I played around with numbers for a few minutes, just for the sake of it... I was assuming a one-way trip to space, given that converting an airliner airframe to reentry-specs would be quite a few orders of magnitudes greater in terms of engineering issues than just putting the thing into orbit...
I was playing with the idea that maybe taking a 4-engine airliner and replacing the inboard two of those with rocket engines in pods and putting explosive bolts on the outboard engine to jettison them once they are no longer needed. It was interesting, to say the least!
A few numbers: 747-400 is 400 to 410 tons MTOW, equiped with 4 engines each with 280 kN thrust, and 241 cubic meters of fuel tank, of which about 65 cubic meter is the center tank.
Now let's say we replace the inboard engines with SpaceX Merlin engines, just to have some specs to work with, with a thrust not too extremely different from the 747 GE or Pratt&Whitney engines, which have a 616 kN thrust rating, twice the turbofan's rated thrust. LOX could be fed from the center tank, converted for cryogenics with an insulated bladder or something along those lines, while using RP-1 directly both in the outboard jet engines and the inboard rocket pods... With an ISP of 305s for Merlins, this yields a propellant flowrate of 206 kg/s. Assuming a 2.25 RP-1/LOX ratio, that yields us 127 kg/s of LOX for the 2 rocket pods, and 285 kg/s of RP-1. So that is 160 L of RP-1 each second and 55 L of LOX.
At that rate, the center tank would have enough LOX for 585 seconds. However, the remaining tanks would only hold about 555 seconds of fuel, not counting whatever would be used for takeoff and climb to the turbofan's service ceiling. Taxi, take-off and climb to cruise altitude takes slightly less than 17650 L of fuel, so let's take that out... only 500 seconds of RP-1 are left to feed the two Merlin engines -after- reaching 35000+ feet with the outboard turbofans... That would leave us with 10.8 tons of LOX for cabin pressurisation and occupants, which is enough to replenish the cabin completely a few times...
Now the killer question: Would such a converted airliner have enough ooomph to get to orbit. Let's crunch a few more numbers.
Climbing to FL350 would lighten us by 16 tons of fuel. Jettisoning the outboard engines with explosive bolts at the pylon attachments as we light the Merlin engines for the climb to orbit would cut out another 7.25 tons. The Merlin engines themselves are 1300 kg a pair, which makes the 747 lighter by 6 tons, since the inboard engines have been replaced by these. Aircraft total mass is now down to 380 tons. At full power, we get 0.33G of initial acceleration. Replacing the inboard engines with the Merlin rocket engines, we might not need too much extensive structural modifications, given that they are much closer to the centerline of the airframe, and hence have less arm moment than the outboard engines. Since the wing is designed to handle the full thrust of an outboard engine with a failed inboard, it's a wild guess but still might not be too far off...
Unfortunately, applying the rocket equation to this scenario yields a mere 2.3 km/s delta-v, which on top of our cruise speed of 250 m/s at rocket engine ignition gives us only 2.5 km/s, with a final acceleration of 0.72G... for a fully loaded 747-400. We only had 19 tons of cargo/passengers at takeoff, so let's cut that out completely, as well as the extra LOX we carried around. We are now up to 2.6 km/s delta-v, hence 2.85 km/s ground-relative velocity, but without anything onboard! And of course, we are way short of orbital velocity.
---------- Post added at 02:26 AM ---------- Previous post was at 02:20 AM ----------
Hmm... I might have screwed up between the LOX/RP1 ratio... doesn't change the picture much in any case... No, you can't put a 747 in orbit without exotic engineering! Would make a nice fantasy add-on however ;P
---------- Post added at 02:32 AM ---------- Previous post was at 02:26 AM ----------
Don't have the time to crunch the numbers much, but even with hydrogen instead of RP-1, without considering volume, still coming short at only 4.3 km/s if using an SSME stuffed into the tail instead of an APU
I was playing with the idea that maybe taking a 4-engine airliner and replacing the inboard two of those with rocket engines in pods and putting explosive bolts on the outboard engine to jettison them once they are no longer needed. It was interesting, to say the least!
A few numbers: 747-400 is 400 to 410 tons MTOW, equiped with 4 engines each with 280 kN thrust, and 241 cubic meters of fuel tank, of which about 65 cubic meter is the center tank.
Now let's say we replace the inboard engines with SpaceX Merlin engines, just to have some specs to work with, with a thrust not too extremely different from the 747 GE or Pratt&Whitney engines, which have a 616 kN thrust rating, twice the turbofan's rated thrust. LOX could be fed from the center tank, converted for cryogenics with an insulated bladder or something along those lines, while using RP-1 directly both in the outboard jet engines and the inboard rocket pods... With an ISP of 305s for Merlins, this yields a propellant flowrate of 206 kg/s. Assuming a 2.25 RP-1/LOX ratio, that yields us 127 kg/s of LOX for the 2 rocket pods, and 285 kg/s of RP-1. So that is 160 L of RP-1 each second and 55 L of LOX.
At that rate, the center tank would have enough LOX for 585 seconds. However, the remaining tanks would only hold about 555 seconds of fuel, not counting whatever would be used for takeoff and climb to the turbofan's service ceiling. Taxi, take-off and climb to cruise altitude takes slightly less than 17650 L of fuel, so let's take that out... only 500 seconds of RP-1 are left to feed the two Merlin engines -after- reaching 35000+ feet with the outboard turbofans... That would leave us with 10.8 tons of LOX for cabin pressurisation and occupants, which is enough to replenish the cabin completely a few times...
Now the killer question: Would such a converted airliner have enough ooomph to get to orbit. Let's crunch a few more numbers.
Climbing to FL350 would lighten us by 16 tons of fuel. Jettisoning the outboard engines with explosive bolts at the pylon attachments as we light the Merlin engines for the climb to orbit would cut out another 7.25 tons. The Merlin engines themselves are 1300 kg a pair, which makes the 747 lighter by 6 tons, since the inboard engines have been replaced by these. Aircraft total mass is now down to 380 tons. At full power, we get 0.33G of initial acceleration. Replacing the inboard engines with the Merlin rocket engines, we might not need too much extensive structural modifications, given that they are much closer to the centerline of the airframe, and hence have less arm moment than the outboard engines. Since the wing is designed to handle the full thrust of an outboard engine with a failed inboard, it's a wild guess but still might not be too far off...
Unfortunately, applying the rocket equation to this scenario yields a mere 2.3 km/s delta-v, which on top of our cruise speed of 250 m/s at rocket engine ignition gives us only 2.5 km/s, with a final acceleration of 0.72G... for a fully loaded 747-400. We only had 19 tons of cargo/passengers at takeoff, so let's cut that out completely, as well as the extra LOX we carried around. We are now up to 2.6 km/s delta-v, hence 2.85 km/s ground-relative velocity, but without anything onboard! And of course, we are way short of orbital velocity.
---------- Post added at 02:26 AM ---------- Previous post was at 02:20 AM ----------
Hmm... I might have screwed up between the LOX/RP1 ratio... doesn't change the picture much in any case... No, you can't put a 747 in orbit without exotic engineering! Would make a nice fantasy add-on however ;P
---------- Post added at 02:32 AM ---------- Previous post was at 02:26 AM ----------
Don't have the time to crunch the numbers much, but even with hydrogen instead of RP-1, without considering volume, still coming short at only 4.3 km/s if using an SSME stuffed into the tail instead of an APU
