Tuesday, 5 April 2016

Titan V and the flight of the Phoenix

The Phoenix, Zephram Cochrane's first warp vessel, was a pretty great design. It looks like a very realistic modern rocket, then transforms into an unmistakeably Trekkish-looking spacecraft, which is clever, since those are often two very different looks. And the name really fits beautifully with the swords-to-plowshares metaphor of its construction.

(Twenty year old spoiler warning: Clip from Star Trek: First Contact)

While reading about it, what caught my eye was that the First Contact designers built their Phoenix around the Titan II missile. This is interesting in a few ways. The Titan II launched the Gemini missions from 1964 to 1966, a century before the Phoenix (which every young Trekkie now knows went up on 5 April 2063). The Titan II was in use for 41 years, and the whole Titan family of rockets were launched for 46 years over all, with the last Titan IVB going up in 2005. When First Contact was released in 1996, Titan II had been going for decades and Titan IV was still quite new, so it wasn't crazy to assume that later versions of the Titan rocket might still be flying many decades later, especially if the fictional Titan V (as the movie-makers dubbed their version) had similar longevity to the Titan II

That said, is over a century of persistence likely for a rocket family? We have no idea. There hasn't been a full century of spaceflight yet. We can say that some families have endured well so far, notably the R-7s and the Atlases. But we can also say that some rocket families are really only families on paper, with early models and late models having very little in common. Sometimes, I suspect, the family name is only transferred from one design to another, totally different, design, as a sort of marketing gimmick, a brand recognition thing. So it's certainly feasible that a future ICBM with roughly the same specifications or role as the Titan II might be given the same name. Even so, I would guess that the Titan V that Cochrane's team launches must have been a museum piece to them, a very old left-over missile. If the 40 year lifespan of the Titan II is a guide to what's feasible, then the Phoenix's first stage may well have been built in the 2020s or 2030s.

What we do know is that the Titan V must have had the same 3 meter diameter as all other Titan first stages, partly because the real Titan II museum piece that represented it is clearly seen on screen with that diameter (you could do the geometry on some screen captures, but we don't have to, because that's a real, retired missile body they filmed with, we already know for certain how big it is), and partly because we're told in the movie that the cockpit section is about 4 meters long, which scales well with the 3 meter diameter.

The Titan V (or 5?), compared with real Titan rockets.
Mixed image credit to Ex Astris Scientia and Historic Spacecraft

The Titan V in the image above fits in well enough, apart from bucking the general trend of moar boosters(!) and increasing height. It also has a distinctly larger first-stage engine bell, replacing the pair of smaller engines of the Titan I & II, and the single small engine on the III & IV. The V's engine bell looks almost as big as those on the SRBs that most IIIs and IVs had strapped to their sides. This was a result of the movie people needing to build one as a physical piece of their set, when people are seen walking around the bottom of the launch silo; apparently the real engines had been removed and no realistic fakes had been installed by the museum. We later see that the Phoenix's second stage engine is roughly the same size, though not quite the same shape. [Edit: If the second stage is intended to operate solely in a vacuum, we can surmise that its similar size actually produces different performance, as explained here.]

(I also listened carefully, and the Titan V sound effects do not include the Titan II's distinctive ignition bwoop.)

Most of the interior of the cockpit section seems to be hollow and unused. It's very spacious in there; compare with the much more cramped Gemini crew module. With the pilot sitting in the front and the engineers in the rear, it's clear that most of the cockpit section's length is dedicated to crew space, and it probably doesn't have a big equipment bay right behind them.

Phoenix cockpit section. Mostly empty space. Approx. 4 meters long. Three crew.

Gemini crew module (greeny-blue in this image) and equipment module (white). Combined length of 5.6 meters. Two crew, cramped. Only the crew module returns to land on Earth, while the equipment module is abandoned.

What's interesting about the Phoenix's one known flight is how we can see it's reached quite an altitude, and Earth keeps quickly shrinking further away, and only then is the first stage jettisoned. We never even see the second stage firing; the warp nacelles immediately extend and it goes to warp without any further chemical boosting. This is an altitude comparable to the highest Gemini flight, which needed two Titan stages and an Agena booster rocket (joined in orbit) to get that far up. How does Phoenix manage it with only one stage, even with a much more advanced 21st century model?

Top: Phoenix, first stage still firing, after travelling/boosting for no more than 55 seconds.
Bottom: Gemini XI, all ascent boosters spent, after travelling for over 40 hours, including total apogee-boosting time of around 7 minutes.

A rough eyeball suggests fairly similar curvature to the Earth, so probably fairly similar altitudes, roughly.

It would be easy to dismiss this as typical Hollywood bad science, but I think there's a better solution. The answer lies in the Phoenix's trajectory, pretty close to straight up. You can also see that it's still over North America, so there's not been much horizontal motion. So, I would guess this was a suborbital launch, pretty close to vertical. The advantage of this is that if the warp drive test fails, you're probably going to fall right back to Earth, without the need for a retro burn. I use this trick for simple test flights in Kerbal Space Program all the time, letting gravity do the hard work of getting you home. And if the warp drive does work, then you can just warp out of Earth orbit, and then presumably either warp back again or fire the still untouched second stage rocket to push back towards Earth's gravity well. (Note that there is no clear, canonical relationship between warp speed and realistic sub-warp momentum, so that's harder to discuss.) If the Titan V can reasonably be assumed to have a more powerful, more efficient engine than the Titan II, then this sort of high-apogee suborbital boost may be wasteful, but it's inherently safer for a quick systems test than a full orbital flight. Assuming the landing systems all function...

I have two thoughts about landing the Phoenix. On the one hand, it's assumed (I think even by original designer John Eaves) that the cockpit section is jettisoned as a traditional, parachute-equipped crew module. Only the crew would come home, and the warp drive, the first of its kind, would be abandoned and burned up on re-entry. It would be sad and wasteful, but much easier. This method has plenty of real precedent.

On the other hand, that second rocket motor is pretty damn big for a mere retro booster. It could instead be a landing engine, for a vertical powered landing of the sort that the New Shepard and Falcon 9 rockets have recently demonstrated. The concept was already known in 1996, with the DC-X testing various necessary systems at lower altitudes throughout the '90s. I have no evidence that anyone connected with First Contact was thinking about this at all, but we can at least consider the possibility that Phoenix could potentially make powered vertical landings.

A compromise possibility is that the warp section and crew section each land separately under their own parachutes, perhaps also jettisoning the heavy second stage rocket motor to make things a little easier.

Image credit: LCARS24, I think, if not J. Anderson.

Since we have no idea how big Phoenix's warp core is, we don't know how much internal space it fills. But if we trust the fan diagram above, then the fuel tanks on the second stage rocket might be relatively tiny. You wouldn't get much delta-v out of that, pushing so much heavy warp drive section plus the cockpit section. But it might just be enough for a suicide burn on a vertical landing, or for a last-ditch emergency de-orbit burn. Does someone want to do the maths?

Remember that when the warp drive works, presumably exactly according to plan, it ends up out beyond Lunar orbit. If there isn't a warp trip back in the other direction, then the Phoenix would have had to take a slow sub-light drift back, something similar to the 3-day return trips that the Apollo Moon missions experienced. I'm not sure where they keep the life support (and snacks) for that scenario. But since Cochrane is back on Earth by nightfall the same day (the movie strongly implies), I think we have to assume that not only did it warp back closer to Earth, but it also landed (by whichever means) with great precision very close to Bozeman, so that Cochrane could get pissed with his (surviving) engineers just in time for the Vulcans.

My last major thought about the flight of the Phoenix is how weird it is that it didn't fry all the spectators on the ground and set fire to at least some of their shacks. I don't know what the minimum safe distance from a Titan launch is, but I've got a hunch it's quite a lot more than the maybe 10 meters between the silo opening and the bar.
He's dead, Jim.

And I also suspect that NASA rules would usually prevent people from standing this close to even quite a small rocket.

Maybe I worry too much, but I've got a suspicion that Newton's laws and a powerful rocket exhaust would at least knock these people over and deafen them, if not actually roast them. Little wonder the camera pans away from them so quickly.