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Archive for the ‘Spacecraft’ Category

NASA’s plans to return astronauts to the moon are dead. So are the rockets being designed to take them there — that is, if President Barack Obama gets his way.

Sayeth Orlando Sentinel.

Haven’t followed NASA’s latest movements. The Augustine panel had some potential but stuff seems to have withered down. The organization seems to be a wannabe monument builder without a job. People might want something more practical than monuments, at least I hope they would. Even when NASA has such huge talent and competence in many areas, it fails to function as a sensible whole in defining strategic human space flight. And then there are the legacy issues. One of which is that of Mike Griffin.

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I always had a different idea compared to the one Jon and Kirk posted, (Kirk Sorensen is now a contributor at Jon Goff’s place, I’m afraid having such top men in the same place might cause a awesomity criticality event). I assume this idea is probably found in some old NASA report from the sixties or seventies, like most things are.

Rendezvous is mostly a 4D problem: 3 space dimensions and time (some more if you take into account that proper attitude must be maintained as well, but that is assumed to be trivial). If you can take out two space dimensions, the problem should simplify greatly. This is possible with the following arrangement:

A boom at both the target and the vehicle, placed at right angles (and both at a right angle to the approaching vector). Basically, this should reduce positioning accuracy requirements hugely. The booms could be short barrels (even inflatable), or really long semi-rigid wires or composite girders or whatever. Depends on design aims.

When they contact, they will both slide until one boom is caught by a hook at the end of the other boom. Then one boom will slide through the hook until the hooks contact. From there on it’s a known geometry. You can reel in the boom, if it is flexible, or just slide it if it is rigid, and get both craft to a configuration you want, for either ordinary robot arm capture for berthing (as demonstrated by HTV, many station modules and Shuttle MPLM:s) or traditional docking (Soyuz/Progress/Shuttle/ATV).

This concept has some problems. For example whipping the target or the vehicle with an improper attitude / position boom. In the pictured boom configuration, approaches should have an offset always to one side. Alternatively one could have multiple booms. That way it wouldn’t matter on which side the rendezvous error would be. Also, the target could have a V shaped bow to avoid having the vehicle hitting dead center with a boom.

Another issue is if there is some kind of failure in the rendezvous, like too high velocity, the boom might rip off. That would result in a very dangerous object co-orbiting with the vehicles. This would be a very bad day for something like the ISS or a propellant depot.

One way to avoid this is to have the hooks have a mechanism to give way if the load gets too high. Another more outlandish is to have a weaker boom attachment in the vehicle. This would sever its boom and leave it hanging to the target in case of a problem.

This all was motivated to make unmanned rendezvous much easier to enable cheap propellant depot tankers. As all know, ATV and HTV are hugely expensive and high dry mass systems. Something like a Centaur or any basic “dumb” already existing restartable upper stage with just mostly a working attitude control system (including a star tracker) could be used instead, if some out of the box thinking is deployed. Most of the smarts should be in the target that is launched only once, but it can’t be the maneuvering party since it is very heavy. This system should get the best of both worlds.

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Someone was asking on ARocket about where to start with building a differentially throttled hovering vehicle. Lots of advice were given by various people. I’ll show some stuff I quickly sketched back in 2007 with Simulink. It’s such an easy to use and awesome software (especially compared to a recent short battling with LabView), that whipping up any control system or any process or system models are really quick jobs.

This is just some really trivial basics, it doesn’t take into account nearly enough things to produce a real hovering vehicle. I guess it’s just a way to show how things could be started.

So, here’s a pic of a simple one-dimensional rotational feedback system. Just tested some feedback coefficient values for the PD controller.

1) You can see the placeholder “guidance algorithm” output at the top left “scope” graph. This is the reference or target value of the angle where we want the vehicle to be. At the start it’s giving a desired angle of zero radians, then from 4 seconds onwards it suddenly wants 0.5 radians, then again zero after 6 seconds. This could be holding a tilt for a while for getting up some lateral velocity for a transverse move.

2)  Below that, you see Theta, the angle that the vehicle actually had during the simulation. It follows the desired angle quite nicely. Below Theta, there’s omega, the angular velocity.

3) The tilting is done by throttling the two thrusters. At top center and top right you can see the values of the throttles. Since this thing only concerns tilt and not stationary hovering, the throttles are at zero when the vehicle is at the right attitude (reference=Theta) and there’s no angular velocity. When the refence is moved, at 4 seconds, throttle 1 shoots up for a while and the vehicle starts tilting. At around 4.8 seconds the other thruster, throttle 2 shoots up to stop the rotation, since the vehicle is nearing the desired angle.

You can see from the block diagram how the error value, E = theta – reference is calculated. And also omega is used. From these the throttle values are calculated, simply by Throttle1 = E*Kp1 + omega*Kd1 and so on. By fiddling with the Kp and Kd values, called gains, one can tune the system. The values currently are just some I quickly tried back then.  This Simulink model actually has a bug, a sign error. The error signal’s difference calculation is backwards, hence Kp1 and Kd1 are negative. It should be the other way around.

You can tune the model. Basically, raising the proportional values makes the system faster (reach desired values quicker) but gives overshooting and can make the system oscillate unstably. Higher derivative gains remove the tendency to oscillate, but the system becomes more sensitive to noise (noise can after all have large derivatives). If the vehicle had problems of always staying some amount off from the final required attitude value, an error integrator could be added that would fine tune the vehicle a little (making it a PID controller). This is probably not needed here though – the guidance system could notice if the vehicle travels somewhere it doesn’t want it to go.

The MDL file for the above is available here.

This is just a very quick first brush analysis, there are much much more things to take into account. The other tilt axis, roll, the need  to actually stay airborne (altitude or vertical velocity feedback loop) that ties into the throttle values too. One could abstract this tilt in one axis to a single control block, and produce another similar block for altitude holding, then couple these blocks into a whole system of altitude and tilt holding (by mixing the throttle values). This should avoid clutter and enable one to isolate problems.

Also, one should add some noise to the sensed angle vs real angle as gyro noise seems to be a real issue for VTVL vehicles. One wouldn’t want to tune a PD controller’s values in a noiseless simulator and then find out it gets wildly out of control in the real world. Even when not simulating noise, one can be quite conservative with the tuning values, and that helps too. The throttles probably would be another source of errors as well, if one used ball valves like here. One could also use pulse width modulated solenoids (John Carmack is fond of those for quick work) and those should be modeled. Digital valves (for liquids) have also improved lately, and hydraulics uses are emerging.

I’ve only done real world PID stuff on lab experiments. But it’s still very cool to see it work. It can balance a ball on an unsteady position on a moving cart, which humans have trouble with using a joystick, and the math in the controller is not complex since linear approximations can be used for that problem.

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Or what you are going to call it, an unrealized proposal from Aerojet around 1984. PDF Found on NTRS.

The idea was to have two turbopumps (like on SSME), but instead operate on the expander cycle. Two heat exchangers, two turbines, two pumps. One for each propellant.

 

aerojet_cycle

Both propellants go through a heat exchanger and an expander driving a pump

 

This is a LOX-hydrogen engine. Also this means that since there is the same propellant on both sides of the axle, in the turbine and in the pump, no elaborate seals are needed. Original intent for these engines was for in-space reusable stuff, that needs to be operated many times and for a long time without maintenance. Size was in the RL10 class, about 70 kN. (RL10 has grown though.)

aerojet_margin
Simplicity and margin were claimed

Think for example if you let a fired turbopump sit in space for a long time. Will some fuel leak to the oxidizer side through the seals? This could avoid that. (You can use helium purges too though but then you’ve got one more fluids you need to tank.)

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The Man. On Space Review. [EDIT: About a month ago, but I only just read it.] This is just excellent. So many things I agree with, that go against the stupid myths of spaceflight and space policy. If you read one space policy interview this year, this should be it!

“NASA is an organization that is dominated by fixed costs. In business terms everything is in the overhead,” he said. The committee found, with some effort, that the fixed cost of NASA’s human spaceflight program is $6–7 billion a year. “The bottom line is that they can’t afford to keep the doors open with they money they’ve got, let alone do anything with it.”

However, he said, if you’re trying to minimize costs, it makes more sense to use a smaller launch vehicle that flies more frequently and has other users and applications. The key to making that work for exploration architectures that require large amounts of propellant—and hence have driven the planning for heavy-lift vehicles like the Ares 5—is the use of propellant depots and in-space propellant transfer. “If you use in-space propellant transfer, it’s no longer true that you have to have a really big piece,” he said.

He said that while he had his own opinions on the right selection of launch vehicles, he didn’t have any insights on what direction the White House and Congress would go. “It’s really up to policymakers whether we have a space program or a jobs program.”

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JAXA’s HTV

It’s in orbit currently. Status updates on spaceflight now and an NSF forum thread. There’s some technical material on NSF L2 about the HTV, for anyone there.

Hope all goes well. This is also exciting, if everything works, there are soon four space agencies that have docked to a space station – a few years ago there were only two. Spacefaring is coming closer, although this is still baby steps in a sense – the vehicles as well as the rockets used to launch them are expendable, and have been developed with ultra expensive national programs.  They are the forerunners, grandparents of real spacefaring hardware that will hopefully come in the not too distant future, and be much easier and cheaper.

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Skimming the document (thanks NSF, Florida Today). Cute how a launch without an upper stage at all in the heavy configuration works out for ISS (burn SM fuel for orbit):

Delta IV Orion options comparison with Ares I and STS from the Aerospace report

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