Polish Eagle wrote:
It's still kind of mind-blowing that they initially planned to use a Learjet. I suppose that underscores just how different the suborbital flight regime is from the orbital or hypersonic one.
Can’t recall the exact speed at cut off but it’s somewhere around Mach-5 to get enough velocity to hit 100km. And more power/speed if your ascent angle isn’t steep enough (70 to 90 degrees). So your airframe has to be pretty robust to handle the loading and stress which is why one of the initial X-prize entrees using a similar jet-aircraft/rocket combo was not given a go-ahead for development. Rather than a Learjet, (bit expensive) the team planned to use a surplus/retired NAA T-39 Sabreliner, (
https://en.wikipedia.org/wiki/North_American_Sabreliner) with a rocket stuck in the tail
Saberliner would never have handled the stress of using the rocket engine, meanwhile the Learjet might have held together going up but would have folded on the way down. That’s why they began redesigning and rebuilding the Learjet with only a few main components being left.
I think I mentioned this before but bear with me. In order to Help clarify “Sub-Orbital” flight we came up with a classification on NSF:
Class-1: Straight up then straight down. May have ‘some’ arc up to perhaps 50 or less km as adding distance requires more energy.
Advantages: With a proper power plant it is the most efficient trajectory using the least energy to broach the 100km as long as horizontal distance and energy is minimized. Beings that the start and landing zone are so close vehicle recovery and reuse are technically enhanced but this greatly depends on the design of the vehicle and the operations plan.
Disadvantages: High stress reentry for aircraft and passengers. Both experience high aerodynamic loads due to the vehicle coming down almost vertically into the atmosphere. G loads for passengers can reach and/or exceed 9gs for higher flights. Short “weightless’ duration. 60-120 seconds for “Low” trajectories like SpaceShip One/Two, “High” flights can get longer weightlessness period at a cost of higher aerodynamic and G loads. Spaceship one experienced 5Gs on entry while Spaceship Two is expected to average 9 due to flying higher to allow up to 5 minutes of freefall. Blue Origin’s New Sheppard should experience similar loading.
Methods suggested to relieve such strain are to increase the vehicle ‘drag’ cross section such as the extending panels of the TGV Michelle-B VTVL vehicle but this has some control and weight and balance issues due to locations of the major mass concentrations in such a design. Note that while the ‘feather’ of Spaceship One/Two enhance stability and control they in fact do no greatly reduce aerodynamic stress or G forces,
Class-II: This has a distance as well as height component, Not only do they rise above 100km altitude but they also typically do not “take-off” or land at the same spot. Sometimes they do but this then requires added ‘trip’ time to the launch point such a a ‘boost-forward’ mission where the vehicle flys subsonic to a point ‘up-range from its base before engaging the rocket and flying upwards. Upon reentry the vehicle should be very near its home port and once landed can immediately be serviced and refurbished. Note Rocket-Plane was slated to fly about 100 miles/160km ‘up-range’ form where it would land to an area designated for it to launch clear of all commercial traffic, light the rocket, fly a ballistic trajectory to 100km and then enter and glide back to base using it’s jet engines for terminal power.
Disadvantages are that it takes more power and propellant to fly this kind of flight and more complex operations and planning. Also the heating pulse it longer and drawn out and the shallow trajectory does not give a lot of time to experience zero G. Advantages are that the aerodynamic and G loading is somewhat less and the horizontal velocity along with vehicle lift can keep them to an acceptable level. Another advantage is it is vastly easier to tap into other commercial and support operations, (hotels, shows, tourism and experiences) since the passenger not only gets to go on a ‘suborbital’ ride but they also went “someplace” when they did it! “Space Travel” rather than a carnival ride.
Take for example my favorite “What if?” involving some casino owners in West Wendover Nevada, (East Wendover is in Utah) who want to make some money and want to tape into the more wealthy and popular resort town of .Las Vegas. (Distance as the bird flies is 321 miles or 516 km, it’s actually a bit shorter going from Los Angeles/Pasadena to Los Vegas at 228 miles or 316 km, Mojave City ito Los Vegas is even shorter at 187 miiles or 301 km)) Tourists on vacation in Las Vegas are flown to Wendover for a few days 'fun' (it can be, and hey you get to visit "historic" Wendover Field where they practiced to drop the A-bomb, ya ok) and are then packed aboard you Class-2 suborbital ferry for an hour-ish long ride up and over and down to a field near Las Vegas and then bused back to the original casino/hotel to 'assimilate' the trip. Fun and profits for all
Nobody is going to pay $200,000.00 for such a 'side-trip' and frankly VG and BO both are looking at the 'trip' to just be an 'event' over a two-week stay and a luxery resort. With "astronauts" giving "briefings" and "training" which is going to really amount to slipping the tourist something to try and avoid the sad fact that 99% of everybody exposed to freefall get SAD (Space Adaption Sickness which would be NO fun in a small space with a bunch of other people) from such a trip. Even RocketPlane was working with a resort chain to arrange a similar package. About the only one that straight out planned to set UP at a resort was XCOR which was what allowed their pricing compared to the others.
Now I've seen a Russian suggestion for a 'hybrid' suborbital vehicle where they actually have a pretty normal aircraft encased in a launch and reentry shell. Goes up and does its thing but once its back down enough the shell splits, (falls away actually as it only covers enough to protect the aircraft) and the plane goes into a regular aircraft airport while the shell is recovered
separately. It actually concentrates the prices, cost and parts in the right places rather than trying to compromise one into the other. Downside is bulk and complexity but not as badly as one might think.
Randy
