March 20th, 2006

Looked so good on paper

Blackstar RLV

Aviation Week came out with a cover story about "Blackstar", a two stage to orbit reusable launch vehicle supposedly being retired by the US government. This would have been built as a "black" program after the Challenger disaster and operated during the '90s. The article didn't offer much evidence, and some of what's in there is probably BS. So even if the thing exists AvWeek hasn't proved it.

I'm more interested in the technical side of the discussion. Various people have disparaged the story on the grounds that the design couldn't work. I think they're full of it. The concept described could be built and flow successfully (whether it'd get a meaningful payload to orbit I'm agnostic on).

The dumbest criticism is that it couldn't work because carrying the orbiter underneath the carrier plane is impossible because the landing gear don't leave enough room between the carrier's belly and the runway. That critic probably spent more time looking at the pretty artist's concept than reading the article. The artist generated his pictures by making a rough orbiter model and combining it with an unmodified XB-70 Valkrie 3-d model. The article describes the carrier with a recess to hold the orbiter, matching the practice of carrier aircraft back to the X-1. The Blackstar carrier supposedly used some XB-70 parts, but it was designed from scratch to match the orbiter. It could have had a huge open bay and landing gear like stork legs if that's what was needed.

Another complaint is that the carrier's top speed of Mach 2 or 3 wasn't good enough to get a payload to orbit. There is a rule of thumb that each part of a multi-stage launcher should contribute an equal increase in velocity, but contrary to those critics it's not a law. The faster the carrier flies the easier life is for the orbiter's designers but they can build it to reach orbit from any starting condition. If the carrier is subsonic the orbiter would be essentially an SSTO, so there'd be a question of whether you're better off not bothering with it. But the Blackstar carrier would be providing 8-12% of the velocity to orbit, which is a bigger contribution than it seems at first glance. Getting the last few percent of performance out of a launcher is always the hardest--it's an exponential function. Now a Mach 12 carrier would be ideal, but not much hardware gets built to ideals.

(As an aside, this is why I don't like The Rocket Company's vertical-trajectory first stage architecture. They're spending a lot of time and money on the first stage and still have to get SSTO-level performance out of the upper stage.)

The orbiter is criticized for having a linear aerospike engine. Superficially they've got a point--the best thing about an aerospike is that it adjust to atmospheric pressure from ground to vacuum, so you don't lose performance. But a rocketship with conventional bell-nozzle engines has a rear end which is shaped like a beer can's. That increases how much space it takes up in the carrier aircraft and requires more weight for aerodynamic shaping. A linear aerospike provides a chisel-shape tail shape (ignore the artist's concept, again), which can be fit flush to the edge of the carrier's belly while taking up less volume.

The fuels for this thing are the most unusual aspect. The main fuel was described as boron-based. Ignition (the best history of rocket propellants out there) is harsh on boron and it doesn't get discussed much these days. The energy released doesn't make up for the toxic byproducts and damage to the engines. It does make sense in context as a way to get a fancy project past reviewers:

"What makes you think you can do better than NASA/DynaSoar/etc?"
"We're using boron fuel."
"Oh, okay, then."

It also makes sense for why the thing would be built, flown just a little, and then retired--most of the problems show up in operations. The AvWeek article included a comment of solid rocket motor pods being added to the orbiter. That makes no sense at all. Between the low Isp and the weight of the casing a solid motor might actually reduce the performance to orbit. Plus a badly cast motor would blow up their fancy RLV. That part of it I have to send to the bit bucket.

So could this have been built? Yes, definitely. Would it work? It would fly. The orbiter would shoot out of the atmosphere. Would it make orbit? Dunno. Depends on how good their design work is, really. Sending the orbiter from Kwajalein in the Pacific to California or vice versa would make for a nice test flight and that wouldn't be much problem. So there could be a few glimpses of it. It certainly didn't do any regular operations, that would have provided lots of chances to get good sightings.

So what's the most this could be? A prototype, one that's too difficult to operate to compete with the other systems deployed or in the pipeline. Most of the missions you need a spaceplane for--on-orbit defense, stealing enemy satellites, flexible ASAT--dropped off the priority list when the USSR went away. The rest of what it can do we have other systems for. If you've got 80's tech hypersonic airframes and boron-fueled engines you'd have to spend a lot of money for each flight, on top of the expense of maintaining the team and infrastructure to operate it. So shutting it down would be the sensible move. Given how many billions of dollars would have gone into such a white elephant I'd expect it to stay a black program until the authorizers have not just retired but died.

Odds are it was just a paper study, or the entire thing is a combination of unrelated programs stitched into a fancy story.

Pity. It would've been nice to have an existence proof of a reasonable RLV when I was working on one. We probably could've underpriced it, too, assuming the gov't charged full price for using Blackstar rather than eating the development costs and only charging for marginal cost.