This picture of an off-road vehicle perched at the top of a dune with the full moon hanging above it and the Milky Way seemingly close enough to touch is particularly appropriate for today’s discussion. Before I delve into why this image matches my topic so well, let me offer an apology.
I know the last episode was a pretty boring rehash of the development of rocket technology over the last several centuries. There was a bit of technical jargon and a slow middle section. We can debate the relative merits of including a prologue versus integrating your backstory into the narrative. Did I make the correct decision to get all this history out of the way up front? You tell me, please. This piece will have a bit more of that, but in the context of promises unfulfilled by an approach known as Single Stage to Orbit (SSTO). Bear with me, I promise that after this we’ll be getting very speculative.
So, let’s circle back to the picture prompt above. How does a big sand dune with a Jeep or Land Rover sitting atop it relate to SSTO? In quite a number of ways, some big and some subtle.
Last episode, we talked about how inefficient and expensive it is to get even a small payload into Low Earth Orbit (LEO). The energy needed to achieve orbital velocity is, well, astronomical. And velocity is the key, not altitude. We think of putting satellites in orbit as lifting them a couple hundred miles above the ground, but altitude is only a small part of the equation. In fact, the only reason they need to be so high is to avoid atmospheric drag which will eventually slow them down until they fall from orbit in a fiery death. Which brings into play the much more important aspect of orbital mechanics, namely speed.
To achieve LEO, a spacecraft has to be going about Mach 25 (twenty-five times the speed of sound) or about 17,500 miles per hour. Given that the fastest jet ever made, the SR-71B “only” goes up to about Mach 3, you can see the challenge. I’m doing a lot of rounding of these numbers but the relative magnitudes still apply. The lesson here is that it’s really hard to put something into space high enough and fast enough to keep it there. It’s kind of like climbing a really steep, high sand dune—see what I did there?
Now, you might think that if it is so hard to get just a couple hundred miles up and stay there, how could we have possibly sent men to the Moon, and space probes all the way out to Pluto and beyond? That’s a great question, and it points out one of the many non-intuitive aspects of space travel, or “orbital mechanics” if you want more jargon. Just getting to LEO takes about ninety percent of the energy it takes to go anywhere else in the Solar System! Getting to LEO means you are almost all the way to the Moon, or Mars, or even Jupiter. All it takes to get there is a little push, a lot of time, and some very clever navigation involving lots of math. Now do you see why the Moon and stars look so close in our picture? Even the stars are reachable from LEO if you’re willing to wait tens or hundreds of thousands of years before you get to one. The Voyager space probes, launched in the 1970s are now on their way, actually, so check back in the year 100,000 (I’m really rounding my numbers this time).
Seems a bit hopeless, doesn’t it? If it costs $11,000 to put a pound of stuff on the Moon ($10,000 to LEO and 10% more to get to the Moon), how could we ever afford to do much in space other than look around and take pretty pictures? Well, there has been a bit of hope on the horizon since the 1960s, though it may be more of a mirage. Which finally brings us to the topic of this piece—Single Stage to Orbit, or SSTO, vehicles.
Back to our picture. Note that there is only one set of tracks leading up the side of the dune, and there aren’t any discarded car parts littering the sand. The Jeep, or whatever, made it to the top in one piece, turned around, and is ready to come back down, hopefully again in one piece. All so it can do it all over again. That is the dream of SSTOs. Launch a vehicle, either vertically like a traditional rocket or horizontally like a jet, go all the way to LEO using just the fuel on board—no discarded boosters or stages—drop off the payload, and come back down to a safe, soft landing. It’s a great dream, but up until now has been a nightmare for investors.
I don’t have the space to outline all of the reasons why, but if you’re interested, Google “Everyday Astronaut” and “Why SSTOs SUCK”. Unfortunately, it’s that pesky Rocket Equation we talked about last time, in combination with a lot of other factors: atmospheric drag and the attendant heating, supersonic combustion issues, atmospheric versus vacuum exhaust nozzle design, residual fuel for deorbiting, reentry heat shielding, “dead stick” maneuverability for landing, and inter-flight refurbishment and turnaround for reusability. Whew! That’s a long list. Not every issue on the list needs a new technology to address it, however.
The Space Shuttle addressed the de-orbit, reentry, landing, and reusability issues, but it was definitely not an SSTO. Its overall design, which incorporated so many independently-developed components, demonstrated its inherent fragility when Challenger exploded during launch and Columbia broke up during reentry.
I’m almost out of space, and there is so much more to say about achieving LEO cost-effectively. Some issues I mentioned above still haven’t been solved or even addressed in any serious way. But, they’ll be “solved” in our next episode. From here on out, it’s all speculation, starting with my Screamer design.
Rob Johnson
May 18, 2020