. It can easily make the return journey to and from orbit once for every two orbits of the space station or every 180 minutes.

The lifter craft and the orbiter craft are both capable of independent operation and flight. The orbiter rides under the lifter until separation above the atmosphere. Both use easy to obtain water as their reaction mass.

Using Nuclear Thermal Rockets in fuel-less air-ram mode the lifter takes off and clears local population centers before going supersonic and accelerating to maximum air speed in an easterly direction. It then boosts towards orbit until it runs out of reaction mass. At this point the orbiter detaches and engages its rockets and completes the climb into orbit. The lifter drops back into atmosphere and flies back to its base using its engines in air-ram mode.

Having docked and exchanged passengers the orbiter re-enters atmosphere and then makes a rapid descent back to Earth. Again using its air-ram engines it lands on its airstrip about an hour after it took off. There it picks up a second Alida orbiter and makes the trip again half an hour later. The Lifter therefore cycles every 90 minutes and each orbiter every 180 minutes.

Typically these craft take off over water and international airspace as few countries want fission craft flying over their territory. So, they typically take off from equatorial coastal countries and islands with several hundred miles of water to their east.

Alida Lifter “Big Al”:

Front
1-6 Upper Stage (Orbiter)

Central
1 Armor (Metallic Laminate)
2-6 Fuel Tank (Water)
0 Control Room

Rear
1-2 Nuclear Thermal Ram-Rocket (Water 3G)
3-0 Fuel Tank (Water)

dST/dHP 30, Hnd 4 atmo/0 space, SR 5/4, HT 12, Move Flight 1G/2500 mph/0.69 mps, Move Boost 3G/2.1 mps, LWt 100, Load 0.1, SM +6, Occ 2+0 SV, dDR 3, Cost M$ 6.1, TL9, Streamlined, Winged, Length 40 yards

DeltaV 2.79 mps (effective 2.67 mps after gravity drag)
-Ram Rocket 2500mph = 0.69 mps
-10 Water Fuel Tanks 10*0.15*1.4 (10 tank adjustment) = 2.1 mps

Alida Orbiters “Little One” and “Little Two”:

Front
1 Armor (Metallic Laminate)
2-0 Passenger Seating (12 seats)

Central
1-6 Fuel Tank (Water)
0 Control Room

Rear
1-4 Fuel Tank (Water)
5-6 Nuclear Thermal Ram-Rocket (Water 3G)

dST/dHP 20, Hnd 4 atmo/0 space, SR 5/4, HT 12, Move Flight 1G/2500 mph/0.69 mps, Move Boost 3G/2.1 mps, LWt 30, Load 1.3, SM +5, Occ 1+12 SV, dDR 2, Cost M$ 1.9, TL9, Streamlined, Winged, Length 20 yards

DeltaV 2.1 mps (effective 1.98 mps after gravity drag)
-10 Water Fuel Tanks 10*0.15*1.4 (10 tank adjustment) = 2.1 mps

Combined Effective Delta-V of Lifter and Spaceplane = 2.67 + 1.98 = 4.65 mps

Economics:

1 flight every 3 hours operating for 7200 hours pa= 2400 flights per year
12 passenger seats at 75% loading = 9 passengers per trip
= 21,600 passengers per year per orbiter
= 43,200 for two orbiters

Variable cost per orbiter trip (includes use of lifter):
Insurance = M$8.0 * 0.001% = $99
Servicing = M$6.9* 0.01% = $690
Fuel: 65 tons Water (50+15) = $1300

Total Variable Cost = $ 2,089

Annual Costs to Amortise from two orbiters and one lifter:
Requires 12 pilots at $7200 pm TL9 Comfortable salary each = K$1,037 pa
Other economics = Insurance + Depreciation + RoI = 10% = K$990

Total Fixed Cost = K$ 2,027

Divided over 4800 trips = $423 per trip

Return Ticket price = $(2089+423) / 9 passengers = $279 (less than 1 weeks net income for an average TL9 job)

There is no significant difference between TLs. The pilots wages increase per TL, which adds only about $10 per TL to the ticket price.

On a backwoods world with only 10% of the demand you only employ two pilots part time and only have one orbiter so annual costs are K$70 + K$800 = K$870 / 480 trips = $1813 per trip. So ticket price per passenger increases to $434 on backwoods worlds

The ticket price should stay approximately the same as ship size increases if you want to scale up to bigger craft and more passengers per trip.

[Agemegos]

Excellent! Can you use one lifter to juggle two or even three orbiters?

[thtraveller]

Originally Posted by Agemegos

Excellent! Can you use one lifter to juggle two or even three orbiters?

Given the fixed space station orbital period probably just two. It only saves about $30 per ticket though.