The "Real Aerospace" Thread

[icekatze]

hi hi

I'm not sure what you mean by "more flexible." Flexibility in exhaust velocity/thrust ratios is one of the VASIMR's big selling points. I'm also not sure what you mean by more economical. The MPDT runs on Lithium, which costs $25 per 100 grams, while the VASIMR runs on argon, which costs $0.5 per 100 grams.

The Princeton 200 kW MPDT demonstrated an Isp of 4000 seconds, or about 39,226 meters per second exhaust velocity. The 200 kW VASIMR demonstrated an Isp of 4900 +/- 300 seconds, or about 48,052 meters per second exhaust velocity.

As far as I can tell, they are both very comparable to each other, with the VASIMR slightly edging out the MPDT in a few different ways.

[Cy83r]

Have we covered fission fragment drives yet? Because Isps of 100,000 to 1,000,000 are amazing on this side of antimatter- Plus! the reaction mass is also the powerplant when you decelerate the ionized medium. Refueling would be an odd affair if one didn't design a revolver sort of system internally, which adds more weight to an already heavy engine.

P.S. Oh! I also realized that both the disc and vaporization chamber designs both scale up fairly well, only really being concerned with surface area which can be designed to work within a linear consumption plan, where fusion chambers seem like creating larger magnetic bottles would suffer from energy input and surface area limitations at higher volumes.

[Mr.Tucker]

What I mean is that you're able to use one engine for ''relatively'' high thrust AND also have high Isp (VASIMR has higher thrust in low gear but much lower Isp). Best of both worlds. And with an efficiency of around 80% compared to VASIMR's roughly 60%. Also worth pointing out that MPDs can work with both hydrogen and helium. But the point is it's not being investigated, and I can't grasp why.

Fission fragment, ah yes. That design is an atomic engineer's wet dream. The problem is...well actually there are more. While extremely promising, they are basically impossible to test anywhere inside the Earth's magnetosphere, let alone atmosphere. And while the scientific principles are well understood, actually building that thing is incredibly difficult because you have no room for error. Like all NTRs the temperatures and pressures can be catastrophic if control is lost. While the concept is actually quite doable in my opinion, it's probably more difficult than it would first seem. Call it an engineer's hunch . Still, an avenue worth pursuing. I'd say it's the final and best possible fission design achievable in a long roadmap of tech that starts with classic NTRs, carries on to more advanced systems like liquid-core, vapor-core, nuclear electric, followed by the safest and most reachable gas core yet designed (closed-cycle, aka ''the nuclear lightulb''), before finally ending with fission fragment. It's the final branch in this tech tree, but we need to make some more steps and gain some more experience till then just to be safe. And with the recent discovery that decay rates may not be as constant as we once though, we still need a heck of a lot more research before we get there .

EDIT: Also worth pointing out that uranium ain't cheap, or plentiful in the solar system (unlike lithium, helium or other light elements).

[icekatze]

hi hi

The thrust difference between the VASIMR and the MPD is really negligible. The MPDT is not the best of both worlds, it is one of the worlds by itself, a low thrust plasma rocket with a high specific impulse. 12.5 Newtons at 4000 Isp is not appreciably different than 5 Newtons at 4900 Isp. Especially when you consider the mass to thrust ratio of the entire craft.

If the VASIMR geared down to produce 12.5 Newtons of thrust, it would probably weigh in around 4000 Isp too, and if the Lorentz-force Accelerator style MPDT readjusted their nozzle to get 4900 Isp, they'd probably drop in thrust to around 5 Newtons. In a medium gear, the VASIMR is going to have around 100 Newtons of thrust and would still be in the mid to upper 3k Isp range. (The relationship between the two for plasma rockets is inversely proportional, but since the tradeoff is not a linear one, I can only make ballpark calculations.)

Efficiency of any magnetoplasma rocket is a function of the power output. Both the VASIMR and others can exceed 70% above 200 kW, while at 100 kW they only get around 40 - 45% efficiency.

But honestly, unless someone is working in a near Earth capacity, high Isp is probably their biggest concern. Gearing down for higher thrust is helpful if someone was trying to go to the moon, cause they'll be there before they finish burning all 500 hours of their total fuel capacity.

Again, the VASIMR is a type of Magnetoplasma rocket, and is largely equivalent to others in that category, like the Li-LFA from Princeton or the one the Russians are working on. They could all potentially run on Argon, Xenon, Lithium (if they can solve the deposition problem), hydrogen and helium. Lithium has proven to be more effective in a lot of different applications. The lower the atomic mass of the element, the lower its efficiency in a plasma rocket.

[Keter]

A shameless snark would say VASIMR is best of both worlds too. All the promise of theory and just enough reality to keep the $$ and enthusiasts going, its immaculate theoretical performance forever spared the ignominy of practical hazards.

[icekatze]

hi hi

A shameless snark might say those things, but it would be really odd, since the VASIMR has conducted thousands of successful, stable test firings by 2013. I'm not sure such a statement would match up well with observed reality.

The Li-LFA style MPDT from Princeton is also a real device that has been tested and observed, and I suspect that if they continue work on that design, they'll probably be able to get an overall higher exhaust velocity than the VASIMR while operating at peak performance.

Whatever problems exist for radiating waste heat and generating electricity, they are problems shared by all magnetoplasma rockets.

[Mr.Tucker]

Whatever problems exist for radiating waste heat and generating electricity, they are problems shared by all magnetoplasma rockets.

Agreed. What's worrying is that the solutions are evident and well-known, yet no one seems to be pursuing them.

A shameless snark would say VASIMR is best of both worlds too. All the promise of theory and just enough reality to keep the $$ and enthusiasts going, its immaculate theoretical performance forever spared the ignominy of practical hazards.

Every drive has a potential niche. What space systems would look like in case of near-term human expansion and space exploration is a menagerie of electrical and other propulsion concepts : simple stuff like arcjets for stationkeeping, multi-use systems like VASIMR or NTR for orbital tugs, high Isp for long voyages,etc. My expectation would be that cargo craft would use different types of sail designs (being propelantless is a big advantage), and that very long range mission probes (like, say, to the solar focal point) would use a combination of such drives (launch with whatever, accelerate with electric, slow down with magsail. Just an example). Fusion is still some time away in my opinion (could be wrong).

[Keter]

I stopped keeping up when I read... I think a year or three ago that they were still stuck on that plasma detachment issue. So I guess that's not a problem anymore?

Whatever the case, they need to put it up in space for TRL 7.
edit-
http://sen.com/blogs/irene-klotz/nasa-n ... ce-station ...

[Mr.Tucker]

I stopped keeping up when I read... I think a year or three ago that they were still stuck on that plasma detachment issue. So I guess that's not a problem anymore?

Whatever the case, they need to put it up in space for TRL 7.
edit-
http://sen.com/blogs/irene-klotz/nasa-n ... ce-station ...

This forum discussion may be relevant: http://forum.nasaspaceflight.com/index. ... =34788.135. Seems they're orbiting it exactly because they're not sure if it's still a problem. If it works, I just hope they do something with it .

[Keter]

Yes and as you can see at the (current) end of that thread you have that same link brought up. Maybe they can get a ride to some other demo platform...

I now also remember that a number of people from the industry thought the whole project was dubious (for reasons I can't recall) and that it was basically Diaz exploiting his position/reputation. In itself it's not basis for anything, but it struck me because the people saying so were not the kind to say something like that just for the sake of voicing disapproval. I can't recall if it was on NSF or at one of the NASA facilities from one of the lab researchers.

[Mr.Tucker]

I believe Bob Zubrin became an adamant opponent of VASIMR, and pointed out some inconsistencies in the design, which has spurned others to look into them and point out issues. I think he doesn't have anything against the drive itself, but against the idea that it's necessary for Mars. This would be a relevant debate by him (in his unique style ): https://www.youtube.com/watch?v=myYs4DCCZts

[El-Hazard]

After reading various discussion on Outsider posts perhaps someone can cure my curiosity concerning gravity. How in so many sci-fi flicks and books
does rotating a cylinder in space create gravity?

[icekatze]

hi hi

Technically speaking, rotating a cylinder does not create gravity. It does, however, create "Centrifugal Force", which is a close enough approximation for people. It is only a simulation of gravity though, and not quite the same. If you knew where to look on a rotating space habitat, you would probably be able to see some differences. (When dropping an object, it wont fall straight down, for example.)

[Razor One]

After reading various discussion on Outsider posts perhaps someone can cure my curiosity concerning gravity. How in so many sci-fi flicks and books
does rotating a cylinder in space create gravity?

You can actually test for this if you have enough space to swing a bucket of water.

Take a bucket, fill it about 1/4 of the way with water.
Hold the bucket with a firm grip. One hand ought to be fine, but two hands for safety.
Now start spinning.
You'll feel a force trying to pull the bucket away from you. It will rise up to the horizontal and the water won't spill out. If you watch the water, it will be flat to the bottom of the bucket.
If you're feeling particularly brave and don't want feel dizzy, this works just as well vertically as it does horizontally.
Swing your arm in a circle vertically with the bucket at the end. As long as you're going at a decent clip, the centrifugal force that Icekatze mentioned will keep the water flush with the bottom of the bucket. Go too slowly, and you'll get a nice cheap shower for your efforts. If you swing too swiftly, the strength of the centrifugal force may actually outpace your grip and the bucket will go flying out of your hand and give someone else a nice cheap shower for your efforts.

You can also enjoy this phenomenon at carnivals or fairs. When I was young, I used to love the ride called the Round Up, which effectively demonstrates this force in action. Once it got up to speed, it honestly felt like lying flat on the ground. The only way you could tell you weren't was the rushing wind, the spinning background, and that the Earth's gravity hadn't entirely disappeared. At the top of the arc you felt lighter (Centrifugal force - gravity), at the bottom of the arc you felt heavier (Centrifugal force + gravity). Sadly they retired it while I was still young and I haven't been to many carnivals or fairs since, but if I come across one anytime soon I'd happily ride it.

You can also see its effects on carousels. Like so:

[Mr.Tucker]

An interesting presentation by Dr. Landis. Feel free to watch the other presentations in the Starship Century symposium: https://www.youtube.com/watch?v=ogKKjpQvfuM

[Karst45]

https://www.youtube.com/watch?v=9QuLgl2vKis

explained here in a computer asisted simulation AKA Kerbal Space Program

[Mr.Tucker]

Well....:
http://www.engadget.com/2015/03/23/boei ... ma-shield/
Use this as blast protection and use the AMAP-ADS for kinetic protection, and MBT's still have a future. Would be too heavy for anything lighter than a Centauro though.

[RedDwarfIV]

Meh. I'd say a Challenger II tank with SCAR* shielding would be pretty much invincible (barring direct nuclear strikes of course.)

* My own term, standing for SuperCapacitor Armour (Reactive), for a type of electromagnetic shield where, upon detection of an incoming RPG round, the charge held in a supercapacitor layer is released into the tank's hull. This forms a momentary electromagnetic field around the tank which destroys the round. Technically, this system could be used to deflect bullets, but MBTs have no issue with those anyway.

Yes, this is being developed in real life.

[Zakharra]

Meh. I'd say a Challenger II tank with SCAR* shielding would be pretty much invincible (barring direct nuclear strikes of course.)

* My own term, standing for SuperCapacitor Armour (Reactive), for a type of electromagnetic shield where, upon detection of an incoming RPG round, the charge held in a supercapacitor layer is released into the tank's hull. This forms a momentary electromagnetic field around the tank which destroys the round. Technically, this system could be used to deflect bullets, but MBTs have no issue with those anyway.

Yes, this is being developed in real life.

It wouldn't even come close. Anti-tank mines would still kill the tanks. Incoming missiles/RPGs and shells would still hit and incapacitate/kill the tank. The shield mentioned in the article might work for one incoming projectile, but not for all (where would the system get the energy? Unless this SCAR shield can physically -stop- projectiles in mid-air (doubtful since velocity alone is going to be a stone bitch to overcome) and defeat the shaped charges going off, all the SCAR is is expensive useless junk. And if a SCAR is up, wouldn't that both interfere with radio communications and send up a huge 'I AM HERE!' sign for radar and electromagnetic sensors? It would also be able to be beaten down with enough hits. No shield is imperious to everything.

[RedDwarfIV]

-snip-

It wouldn't even come close. Anti-tank mines would still kill the tanks. Incoming missiles/RPGs and shells would still hit and incapacitate/kill the tank. The shield mentioned in the article might work for one incoming projectile, but not for all (where would the system get the energy? Unless this SCAR shield can physically -stop- projectiles in mid-air (doubtful since velocity alone is going to be a stone bitch to overcome) and defeat the shaped charges going off, all the SCAR is is expensive useless junk. And if a SCAR is up, wouldn't that both interfere with radio communications and send up a huge 'I AM HERE!' sign for radar and electromagnetic sensors? It would also be able to be beaten down with enough hits. No shield is imperious to everything.

I refer you to the fact that one Challenger 2 survived 70 RPG hits. Another survived fourteen hits plus a MILAN anti-tank missile. The only one to ever have been destroyed in combat was killed by another Challenger. This is because the British armed forces favour armour over mobility in their tanks, hence why it's much tougher - but slower - than the Abrams. As for mines, I thought I'd read somewhere that an Abrams attempted to drive along a road that had numerous IEDs and was destroyed, while a Challenger made it over fine, However, since I can't find where I read it, you can take that with a grain of salt. In any case, in all the times a Challenger's armour has been penetrated by IEDs, it resulted in at most the loss of a leg for one of the crew. The Streetfighter upgrades should see to that, as they will be putting armour on the underside where the tank is weakest.

On SCAR:

Researchers at the Defence Science and Technology Laboratory (Dstl), which is the research and development arm of the Ministry of Defence, claim it is possible to incorporate material known as supercapacitors into armour of a vehicle to turn it into a kind of giant battery.
When a threat from incoming fire is detected by the vehicle, the energy stored in the supercapacitor can be rapidly dumped onto the metal plating on the outside of the vehicle, producing a strong electromagnetic field.
Scientists behind the project claim this would produce a momentary "force field" capable of repelling the incoming rounds and projectiles.
Although it would last for only a fraction of a second, if timed correctly it could prevent rocket propelled grenades, which detonate on impact, from reaching their target. The supercapacitor could then be rapidly recharged ready for another attack.

"The supercapacitor material can be charged up and then discharged in one powerful event to repel incoming fire.
"You would think this would require huge amounts of energy, but we have found it can be done with surprisingly small amounts of electrical power.
"Conventional armour is just a lump of metal but an RPG round can punch through more than a foot of steel. Carrying around enough armour to protect against that is extremely heavy.
"The real advantage to the electric armour is how light it can be by comparison."
Sophisticated tracking systems will also need to be developed to work in conjunction with the new armour so that incoming threats can be identified and the electrical discharge timed correctly to repel the rocket.
It is unlikely that such a system would be used against fire from small arms as the outer skin can be made to be bullet proof.

(Source: http://www.telegraph.co.uk/technology/n ... tists.html)

Since it's momentary, it's not going to interfere with communications. Besides, you know what else interferes with communications? Being dead. As for the 'I AM HERE' thing, I'll point out that we don't exactly build tanks for stealth.