Talk:Orbital airship
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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)
Contents |
[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)
[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)
[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.
