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Why is a carrier airplane (White Knight Two) used for launching Virgin Galactic's spaceship? Isn't it possible for the spaceship to take off from the ground?

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The carrier plane is acting as a minimal first stage. By getting the vehicle to 40,000 feet and to a couple of hundred miles per hour of speed, it takes away the hardest first part of flight.

This means the booster is in thinner atmosphere and can use a more optimized outlet for lower air pressure.

All these things contribute to making a lighter/cheaper passenger vehicle.

Is developing the carrier cheaper than a first stage booster? Maybe? Maybe not?

But is it simpler to operate a plane, fly to a reserved air corridor and then launch? Possibly.

When planning on high flight cadence this can make a big difference that negates possible costs.

Pegasus is a commercial space launch vehicle launches from a Lockheed L-1011 carrier plane. It is not particularly cheap in comparison to alternatives. But it can fly to many locations to launch into specific orbits.

There is a 747 from Virgin airlines that is launching LauncherOne, much like Pegasus, to launch small payloads to orbit.

It is a trade-off. The particular design of SpaceShipTwo is not really for lift based flight from the ground and up, so it would need to be much bigger to fly from the ground, and a total redesign. It is basically designed for reentry flight.

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    $\begingroup$ Also, the carrier airplane is powered by jet engines. That means you don't have to use rockets to get to the release altitude & speed, so you don't have to carry oxidizer (probably LOX), don't have to have tankage for it, don't have to deal with handling & storing a cryogenic liquid. You're also using engines that have perhaps millions of hours of testing in commercial aircraft, as opposed to spending lots of money developing your own first stage engines. $\endgroup$
    – jamesqf
    Jul 14, 2021 at 15:30
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    $\begingroup$ @jamesqf the jet engine part is imho a very important factor. Way easier to manufacture, maintain, reuse, far more efficient and cheaper in fuel. $\endgroup$
    – Hobbamok
    Jul 15, 2021 at 8:41
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    $\begingroup$ The airplane basically acts as a very easily reusable first booster stage, thus doing what SpaceX does, but with cheap and proven technologies (the Soviets already experimented with launching their "Space Shuttle" off of an Antonov), and Airplanes are just far more common and known about than rockets $\endgroup$
    – Hobbamok
    Jul 15, 2021 at 8:43
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    $\begingroup$ "The particular design of SpaceShipTwo is not really for aerodynamic flight' ???? This answer makes no sense. It's a Mach 3 airplane. $\endgroup$ Jul 15, 2021 at 13:53
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    $\begingroup$ That comment is completely wrong. Aerodynamics =/= lift. $\endgroup$ Jul 15, 2021 at 14:02
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With some technical modifications it is certainly possible to launch SpaceShipTwo without carrier airplane like it was planned with XCOR Lynx

or vertically next to launch tower like it was planned with VentureStar SSTO or you can see now with F9 and BO New Shepard.

But you will need more powerful engines with bigger tanks and generally bigger spaceplane with bigger wings, so you can launch and land 8 people ( 2 pilots + 6 passengers ) above the Kármán line without need of carrier airplane, like VG always planned.

Don't see reason why it shouldn't be technically possible for suborbital flight carrying only 8 people. Reasons why VG decided to develop rather launch system with SS2 and carrier airplane ( White Knight Two ) can be lower development cost, faster development time and higher passengers safety when using SS2 only for second part of the flight. Unlike BO New Shepard SS2 doesn't have launch escape system, so passengers safety could be important factor in decision to develop this launch system.

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    $\begingroup$ Much bigger wings. The current SS2 cannot lift itself from a runway with a full fuel load. So bigger wings to lift it. This is more mass and drag, so more fuel needed. So bigger wings still to carry that fuel. Now bigger undercarriage to carry this weight. Now more fuel to lift the heavier undercarriage, then... etc, etc, etc. It's not impossible, but would require a 2-3x larger vehicle for the same passengers, and would be much less economical. And likely less safe. $\endgroup$ Jul 14, 2021 at 9:24
  • $\begingroup$ Why not launch it vertically like was planned with Lockheed Martin X-33. It was design for vertical take off and horizontal landing. You will still need more powerful engines for take off and bigger tanks to reach planned height, but not much bigger wings for landing. $\endgroup$
    – David Cage
    Jul 14, 2021 at 9:49
  • $\begingroup$ @DavidCage Thanks for the comment. Why using SS2 for the second part is safer? $\endgroup$
    – user15847
    Jul 14, 2021 at 17:42
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    $\begingroup$ White Knight 1,2 is very similar to standard commercial airplane and has regular jet engines, which were proven so far to be be much more reliable and safer than any rocket engine. To reach targeted height with 8 passengers without carrier airplane VG would need bigger spaceplane with bigger tanks, bigger wings for landing and more powerful rocket engines. Spaceship 1 development started in early 2000 and using launch system with carrier airplane and Spaceship 1 was seen be VG as safer option than developing single larger spaceplane powered with more powerful (and more risky) rocket engines. $\endgroup$
    – David Cage
    Jul 14, 2021 at 21:18
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    $\begingroup$ That doesn't mean that Spaceship 2 with White Knight 2 is best/cheapest possible option for suborbital flight. Scaled down X-33 single stage with vertical launch and horizontal landing with more powerful engines still could be cheapest option overall. But I understand that in early 2000, it looked like safer option for VG to develop launch system with carrier airplane and spaceship 1, than try to develop much larger spaceplane with more powerful rocket engines. $\endgroup$
    – David Cage
    Jul 14, 2021 at 21:21
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My vote is safety.

I'd suggest the flight profile of SS2 is inherently safer than anything which might be launched off the ground. To travel above the ceiling of a commercial jet, you need a rocket engine.

Launching a rocket plane from the ground has more safety challenges than launching a glider from an aircraft eg:

  • the glider can land (since it already has altitude & airspeed) but a rocket plane taking off vertically would have to reach a certain minimum altitude to either land or for parachutes to become effective.
  • the carrier plane uses proven safe jet engines which are less energetic than a rocket engine, requiring less exotic less energy dense fuel

If you want to get anywhere near space from a standing start on the ground using a reaction engine (rocket engine) and no wings, you need an enormous amount of energy (= rocket fuel, potentially explosive, etc etc). If you add wings and jet engines, the situation improves but as others have said now you have a power:weight challenge that means you need big wings and you need to carry fuel on your plane. If you take away the big wings and add a hybrid rocket motor you basically only have to carry an oxidiser (nitrous oxide in this case). This is a pretty high energy-density setup so you don't need big tanks of liquid fuel, though obviously you still have a pressurized oxidiser hanging around so there is that.

The approach to hazard mitigation here sounds to me like the "first, eliminate the hazard" solution. This makes sense to me considering they want to commercialise these flights - this is not just a one-off trip.

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    $\begingroup$ Re "exotic less energy dense fuel", many rockets use RP-1, which is basically just the same petroleum-derived kerosene used in jet airplanes. en.wikipedia.org/wiki/RP-1 And not all that different from automotive diesel. $\endgroup$
    – jamesqf
    Jul 16, 2021 at 17:07

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