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Reusability

23 Comments

Jeff Foust writes about the unheralded 25th anniversary of the DC-X flights, and what has happened in the past half decade to see the promise that it offered a quarter of a century ago finally coming to fruition. I attended the 20th anniversary, but the only thing happening this year is a dinner in LA later this month.

I would note, per the criticism of the “purists,” that SSTO is highly overrated. Two-stage systems are much more flexible and efficient, particularly for off-nominal missions (e.g., high inclination or high altitude). SSTO would make sense only for a large traffic model to a single destination, probably equatorial.

23 thoughts on “Reusability”

        1. What about having boost ability on mothership, so it can climb more vertically when launching rocket,
          And your hold before the launch is something like 1/2 second time period?

          1. I seem to recall that back in the early 80’s some outfit – I don’t remember which – was looking seriously at putting hydrolox tankage and an SSME in a 747 in order to do exactly that when launching a small piggyback spaceplane.

  2. Well, launching from altitude can retire ~1km/s from the rocket’s delta-v budget, mostly due to enabling an increased nozzle expansion ratio and reductions in drag and gravity losses. This also draws you back down the exponential curve of the rocket equation and so reduces sensitivity to mass margin growth, which is especially critical to SSTO designs.

    1. Kistler originally wanted to air-launch their planned ‘SSTO’ in the 90s for that very reason. But as far as I remember they were going to lift it to around 80,000 feet, which is a bit too high for a 747 :).

        2. This one intrigued me enough to search. Didn’t take long to find this article from Dwayne Day for the Space Review:

          http://www.thespacereview.com/article/1569/1

          From the early 80’s it was a study program by the USAF initially called the Space Sortie Vehicle later renamed to the Air Launched Sortie Vehicle.

          Today instead seems as if we have the X-37B launched from the Atlas V EELV.

    3. Also, if you launch from high enough altitude, you can eliminate the aerodynamic fairing around your spacecraft… a non-trivial mass budget benefit. Of course, you’re probably in the realm of stratospheric balloons and not winged motherships at that point.

  3. I still think SSTO is needed. Especially if you want to launch millions of people a year. We also need to do launches that don’t have high G loads. With either the shuttle, or the Soyuz, I think it is about 3 Gs. We need to get that down to 1 G. Get it down to one G, and millions of more people can then go into space. Not thousands of people, but millions of people. To do that, you need cheap access to space.
    Now I have heard that if the BFR is fully fueled, that it can launch SSTO. If so, then that would be great. But how many Gs is the BFR? Is it 1 G, or more than 1 G?

    1. When going up at 9.8 m/s/s, a 200 lb person weighs 400 lbs.
      And 3 gees is 200 lb person weighing 600 lbs.

      I don’t you get person to space if can’t have a 200 lb person
      effective not weigh less than or only 400 lbs during trip to orbit.

      But if traveling at 9.8 m/s/s for 10 seconds this goes from 0 to 98 m/s and 98 m/s 342.8 kph or about 218 mph.
      And distance traveled is 490 meters.
      So if had 1/2 km tall pole which accelerated a rocket at constant acceleration of 9.8 m/s and when rocket left the top of pole and accelerated with rocket power at 1/2 gee and as rocket fuel was used increased in acceleration until it was going 1 gee, and that point separated from first stage, and had second state accelerate at 1/2 gee, and by time it reached 1 gee [due loss of burnt rocket fuel] you going at more horizontal trajectory and thereby not weigh twice as much and continue horizontally up to 2 gees acceleration [weighing same weigh as 1 gee pole launch].
      That might be possible.
      But instead of a pole, I would use pipelauncher- which is a big pipe which accelerates a rocket by displacement water. And it would need to be big/tall pipe to accelerate at 1 g and reach 218 mph. So I limit the size so only about 150 mph so as to have shorter pipe, and accelerate faster than 1 gee as option to faster launch speed.

      But evenually one could make taller pipelauncher and could have them only accelerate to 1 gee- for passenger comfort or things not designed to withstand more than twice their weight.

    2. 1 gee is what you experience standing on the ground. Any acceleration at all is going to make it higher than that. Anyone that can’t take a few minutes of 3 gees on their back needs to be in a hospital instead of a spacecraft.

      Also, the less acceleration used, the higher the gravity losses, which SSTO can’t afford even if a feasible design is considered. A minimal gee profile could easily make SSTO impossible. 11,000 m/s including extreme gravity losses is much harder than 9,000 m/s including normal gravity losses..

      1. How many gees does a commercial airliner have? Like a 737, or an A 320? Back in the early 90s, I was hoping that the DC-Y would be built. I think you need at least two flights a week. But 10 flights of week would be ideal. That would be a launch every 10 hours. If you can do that many flights a week, then you can have flights to any where in the world, in one hour, or less.
        So I would like to see thousands of flights a year.

    4. If I remember correctly (these numbers were worked out elsewhere), the BFR second stage (the BF Spaceship, or BFS) might be within rounding error of being a SSTO with no cargo. Ground-level thrust/weight ratio is dubious, and it’s doubtful it could do a payload even in the Falcon 9 class best-case. Conceivably it could make sense as a shuttle but not if the 1rst stage RTLS is working cleanly.

  4. SSTO would make sense only for a large traffic model to a single destination, probably equatorial.

    I think this is too strong (of course I do, as I’m working on an SSTO!). You can make much the same argument for the 737, that if you want to make it fly further than it’s design range it loses a lot of performance. That doesn’t make the design bad, though. Almost no 737s fly fully loaded, after all. One can only hope that we get to that point with spacecraft!

  5. I think the Falcon 9 RTLS and also to the boat has demonstrated that the risks of staging are now over rated and that TSTO gets you lots more performance.
    Think of the Booster as a tugboat getting the ship out of harbor.

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