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Archive for the ‘Suborbital’ Category

Clark Lindsey comments on how Arianespace just raised prices instead of lessening costs. I’m reminded of Rob Coppinger’s recent visit to Kourou, their launch site. It just costs a lot to keep a city going in the middle of jungle, and when that cost must be paid by the monthly rocket flight customers only, it gets expensive per flight.

A lot of ideas have beend expounded on expendable rocket manufacturing and reusable refurbishment costs. But it seems the integration before launch is terribly expensive as well, if not the most expensive part. And the launch control costs as well, as does mission control.

Airports are expensive facilities as well, but they are still cheap per trip since the throughput is large. Though I don’t know how smaller airports manage, if they still need radars, passenger and aircraft services etc…

Anyway, this should be a very important focus. It’s less sexy than the sleek fast machines, but lays the important ground work for space access.

The suborbital trips might be good training and experimentation for this. Virgin galactic seems to opt for grandeur with huge custom facilities, making it into something like a theme park. I do wonder if XCOR’s working on something more modest. This might make a huge difference to their profit margin per flight…

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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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Armadillo finally won L2 already.

Masten and Unreasonable are still flying for second place I think (I’m not 100% clear on the rules) today!

Spacetransportnews is the place to watch all this. (Or it has the links collected.)

It’s historical in a sense. These rockets will serve as the basis for reusable sounding rockets, possibly high altitude tourist vehicles and later orbital system lower or upper stages. When the operations are routine and landings safe, the cost per flight goes down orders of magnitude, compared to ordinary rockets.

A new era for rocketry is dawning.

 

Update: This is the twitter account to follow: http://twitter.com/mastenspace

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62 mile club has a writeup of a beta “customer qualification program” for XCOR’s Lynx suborbital craft. This highlights the differences and current state of play. Rocketships will not be as safe as airliners in the near future, and they don’t need to be. There are millions of things that are less safe than airliners – scuba diving, ballooning, general aviation, motorcycling, probably even driving a car. And yet people do those things because they have the judgement and can decide for themselves.

The key difference is an informed consent. The suborbital rocket traveler should be told of the risks truthfully so that they can decide for themselves if they want to do it or not. This, I gather, has always been XCOR’s principle.

Airlines are not directed at such customers because they are a mass means of travel – the customer is not briefed specially but is expecting reasonably good safety – and there are thus governmental and intergovernmental bodies regulating the airlines and trying to constantly improve safety.

It would not make the slightest sense to regulate suborbital passenger rockets at this time at airline level – there are only a few passengers and the companies should have the time and resources to screen and brief them very well on what it will be like and what the risks are. (This is a must though – you shouldn’t advertise the service as something as safe as airlines.)

There should be some very simple regulation of rocketships regarding the risk to the uninvolved public of course – most companies deal with this adequately by just flying from remote enough locations. And of course there’s environmental regulation – it’s not cool to put tonnes of methanol into the ground water for example. Yet these are small no brainer issues (I’ve heard stories of over-eager environmental protection agencies though).

Nontoxic (or those that quickly decompose to such in nature) fuels and oxidizers should help a lot in this regard. Suborbital rocketry is not that performance critical anyway – it is a great way to find the lowest investment and operating cost approaches to rocketry – and these drive towards “nice” systems. Safe, easy, nontoxic, nonhazardous, redundant.

Regeneratively cooled LOX-ethanol or LOX-methane engines could be good in this regard – propellant spills and dumps are not that horrible to the environment or to the public, and the engine could in theory run indefinitely without any parts replaced if it is just refuelled.

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By ATK in 2005. Shows how little I know.

EDIT: Spaceref had the details, Mach 5.5.

(Scramjets are still not a space application.)

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Ongoing.

Norm AugustineNorm Augustine

You can stream NASA TV with VLC, just paste this link into it:

http://www.nasa.gov/55644main_NASATV_Windows.asx

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And partly what this blog is about (I realized in the middle that I’m typing like in a slide show, so I changed it into bullet points, as it’s an overview and not a deep text). I present my vision that should be aimed for:

What should NASA do?

  • In the near term, NASA should change to EELV:s (Atlas V, Delta 4) and COTS (Falcon 9, Taurus 2) as launchers for the ISS and lunar programs.
  • At the same time, NASA should do basic research and cheap small tech demonstrators for space technologies that give more for less.
  • This should move humanity closer towards spacefaring.

Spacefaring? Spacefaring is making space operations routine.

  • Space faring requires that space access is cheap, reliable and hassle free.
  • Launch is only part of the spacefaring,
  • But only from that point on can the better in-space technologies (tethers, ballutes, sails, ISRU, slings, whatnot) be developed.
  • Hence launch improvements are absolutely crucial for spacefaring

How can cheap and reliable space access be reached? There must be:

  • Many independent providers of space access.
  • It is done largely with well reusable vehicles.
  • The architecture – more of a market – is multi-faceted and the launchers can be improved, new ones can enter the market and old ones can be scrapped

This coal can be reached, in the next few decades.

Things to avoid:

Technically unrealistic choices at the highest level:

  • In the NASP program, the early performance numbers were fudged and there were unacceptable internal politics meaning no real independent technical criticism would be heard at the top
  • In the “Safe Simple Soon” Ares rockets vs the already flown EELV:s debacle, OMB has lacked the expertise to keep NASA on a leash so they are a “loose cannon” controlled too much by the whims of a leadership that fires all who disagree
  • Countless other examples…

Program mentality:

  • Apollo was ended since it was just a short unsustainable program with a specific stunt style goal, not fitting in any overarching smart picture as a sustained capability
  • STS has been an unimprovable yet critical massive monolith, barely sustainable, for various reasons
  • Danger of having yet another single solution launcher (or two) just for a definite program

Lack of motivation:

  • Has NASA become too big and corrupt by internal politics to really do technical or economic choices? Has it just become pure politics and internal struggles for personal or group benefits? (ESMD) There are great and talented people working there, but does it make a difference?
  • What does the whole agency exist for anymore anyway? Or its current lunar program? Is it just a relic from Apollo?
  • How much actually flying a few people to space every year conflicts and directs efforts away from the goal of reaching real spacefaring?

Summarized, NASA’s goal should be a spacefaring humanity in the future, not having a narrow minded program after another.

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