There’s been considerable debate in the last few years about the relative merits of orbital assembly versus superheavy lift.

The argument for superheavy lift is pretty straightforward. Very large rockets enable payloads to be launched in one piece. Their advantage is not simply in the weight they can carry but, more importantly, in their large-diameter payload shrouds. Those very-large shrouds come in very handy for launching things like large-aperture telescope mirrors for future very large space telescopes.

In 2007, for example, NASA’s Marshall Space Flight Center did a study of how future space telescopes could be launched on the then-proposed Ares V rocket.

Ares V launches 8-meter space telescope

The picture displayed here came from that study. It shows Ares V launching a very impressive 8-meter-aperture telescope.

This was one of a number of telescope concepts considered in the study. NASA found that a Delta IV, with a payload capability of 13,000 kg to L2, could launch a telescope with a 4.5-meter aperture. Ares V, however, could launch a payload of 60,000 kg to L2 or 130,000 kg to Low Earth Orbit. This would allow the launch of an 8-meter telescope to L2 or a 12-meter telescope to LEO.

Even larger apertures would be possible using segmented mirrors, based on the technology being developed for the James Webb Space Telescope. With its baseline 8.4-meter fairing, Ares V could accommodate a 10-meter class segmented-mirror telescope. With a new 12-meter fairing, it could accommodate a 15-meter segmented-mirror telescope.

This sounds very impressive. Astronomers would surely love to have 15-meter-aperture space-based telescope. How does this compare to orbital assembly, however?

The following picture is from the early 1980’s, an era when NASA thought big and was not afraid of orbital assembly. It depicts a 100-meter Thinned Aperture Telescope, described in the study found here. Note the Shuttle orbiter for scale. (Apologies for the quality of the picture. It’s the best available.)

NASA 100-meter Thinned Aperture Telescope concept

It’s interesting to note that this behemoth telescope has a mass of just 85,000 kg. Although it has an aperture 12.5-times larger than the 8-meter telescope in the Ares study, the mass is only 41% greater. Ares V could launch all of the components for this telescope in one flight.

On the other hand, the components could be launched on several flights of an existing rocket such as Delta, Atlas, or Falcon 9 or a future commercial reusable launch vehicle. In that scenario, however, NASA would not be tempted to raid the space science budget to pay for superheavy lift development, so projects like this Thinned Aperture Telescope would have a better chance of being built.

The cost of space systems typically scales with the weight. This suggests that the 100-meter Thinned Aperture Telescope might not cost much more than the 8-meter telescope proposed in the Ares study. The performance, however, would much greater. To show what on-orbit assembly means for science, the following pictures shows simulated images of an exoplanet imaging with a 10-meter space telescope and a 100-meter space telescope.

Simulated images of an exoplanet imaged with 10-meter and 100-meter aperture space telescopes

Anyone with an interest in astronomy, astrobiology, or space exploration wants to see pictures like the one on the right. The question for NASA is, how can planners fit such capabilities into NASA’s budget?

The question for citizen scientists and others is, if NASA doesn’t build instruments like this, who is going to? Unfortunately, that’s not a rhetorical question. This is certainly beyond the range of ordinary citizen science, but perhaps, with on-orbit assembly and low-cost commercial launch systems, the cost for such instruments could be reduced to something comparable to large ground-based telescopes. Would private foundations such as Keck be willing to take the risk involved in large space projects like this?

Written by Astro1 on April 10th, 2012 , Astronomy

Leave a Reply to Mike Loucks Cancel reply

Your email address will not be published.

COMMENTS
    Mike Loucks commented

    The argument against heavy lift is even more straight-forward. There are no other customers so NASA has to pay big bucks to keep the rocket available even when it’s not launching.

    You mention the development costs to pay for “super-heavy lift” but leave out the ongoing costs of keeping the super-heavy rocket operational. Recall that during the space shuttle days, the cost of the shuttle program every year was the same if they launched 0 or 10 shuttles. That is because the people and facilities that are required to build and operate the rocket have nothing else to do when the thing isn’t flying. There are no other customers.

    Unlike a commercial vehicle that has other customers, where you just pay when you buy one, with a super-heavy lift rocket, developed by the government, you keep paying whether you need one that year or not. Once you stop paying, the rocket is no longer available.

    So, not only are you stuck with the development costs, you just took $2+ billion/year out of NASA’s budget that cannot be used for anything else as long as you plan on keeping this rocket available.

    So you’ve got this cool rocket that can lift heavy things, but no money to build the heavy things you want to launch with it, unless you increase the NASA budget (which doesn’t appear to be happening).

    The only way we can afford to fly these instruments is by using commercial rockets that have other customers and don’t require NASA to pay when they aren’t flying anything.

    If I had an existing Heavy-Lift capability, and I didn’t have to pay when I wasn’t using it, then yes, it would be great to have it and it would be a no-brainer

    It doesn’t exist, so I have to pay to develop it, and then continue to pay to keep it operational.

    Reply
    April 10, 2012 at 1:18 pm