Thursday, January 26, 2006

Good news

  • Delta IV has been canned in favor of Atlas V. I predicted it would go the other way, because the DoD has yet to launch on Atlas V. I had presumed there was some sort of issue with the Russian (Ukranian?) origin of the engine. I now presume the issue has been dealt with somehow, and that's good news all around.

    So now, does the Air Force think it's safe from a Boeing/LockMart monopoly because SpaceX is coming along well enough to scare them into better prices? Or is the Air Force figuring that keeping the Delta IV on life support is going to be more expensive than monopoly overcharges on Atlas V?

    And, for those who might protest at me treating the whole thing as a spectator sport, let me point out that it costs me something like the cost of a movie ticket every time they launch the Shuttle or Titan IV.

  • Other good news, might be old though: John McCain is being clear about our foreign energy dependence problem, and how to deal with it: build nuclear power plants. Of course, McCain is also in favor of hydrogen-fuelled vehicles, which is a boondoggle as far as I can see.
  • Wednesday, January 25, 2006

    Men in Space, II

    I thought I'd learned a long time ago that if you're going to write in public, you need to start with a large backlog of already written pieces, so that when a dry spell comes (christmas vacation or whatnot), you can post the prewritten bits and not lose your audience. Well, you have to remember to post those prewritten bits.

    I just read Giving future human space explorers the credit they're due. I think it nicely, if accidentally, summarizes the pitiful reasoning behind manned space exploration.

    Mr. Kendall's main point seems to be that humans can cover more difficult terrain, and more of it, than robots. Fine, we'll build a rover with legs. This is orders of magnitude cheaper than sending people to Mars. Note that people on Mars are going to need vehicles to get any great distance anyway, so I suspect there is no real advantage for people here.

    The Mars rovers have driven only kilometers because they are small, low power vehicles. If we want to spring for more payload for a faster vehicle, we can do that without also spending orders of magnitude more for a person to ride on that vehicle.

    Second point: people can be more detailed than robots with the same instruments. Given the vastly superior stay-time of robots, I find this unbelievable. His point about people staying 500+ days on the Martian surface presumes a very expensive mission.

    Third point: Excavating partially buried rock, drilling hundreds of meters... These seem like things not done by people but by their equipment, and are an excellent argument for robots.

    Fourth point: repairing broken parts, creating new tools. How? With what? In a machine shop? How much will that weigh?

    Fifth point: pilots can step in if mission control miscalculates. No way. No pilot is going to override a reentry burn successfully. What data would he base his intervention on, that mission control does not already have?

    Sixth point: cultural importance, human triumph. Now we're back to "footprints in the dust". This is bogus. Footprints are symbolically important when they are the first footprints of many. When we have a reason to put people on Mars, the first footprints there will be (temporary, due to wind) symbols of our purpose there. What are the lunar footprints symbols of? Peace?

    Seventh point: educational importance. How do we make Mars interesting enough for a fourth grader to pay attention? Step 1: put in a sense of scale by placing a person in the picture. This helps, but it's not worth the money. I have a better suggestion: concentrate on movie cameras that take pictures that people care about. Make sure the mission definition team has an experienced, professional nature photographer with sufficient clout to overrule science goals if necessary and ensure that the mission returns great TV.

    I think we're missing movies of robots in action. Taking pictures of the exploration itself is what communicates the triumph to the taxpayers footing the bill. We need movies of rovers drilling rock and driving around. We need movies of assembly robots piecing together large orbiting telescopes. We need movies of Cassini cruising through the rings.

    Movies aren't usually included because the bandwidth required is appalling, and wouuld swamp the science returned. This is not a problem anymore. The cameras and recording system can be lightweight and solid state. They should be standard NTSC resolution, and use compression like any non-science TV show uses, as the point isn't to enable some scientist to make verifiable conclusions by closely examining the pixels. Finally, the cameras can shoot dozens of hours of footage, store it locally in a few hundred gigabytes of flash memory (lightweight and cheap nowadays), and the probe can send back a tiny fraction of that selected by a movie director.

    The other problem with movies is that there hasn't usually been a vantage point from which to film the robot. But this need not be the case either. The Deep Impact mission actually had two probes. The one that survived could have sent back a nice high-res, slow-motion movie of the moment of impact in the months after the mission was over. The Mars rovers all had landing vehicles that could have shot movies of them leaving the lander. Heck, I think it would be reasonable for the rovers to cart around a monopod with a remote camera on it. Find a nice rock, stick the monopod nearby, then drive over and look at the rock while being filmed from the monopod. When done, pick up the monopod and drive on.

    Monday, January 16, 2006

    Slow Launch

    Every time I see a rocket vault off the pad, I wonder why the engineers didn't make the propellant tanks bigger. Adding propellant to any stage always increases the delta-V delivered, so long as you ignore gravity losses.

    It takes a lot of thrust and energy expended just to get the rocket to hover stationary above the pad. The delta-V lost is called the gravity loss. As the rocket gets closer to orbital velocity, gravity losses drop. At low velocities, gravity losses are just time * acceleration by gravity.

    Gravity losses, as it turns out, are a big deal. As you add propellant, you logarithmically increase the delta-V, but you linearly increase the gravity losses. At some point, gravity losses overtake the increased delta-V.

    The tradeoff between the two is different for different kinds of rockets. In a solid-fuelled rocket, the entire stage is a big combustion chamber, that must contain the gas pressure used to accelerate the vehicle upward. Large pressure vessels are heavy, and so the fuel container is a large fraction of a solid rocket's mass. Pressure-fed liquid-fuelled rockets don't actually have the combustion chamber in the tanks, but the tanks must hold higher pressure than the combustion chamber, so the mass penalty is similar.

    Pump-fed liquid-fuelled rockets hold their propellants at a small fraction of the combustion chamber pressure, and so their tanks are a small fraction of the weight of a similarly-sized solid rocket.

    Let's take a look at the two extremes. First, a kerosene/LOX liquid-fuelled rocket, like the Saturn V or Falcon 9. I've made up a table to show the decreasing performance return of steadily larger and larger tanks. Here I've presumed a base rocket with an Isp of 290, a first-stage thrust of 750,000 kg, a Gross Lift-Off Weight (GLOW) of 500,000 kg, and a first stage burnout weight (this includes the upper stages) of 150,000 kg. This rocket has an initial acceleration of 1.5 G (but remember you lose 1 G to earth). Incremental tankage weighs just 2.5% of the incremental propellant stored. I'm assuming that the engine thrust stays fixed. Delta-V numbers are in meters/sec. Tower clearance times are a little high, as they assume no acceleration beyond the initial acceleration.

    Isp 290
    Ve 2842
    Tankage 0.025
    Thrust 750000
    Burn rate 2586
    Tower 50
    inc inc delivered tower
    G Mf Me delta-V time G-loss delta-V clear
    2 375000 146875 0 0 0 0 3.2
    1.9 394737 147368 136 7 73 63 3.4
    1.8 416667 147917 143 8 81 62 3.6
    1.7 441176 148529 151 9 91 60 3.8
    1.6 468750 149219 159 10 102 57 4.1
    1.5 500000 150000 169 12 115 53 4.5
    1.4 535714 150893 179 13 132 47 5.1
    1.3 576923 151923 191 16 152 39 5.8
    1.2 625000 153125 205 18 178 27 7.1
    1.1 681818 154545 221 21 210 11 10.1
    1 750000 156250 240 26 252 -12 N/A

    You can see why the Saturn V initial acceleration was just 1.13 Gs. You can also see why launching the Saturn V in a strong wind could have been a problem: it takes a long time to get past the tower.

    Now let's look at the other extreme, a solid rocket first stage, like the Titan 4 or the Stick proposal. (Because most of the Shuttle's liftoff thrust is from it's solids, it fits in this category too.) Here the Isp is a bit lower, but more importantly the tankage fraction is far higher: 12%.

    Isp 242
    Ve 2371.6
    Tankage 0.12
    Thrust 750000
    Burn rate 3099
    Tower 50
    inc inc delivered tower
    G Mf Me delta-V time G-loss delta-V clear
    2.5 300000 126000 0 0 0 0 2.6
    2.4 312500 127500 69 4 35 34 2.7
    2.3 326087 129130 71 4 38 33 2.8
    2.2 340909 130909 73 4 41 32 2.9
    2.1 357143 132857 75 5 45 30 3.0
    2 375000 135000 78 5 50 28 3.2
    1.9 394737 137368 80 6 55 25 3.4
    1.8 416667 140000 83 6 61 22 3.6
    1.7 441176 142941 86 7 68 18 3.8
    1.6 468750 146250 90 8 77 13 4.1
    1.5 500000 150000 93 9 87 6 4.5
    1.4 535714 154286 97 10 99 -3 5.1

    You can see why a Titan 4 gets off the pad with nearly 1.5 Gs of initial acceleration.

    Solid rockets are a good match for first stage engines. It takes relatively little engineering work to produce a large amount of thrust from a solid motor, which is the biggest cost driver for first stage engines. But because the casing weighs so much, and also because solid propellants have lower Isp, the delta-V of a solid first stage is never going to be as good as a liquid-fuelled analog. That leaves more work for the second stage, which means thinner engineering margins and more desire for high-Isp propellants, like liquid hydrogen. And so, cheap solid rocket first stages drive more cost into the upper stage.

    Tuesday, January 10, 2006

    Launching in 50 MPH winds

    I was a little curious about the idea of launching in 50 MPH winds. Could the vehicle be blown into the tower?

    The Falcon gets off the pad with something like 1.3 Gs of acceleration, so to clear a 21 meter tower will take about 2 seconds.

    Plain horizontal translation is not enough to worry about. I'm not sure what the horizontal drag coefficient of Falcon is, but it might be around 0.5. Assuming that, a horizontal 50 MPH wind would accelerate the vehicle laterally at about 0.3 m/s. By the time it gets off the pad, that's a little over half a meter.

    One wonders how much the vehicle sways under wind. If the nose was deflected a meter and a half towards the tower when the holddown clamps release, that would add a meter of translation by the time the rocket clears the tower. I think the tower is more like 2 meters away, and I find it pretty hard to imagine pushing the launch button on a vehicle whose nose is whipping around anything like the complete distance to the tower.

    So, no problem, but I thought the numbers were interesting so I'm posting them.

    P.S. I think the SpaceX LOX boiloff solution of insulated panels velcroed onto the rocket, that get ripped off at launch, is cheap and cute. But this isn't going to change their procedural problem that they need to unload a bit of kerosene if they need to sit on the pad for any length of time. It was an error during the unloading of kerosene due to a launch delay that caused the tank damage.

    And I'll bet those panels are going to get blown to bits when the rocket blast hits them. Not that it matters a lot, but they are going to get polystyrene bits all over their lovely tropical isle. If they ever want to clean that up for a VIP visit it's going to take a lot of time, and where are they going to find low-wage workers out in the middle of nowhere?

    And finally, I think SpaceX gets my vote as best nerd entertainment of the decade. At some point, some producer is going to realize that real-time updates on spectacular engineering projects are better than "reality" TV programs, and then I'm going to have to start watching TV again. I wonder how much the buzz they're getting is worth, in term of popular support when they need to negotiate with the government?