Talk:Orbital airship

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[edit] Which unit?

"one dollar per ton per mile of altitude" - which type of ton is this referring to? That should be pointed out, also, a si-conversion should be available as well. Ran4 (talk) 21:21, 29 January 2008 (UTC)

[edit] Talk

I think that it will be a long time before we have the technology to make something like this work. Current ion propulsion spacecraft would not be able to get anywhere near orbital velocity in just 3-9 days, even if they did not have to worry about atmospheric drag.

Also, it seemed strange that they are planning on using fuel cells to power an ion engine. Wouldn't it be simpler and more efficient to just to use a conventional chemical rocket engine?--Todd Kloos 01:34, 31 May 2004 (UTC)

Well, I'm not the one to do these calculations, so you'd have to talk to JP Aerospace for more details. However, as for the fuel cells issue, that's an interrim step; the final design is to have thin-film solar cells, which already exist, coating the blimp's surface. They're inefficient, but there is tons of surface area to collect light.
I can do some quick calculations for the ion engine here (if you'll let me neglect air resistance! It should be negligable for most of the trip if you're past the critical thrust/resistance threshold; the main difficulty would be early on). A LEO orbital velocity is generally around 7500 km/h. Lets assume that the blimp is to carry 500 kg of ion thrusters, and it itself weighs a total of 10,000 kg including helium and payload (just pure guestimation here, I don't have any numbers for it). Using modern ion engines or hall effect thrusters, you could probably get about 1 newton per 20 kg of thrusters/fuel combined. That would mean that it would provide 25 newtons of thrust to the blimp. In 9 days (777600 seconds), the blimp would be going 1944 m/s. So, while we get numbers that are pretty small, they're not completely out of the ballpark or anything. If the power/mass of ion or hall engine propulsion improves, or if the thrusters take up a larger percentage of the blimp's mass than 1/20th, 9 days seems quite reasonable. I'm not so sure about 3 days, though.... Rei 16:18, 1 Jun 2004 (UTC)
Doing a quick web search, I think I see how they got better numbers than I did. The ratio of payload to frame/envelope mass is much higher than I thought in most airships. For example, with the airship US LTA Model 138S, we have a gross weight of 4213kg and an empty weight of 2673kg [1]. Even assuming that they're not counting the helium mass, the mass of 510 m^3 of helium at STP (0.176 kg/m^3) is 89.76, so it's almost negligable. I think this explains how they got their "3 to 9 days' number - they probably plan to have at least 1/5 of the mass being ion thrusters. Of course, that would sap a huge amount of power, but thin-film solar cells across the whole surface of such a gigantic (but incredibly thin) helium envelope would produce enough energy to power a small city.
I have my doubts as to whether they'd be able to pull it off, also, but I think their math is at least theoretically sound. Although we have ignored air resistance thusfar... do you know any good way to calculate the air resistance? Rei 17:28, 1 Jun 2004 (UTC)
I wrote a quick space blimp simulator (I'm kinda proud of myself). However, it reaffirmed your concerns, Todd. The problem is that, even in the very thin air, there's just way too much resistance for an object with such a large cross section. I've mailed JP Aerospace to ask them for comment, so hopefully I can get hard figures to run in the simulation instead of just guesses. I'm attaching the simulator below. The physics should be good in almost all cases, although I had to guess on how to handle the lift in such a case (I just simply allocate a percent of the drag to lift), and it doesn't take into account the change in atmospheric composition in high altitudes (for example, you start getting more elemental oxygen, less CO2, etc). Also, the ion engines/fuel here are assumed to stay the same weight for the whole trip, even though they are burning up their fuel (however slowly):
I'm not sure how they are planing on overcoming the air drag. At 60km up there is very little air, but since the ship has to float that far up using boancy, there isnt going to be a gain from the thin atmosphere. the higher they start, the larger the ship has to be, and therefore the total drag inscreases from skin drag and frontal area, wiping out any high altitude gains. When they start the engines there is still going to be a huge amout of drag. I think the real desgin factor is creating a wing with an insanely good lift to drag ratio. The help from the boancy will be quickly exausted as they start to climb and it will be up to the wing shape to provide the nessisary lift. This, from my understanding is because lift, drag, and air density scale linerarly. Going 7800 m/s at 100 km up, is roughly (extrem symplification) as going 1 m/s at 50 km up, so if your wing can't lift you at 1 m/s 50 km up, it won't get you up to orbital speed without larger drag inseases, wich would probably start to melt the wing. Cchandler 15:54, 25 June 2006 (UTC)
#!/usr/bin/python

from math import *

#User-configured constants
E=0.05          #Engine efficiency (N/kg)
Me=1000.0       #Mass of engines and propellant combined (kg)
Mp=1000.0       #Mass of payload (kg)
Mf=1000.0       #Mass of frame and envelope (kg)
C=0.015 #Numerical drag coefficient (horizontal motion)
C2=0.04         #The same, but for vertical motion.
DragToLift=0.4  #The amount of horizontal drag that is converted to vertical lif
t.
DragToVel=0.2   #The amount of vertical drag that is converted to horizontal thr
ust
#Assume two cylinders in a V, each 2000 meters long and 150 meters wide, spaced
to 1500 meters in the end.
volume=71000000 #Volume of the envelope (m^3).  For an ellipsoid, Pi/6*d1*d2*d3
A=225000        #The cross sectional area (front) (m^2).  For an ellipsoid, Pi*d
1*d2/4
A2=400000       #the cross sectional area (top) (m^2).  For an ellipsoid, Pi*d1*
d3/4

#Other constants
Pi=3.14159265                   #Pi
e=2.718281828                   #e
G=6.672e-11                     #Universal gravitational constant
d0=1.29                         #Density of air at sea level (kg/m^3)
Mearth=5.97e24                  #Mass of the earth (kg)
Rearth=6.3781e6                 #Radius of the earth (m)
HAratio=.179/1.29               #The ratio of the densities of helium and air
endday=3                        #When to stop thrust (4 entries)
endhour=0                       #
endminute=0                     #
endsecond=0                     #

#Starting conditions
alt=40000
altvel=0
v=0
seconds=0
minutes=0
hours=0
days=0
thrust=True

#Loop!  One click equals one second.  Print every hour
while True:
        Da=d0*pow(e,-alt*1.43e-4)
        Dh=Da*HAratio
        Ma=Da*volume
        Mh=Dh*volume
        mass=Me+Mp+Mf+Mh

        drag=0.5*C*Da*A*v*v/mass
        altdrag=(0.5*C2*Da*A2*altvel*altvel)/mass
        if altvel<0:
                altdrag=-altdrag
        if altdrag>abs(altvel):
                altvel=0
        else:
                altvel-=altdrag

        if thrust==True:
                accel=E*Me/mass
        else:
                accel=0
        v+=accel+DragToVel*altdrag-drag
        if v<0:
                v=0

        ovel=sqrt(G*Mearth/(alt+Rearth))
        gaccel=(Mearth*G)/((alt+Rearth)*(alt+Rearth))
        lift=(Ma-Mh)*gaccel
        weight=mass*((ovel-v)/ovel)*gaccel
        altvel+=(lift-weight)/mass+DragToLift*drag
        alt=alt+altvel
        if alt<0:
                alt=0
                altvel=0

        seconds=seconds+1
        if seconds==60:
                seconds=0
                minutes=minutes+1
                if minutes==60:
                        minutes=0
                        hours=hours+1
                        if hours==24:
                                hours=0
                                days=days+1

                print "%d:%d:%d.%d) velocity=%f m/s, altitude=%f km" % (days,hou
rs,minutes,seconds,v,(alt/1000))

        if days>=endday and hours>=endhour and minutes>=endminute and seconds>=e
ndsecond:
                thrust=False

(To not have the code formatted all strangely by the wiki, you'll have to edit the window). Rei 23:48, 4 Jun 2004 (UTC)

[edit] Title of article

Is "space blimp" the right title for this article? JP Aerospace calls it "Airship To Orbit" (ATO), so maybe "orbital airship" is a better title. Astudent 11:34, 2004 Jun 14 (UTC)

Completed move to "Orbital airship" Astudent 07:35, 2004 Jun 17 (UTC)

[edit] Confusion

In the second section, there's a sentence claiming that the proposed method of propulsion is faulty and that: "This is not a matter of technology, it is a matter of fundamental physics." However, the following explanation rests on practical limitations of technology (e.g., current inefficiency of spray-on solar cells) rather than fundamental science; can someone fill in why this is not simply an engineering problem? siafu 23:34, 3 August 2005 (UTC)

It would seem to me that 10Mwe is plenty of energy (although I really don't know if you could get that much). The efficiency of current thrusters may be low, but what about VASIMR technology? That would be the ideal thruster: it can handle high power levels and can trade off Isp and thrust, and seems to be very efficient. Currently these must use hydrogen, but perhaps atmospheric nitrogen could be captured/compressed and used instead. It may be that JP is waiting for that technology to develop.

Which really makes it a simple engineering limitation and not a fundamental physical one. siafu 21:20, 24 September 2005 (UTC)

[edit] NPOV?

Is the Skepticism section unbiased? Maybe a few changes of word choice would be appropriate. Scourgeofsmallishinsects 15:06, 27 March 2006 (UTC)

Contrary to many peoples assumptions, NPOV in the wikipedia works by people expressing their contrary opinions, not by aiming to be unbiased (although it is good to do that). Unless what is written is provably inaccurate it probably should be left. Feel free to add well-based contrary opinions.WolfKeeper 19:56, 27 March 2006 (UTC)

I was just thinking of a few grammatical changes. Scourgeofsmallishinsects 15:15, 29 March 2006 (UTC)

I would have to recommend more than a few grammatical changes in the skepticism section. References would be desirable. Is Robert Pickar's independent analysis available? To my knowledge, the figures given in this section of the article were never released by JP Aerospace. Are they from Pickar's analysis? Also, while JP Aerospace has released statements about fuel cells in their suborbital testbed vehicles, I know of no JPA statements indicating that their final design would employ them for nighttime power. Does this claim have a source? Definitions for the phrases "overcome atmospheric drag" and "Thrust lower than atmospheric drag" would also help clarify the article. (Drag is created by motion through a fluid. Any discussion of its intensity requires information about that motion.) The definition problem might be solved with a simple reference back to the original analysis, though. --C M Edwards 19:24, 22 May 2006 (UTC)

I've edited the skepticism section. I've deleted the unreferenced figures for the available thrust and the atmospheric drag, and replaced them with elementary calculations. If anyone knows where Robert Pickar's analysis is, then please put a link to it. I would particularly like to know where the figure of 11,000N drag came from. —Preceding unsigned comment added by 213.162.124.166 (talk) 15:52, August 28, 2007 (UTC)

[edit] Hypersonic issues

I'm loathe to add anything based on this because I don't have any references, but has anyone considered the effects of hypersonic heating on something trying to achieve orbital velocities within the atmosphere? I know that 40+ km up is decidedly rarified but one would think that Mach 25 heating would be a concern. --The Centipede 20:07, 3 June 2006 (UTC)

I belive that they are atempting to get around the heating issue by starting from 60 km up at a standstill. then the ship will slowly begin to acellerate and climb at the same time. I don't know it this will work, neutral boanycy at that alltitude is a trick in it of it's self. However, if it did work, there is no reason they couldn't climb fast enough so that the speed increase and it's speed cancel themselves out. 24.137.78.34 23:06, 21 June 2006 (UTC)

Not to mention, the faster the airship goes, its weight will decrease as it gets closer and closer to freefall. So as it gains speed, it increases drag, which you offset by increasing altitude and thinning out the atmosphere. But thinner air decreases buoyancy from the helium, which you offset by going faster and decreasing gravitational weight. A neat, cyclical little problem. They have to find the sweet spot between air resistance, thrust, altitude, payload, airship size, and travel time that will allow them to accelerate constantly up to ~29,000 kph. Dyolf Knip 05:34, 28 January 2007 (UTC)

There is an important component missing in the above analysis, namely lift. As soon as the ship picks up speed, it will be subject to drag AND lift. The lift/drag ratio of even a flat airfoil is at least 5 at all speeds, if the optimal attack angle is maintained. At low speed, with optimized airfoil shape, lift/drag ratio can be above 20. I think that means that lift starts to dominate buoyancy even at fairly low speed, and can be used to move the airship as high as needed to overcome drag. I would not be surprised if it would turn out advantageous to get rid of the lifting gas entirely and operate the orbiter as a powered kite skimming the upper reaches of the atmosphere. Andreas 14:55, 21 June 2007 (UTC)

On the other hand, even with a lift/drag ratio of 20 (which cannot be maintained at supersonic speed) you would still need 1/20th of a g in acceleration just to counteract the drag while keeping the ship from sinking (without buoyancy). Ion engines today can produce no more than 1/1000th of a g. Thus, absent much better propulsion technology, buoyancy needs to play a major role throughout the flight. I am skeptical that any shape exists that can produce the needed combination of high buoyancy and low drag at hypersonic speeds, much less one that can be engineered to hold together and carry propulsion system and payload. Since buoyancy is proportional to volume, and drag proportional to cross-section in the direction of travel, I imagine the best bet would be a single very long cylindrical tube moving longitudinally like an arrow. It appears critical that slip resistance be kept low, which would dictate a very smooth outer surface. Andreas 17:37, 21 June 2007 (UTC)

[edit] LEO orbital velocity

REI mentioned that LEO orbital velocity is generally around 7500 km/h. I think it is 7905 m/s or 28,458 km/h.

I was just looking through this article about how helium starts to weight something once you get near orbit and I was wondering whether a 'vacum' air ship would work any better. Also to deal with the 'impossible to deal with pressure' why to only make the secondary ballon (one that goes into orbit) a 'vacum' airship (built in orbit of of course) and get the first airship to bring the cargo up to a level that the second airship can with stand. Well thats my idea anyway.

Yes, a vacuum airship would technically have more lift, but it's not really worth the hassle. Normal air is 78% Nitrogen (molecular weight 28) and 21% Oxygen (molecular weight 32); ignoring the trace elements and assuming that ratio doesn't deviate overly much in the upper atmosphere, it is a mixture with a molecular weight of 28.56. Hydrogen, on the other hand, has a molecular weight of 2. Which means that at the same temperature and pressure, a given volume of H2 will mass 93% less than the same volume of air. Vacuum of course would have no mass at all, but for all the muss and fuss of a shell capable of withstanding outside pressure, you get an extra 7% lift. Whoopty do. Dyolf Knip 20:04, 21 May 2007 (UTC)

[edit] Hydrogen?

Why not use Hydrogen for both lift and power?

Any LTA vehicle must vent lifting gas as it climbs and atmospheric pressure goes down. If gas is not vented, a pressure differential will develop that could rupture the envelope. If the envelope is strong enough to withstand the pressure and remain at a constant volume, eventually the weight differential between the thinning air outside and the gas inside will be reduced to zero and no more lift will be generated. Some balloons have been designed to take advantage of this to remain floating at a fixed altitude for extended periods of time. For most LTA craft, the only practical solution is to vent lifting gas as altitude is gained.

If the first stage Ascender used Hydrogen, instead of Helium, it could use the excess gas created by the climb to altitude to power jet engines. If the Ascender managed to dock with the Station in a slightly overpressured state, excess Hydrogen could be transferred and stored for use in second-stage Ascender vehicles.

Second-stage Ascenders might be flown in an initially-overpressured state, using their fuel to add rigidity and strength to the hull until extremely high altitudes are reached. Engines able to function as either turbojets or rockets would be required. For the final insertion into orbit, far above altitudes where jet engines can be used, rocket power would require an onboard supply of pressurized oxygen.

The appeal of this scheme is that it: 1. Uses otherwise-wasted lifting gas to generate thrust; 2. Allows use of Hydrogen-burning technologies we have already mastered (at least on the rocket side); 3. Provides the higher thrusts that are available from chemical reactions (combustion); 4. Avoids the need to carry either extra fuel, enormous photocells, or other heavy electric-drive components.

This is all very reasonable, but if you think it through, you end up with the old rocket equation problem. You have to bring enough fuel to accelerate the ship to orbital velocity, and accelerate most of the fuel as well. You might as well use a conventional rocket, without all the LTA complications. The only thing that makes the ATO idea potentially useful is that it enables the use of high ISP engines to avoid the shackles of the rocket equation. Andreas 17:52, 21 June 2007 (UTC)

Safety? Hopefully modern technology could allow us to avoid the “Hindenburg scenario”.

By the way, the problems of returning from orbital speeds to the relatively unmoving Station have not been given as much attention here. It has been said that the second Ascender could be much less robust than the first stage, because it does not have to survive passage through the lower atmosphere. But it would have to survive some form of aero-braking to decelerate from orbital speeds, which could be quite a stressful ride in its own right.

Drunyan8315 17:43, 31 March 2007 (UTC)

Cool idea. Though I think many ultra-high altitude balloons are made to expand as they go up, so as to automatically offset the decreased mass of displaced air by increasing the contained volume of lift gas. It contains the same (or nearly) amount of gas from low to high, but the widening pressure differential makes the balloon expand like, well, a balloon, maintaining both pressure equilibrium and constant lift. But no reason H2 as lift gas couldn't be used as fuel in stages where thrust is more important than lift, or as an emergency fuel source or whatever. Dyolf Knip 20:19, 21 May 2007 (UTC)

One reason the H2 could not be used for fuel or whatever is that at the altitudes envisioned, the gas would be extremely thin and large pumps would be needed to compress it into a form that would be useful for burning or anything else. Andreas 18:28, 21 June 2007 (UTC)