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back to article GPS trigger will light LOHAN's fire

There's been a veritable jetstream of activity at the SPB's mountaintop headquarters this week as our quest to find a method to fire our Vulture 2 spaceplane's solid rocket motor at altitude nears its end. Click here for a bigger version of the LOHAN graphic Followers of our audacious Low Orbit Helium Assisted Navigator (LOHAN …

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Pint

Thank you

Your preemptive clarification about the "AND" is exactly correct. I read the headline and excitedly half-expected to find an incorrect comment that would need correction. But you've successfully preempted the whole thing. Congrats.

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Silver badge
Headmaster

Re: Thank you

On the contrary, in the language used by Lester, "or" is actually the correct word to use, not "and". If the devices are restricted to both 18,000m AND 515m/s, then they cannot exceed either. If, on the other hand, they are restricted to either 18,000m OR 515m/s, than they can exceed one, but not both.

If Lester had instead used the phrase "cannot exceed" rather than "are restricted to", then "and" would be the correct term to use. Remember, A && B == !(!A || !B); A || B == !(!A && !B).

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Silver badge
Pint

Re: Thank you

@Steve Knox

Well done. Have a beer.

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Coat

Cruise?

18,000m AND 1,152mph isn't a problem for cruise missiles, but it would prevent GPS guidance of a ballistic re-entry vehicle.

It's kind of you to ask and yes I am actually planning to get out a bit more...

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Facepalm

Re: Cruise?

I see I'm a bit too late to point out that we're talking about ballistic rather than cruise, but I think that a few more comments could be added.

The greatest period of concern about accurately targeting ballistic missiles was probably the 1970s. In the 1960s bombers were still common - by the 1980s the cold war was winding down.

GPS was starting to be developed in the 1970s, but only went operational in the 1990s. What we are seeing is the result of a committee decision made in the 1970s, probably set into the standards in the 1980s, and now being pointlessly applied thirty years later. Does anyone think that an organisation which is capable of building a device which can exceed 18,000m AND 1,152mph is incapable of designing their own electronics to read GPS signals? On the other hand, slower moving autonomous flying vehicles capable of carrying a fair weight of explosive are well withing the capability of the average model aeroplane enthusiast. If there were any threat, that would be where it would come from. Perhaps in 40 years we will get a ruling preventing us from building cruise missiles - of course by then beam weapons would be readily available....

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Thumb Up

Re: Cruise?

Some researchers have just managed to get a location using a re-purposed DVB dongle and a software radio:

http://hackaday.com/2012/08/19/real-time-gps-decoding-with-software-defined-radio/

Still early days of course.

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Silver badge

Dog

I like Patterdales, very much a one-man dog. Can be grumpy when cold and wet.

What's not to like?

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Stop

Re: Dog

[off topic] @Robert E

Not all Patterdales are one-person dogs. We got ours when she was a year or so from another family, settled in just fine. Agree they don't like the wet.

They do however, make the Duracell Bunny look lethargic, like a Jack Russel on speed. Maybe Mr Stirk potters in his shed to get away from the frantic, noisy dog?

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Silver badge
Coat

He swiftly put us in touch with Anthony Stirk

didn't he invent an flying suit?

*pocket stuffed with the Marvel dyslexia edition

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Silver badge
Boffin

Can be grumpy when cold and wet

Sounds like my car...

One thing we're looking for a bit of a brain storm on, folks, are the parameters under which we can launch, and perhaps as importantly, when we should definitely abstain from launching... in particular, there are two occasions at least when we expect the payload to get a proper kicking - as we pass through the jetstream between 30-60,000 feet, and once the balloon pops - at which point everything goes into free-fall and starts tumbling without benefit of much in the way of aerodynamic stabilisation. Note that the parachute will do very little for the first few thousand feet on the way down... any thoughts are good thoughts.

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Re: Can be grumpy when cold and wet

"any thoughts are good thoughts"

As someone who has worked with these kinds of projects before, I can confirm this is not true.

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Anonymous Coward

Re: Can be grumpy when cold and wet

>Sounds like my car...

You're lucky, sounds like my wife...

...and quite often being cold and wet is not a requirement.

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Anonymous Coward

Re: Can be grumpy when cold and wet

>>Sounds like my car...

>You're lucky, sounds like my wife...

You're lucky, sounds like me...

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Black Helicopters

Re: Can be grumpy when cold and wet

Put your payload in a balloon with N2 in it as well that is forced to burst when your out of danger ?

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Bronze badge

Re: Can be grumpy when cold and wet

Will the Swift board be carrying a gyros or accelerometers? If so you could possibly calculate a "bumpiness" factor (Ofcourse referred to as cupsize), being the average absolute acceleration over x seconds. If the bumpiness is above the threshold, prevent launch. I'm not a big fan of simply blocking launch between fixed altitudes. (Or possibly just 2 mercury switches, set at 90 degree angles to detect swaying, but this might be inaccurate and/or unreliable)

I will advertise choosing a "conservative" launch altitude. Choosing 30.000 meters is all well and good, but only if we can be sure the balloon will actually make it. Setting it to 25.000 meters would be a safer choice imho.

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(Written by Reg staff) Silver badge

Re: Re: Can be grumpy when cold and wet

Yup, we're adding an accelerometer. Update on Friday...

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Re: Can be grumpy when cold and wet

The phenomenon that you describe is known in the ARHAB community as PBC (Post-Burst Chaos), and it's quite common, but tamable. The most effective solution is to use a much larger parachute than the typical 2m diameter ones in common use. PBC comes about when the drag of the descending payload string approaches that of the inflated parachute. A conventional spherical parachute is meta-stable in that it likes to dump its high-pressure bubble of air out under its skirt, and that usually happens asymmetrically, thus creating a lateral blast of air that pushes the canopy farther off upright and so on until the 'chute is flying nearly sideways. In that condition, the drag of the payload string can easily exceed the 'chute's; it's not unknown to recover the payload string buried INSIDE the canopy after it fell into it!

But one need only review the videos of the Martian entry of the Curiosity rover to see that it can be done right. The key is to provide a huge canopy that cannot be so easily tipped by a draggy payload; but this also carries a price in terms of an initial investment plus tare weight that eats into the payload weight budget. The other method is to construct the payload enclosures in the form of low-drag bombs with tail fins to keep them falling fast and upright, thus serving as "plumb bobs" to keep the 'chute erect, despite its meta-stability.

Sadly, the LOHAN truss looks as though it would act more like a kite than a low-drag bomb. But then again, if PBC occurs after the aircraft is launched, there's not much downside to PBC relative to the mission requirements.

73 de Mike W5VSI, EOSS

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Pint

Re : Neil Barnes, jetstream etc.

As I understand it, the jetstreams are commonly used or avoided by commercial aircraft, depending on their direction, so are included in aviation weather forecasts. Access to this information should allow the impact of these streams to be avoided, or at least minimised.

As to the other point, I thought it had been decided that launch would best take place before balloon burst, while the truss was still in a stable configuration. After all, isn't that what all this discussion about GPS is about ?

You would still have two points of kicking if the jetstreams cannot be avoided - on entering and on leaving, but I raise a glass to Anthony Stirk

Chris Cosgrove

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Alert

GPS Limits

The GPS limits may be implemented as an OR, not an AND logic condition. GPS units also have G limits that may come into play. If your GPS gets confused, have a backup plan.

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Alert

GPS to trigger LOHAN's rocket motor... Are you sure?

GPS is notoriously inconsistent for reporting Earth-surface-based altitude measurements. It has to do with the inherent error in measuring an extreme vertical distance below the GPS satellites. From Wikipedia the average altitude of GPS satellites is 384,403 kilometres (238,857 mi). I can't imagine that the GPS error would be reduced much at LOHAN's ignite-rocket altitude. I'm not sure what your allowable error-of-altitude measurement is but hope you guys at least take a look at this potential pitfall.

Good luck!

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Re: GPS to trigger LOHAN's rocket motor... Are you sure?

It's true that GPS isn't nearly as good at measuring vertical position as it is at measuring horizontal position, but it's still plenty good enough for this application. An error of 20m say vertically isn't going to make any difference.

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Facepalm

Re: GPS to trigger LOHAN's rocket motor... Are you sure?

Lol, I think you've got a bit confused there Jimbo in Thailand. 384403km is the distance to *The Moon*. Earth orbiting sats are nowhere near that far away, Low earth orbit says are typically about 800-100km altitude, GPS sats are about 20,000km altitude.

Altitude is a bit less precise than lat and lon but still good enough to 50-100m, which is plenty for a rocket going to many 10s of km.

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Yes, some (actually, many) manufacturers mistakenly implement OR rather than AND. However the ones used in high altitude ballooning are ones known to work above 18km, with some having been flight tested at around 44km (rather higher than the LOHAN balloon will get to). As the article says the model on the Swift board (a uBlox 6) works up to 50km once it's put into flight mode.

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Pre-launch stability

Is there anything that rules out gyroscopes?

If so then maybe an accelerometer attached to some compressed gas source or similar that fires some bursts before launch?

Oh and has anyone found 'we're NASA and we know it' yet? (https://www.youtube.com/watch?v=QFvNhsWMU0c)

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Bronze badge

Re: Pre-launch stability

Rules out gyroscopes? It's called weight... the more mass in the truss and payload, the larger the balloon has to be... For this project, every gram has to be justified.

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Paris Hilton

Raspberry Pi?

Atmel? WTF? No more love for the Raspberry Pi?

Paris, not happy.

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lohan rehab

noticed on youtube today (I'm behind the times as always):

http://www.youtube.com/watch?v=RtBVGIs-q-8

for those who can't be bothered it says

Lindsay Lohan Paid $150,000 For Rehab

Very kind of her to support your efforts

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