Tuesday, November 16, 2010

It's Only a Model

So, I've been interested for a while now in Bigelow Aerospace's TransHab-derived module designs. The concept of a module that it is compact when it needs to be at launch, yet larger when its internal volume is put to use in orbit. The solution is elegant in a way that I really like, and so I've been following their designs with great interest. However, nobody is perfect, and they seem to have messed something up in the display model they had at the International Symposium on Personal and Commercial Spaceflight 2010. Specifically, their model seems to be representing something different than what they said it did.

Does this really show a 100-ton 2100 cubic meter module?

I've been working on a calculator for the size and mass of Bigelow modules of certain parameters. I first got interested when I noticed that there's some fairly consistent information about Bigelow's modules, and it plays a role here. The diagram below shows some dimensions relevant to TransHab-derived modules.
Reference Dimensions of Bigelow Inflatable Modules
Model and image created by Rob Davidoff
The inflated diameter is generally about three times the diameter of the rigid core, which means the radius of the circular section of the inflated section is 1/3 the inflated diameter. If the rounded portion's cross section is approximated as a section of a circle (as I have in my model), then the volume of the module can be related to the diameter and the ratio of the length to the diameter. Data on volume and mass is available for several of Bigelow's other designs: Both Sundancer, which is to be their first man-rated module to go to orbit, and the BA-330, which will join it to become part of their first space station, have a launch mass equal to around 45-55 kg per cubic meter. With this value, if the volume of the module is known, the mass can be calculated. (I've been working on a calculator to automate this, but it's not ready yet. Some of the solutions of the equations for volume are a bit nasty.)

So, anyway, at ISPCS, Bigelow was showing off a model of what they claimed to be a 2100-cubic-meter, 100-ton module capable of being launched in an 8-meter fairing. They also said they have a similar design for a module that would weigh 70 tons and be 1150 cubic meters when deployed. How do these claims compare to the math, and does the model they were showing off actually represent what they claim?

First, the density check. Do the volumes cited match the masses? At 100 tons, the 2100 cubic meter module would have a density of 47.6 kg/m^3, which is in the range mentioned above. At 70 tons, the 1150 cubic meter module would have a density of 60.8 kg/m^3, so that may actually be a conservative value for how much volume could be contained by a 70-ton module; something more like 1250 cubic meters is possible if you use 55 kg/m^3. So the figures for what really matter, the mass and volume on orbit, seem right, if maybe even a bit conservative for the 70-ton module.

However, the volume they claim for the model module doesn't seem to reflect these figures. The launch diameter of the BA-2100 showed at ISPCS was supposed to be something like 7 meters (figure some clearance for an 8 m fairing), so I worked up a model of the module in AutoCAD Inventor using the observations from above and images of the model. Have a look at the figure below, which compares the ISPCS display model and a similar image of the CAD model I created.
Left: Bigelow "BA-2100" Model at ISPCS 2010.   Right: AutoCAD Inventor Model Approximating Model to Left
Inventor has the ability to calculate the volume of a solid model, which tells me that the model I created has a volume of 2771.5 cubic meters. Now, the real module that the model depicts would have some skin thickness which would subtract from the volume, but the module the model depicts is almost 600 cubic meters bigger than the volume it is supposed to contain. I don't think the skin makes a difference equal to 3/4 the volume of the International Space Station, so the model does not appear to accurately represent a BA-2100.

The difference becomes even clearer when an actual 2100 cubic meter, 7.6 meter module is compared to the modeled monster module. Below is an image showing representations of a Sundancer and a BA-330 docked to a pair of nodes modeled after Nodes 2 and 3 of the International Space Station. Also docked are the BA-1150 and BA-2100 models I created with accurate volumes, and the monster module they were showing off at ISPCS. All modules have inflated diameters and a launch mass (estimated from density in the case of monster module) noted.
Comparison image of AutoCad Inventor models of various Bigelow inflatable modules
Models and Image by Rob Davidoff
So, it's clear that the module shown in the model and the BA-2100 they talked about are not one and the same. They bear about the same resemblance as the BA-330 does to Sundancer: same diameter, but different lengths. However, the notion of a 100-ton module with a 7 meter diameter and a volume of 2100 cubic meters checks out, and the 70-ton smaller module with 1150 cubic meters of volume is perhaps even conservative (1250 cubic meters may be a better estimate). So the patter was right, but there was discrepancy in the model. What's the source of the erroneous model? My guess is that the model, which shows a module massing about 150 tons, is old and prepared based on a design intended to max out the probably-now-history Ares V. They revised the mass and volume figures to fit on the new SLS launcher (100 tons with upper stage, 70 tons core-only), but they're still using the same old model. Thus, like everything else associated with Ares V, it fails to fit reality.

I care about this because Bigelow is one of the leaders in private spaceflight, maybe even one of the top five or so key companies in my mind. Unlike most entrants to the field, they aren't focusing on the launch vehicles; they're focusing on the payloads and what we'll do when the launch market exists. That's a big deal and a major part of making sure there's someplace to go when the vehicles come into being. The fact that this is possible is a major reason why I am so astounded with the work they do. I am amazed with what they do because it is indeed now possible for a private company to talk about selling space stations for about $500 million on delivery and contemplate building new stations larger than any currently in existence, funded not by governments but paying for themselves. However, I hope that next time they come to an event like this, they can make sure that the models they bring of the cool stuff they're doing are current, or that they at least make clear where the model and the current vision differ. Images of this BA-2100 went all over the place in the space community, from Popular Mechanics to the NASAspaceflight.com forums. I even talked about it here on Engineer in Progress last month, and there's already plenty of misinformation out there stemming from just the actually correct parts.Some people missed the distinction between the 70-ton BA-1150 and the 100-ton BA-2100, and ended up confusing the two. It'd be nice if all those images actually matched what Bigelow is really capable of, rather than a payload for a now-cancelled rocket.

I'm kind of busy today, so your closing link is just a music video for Weird Al Yankovic's great song Everything You know Is Wrong.Why? Because it's a fun song. Enjoy.


  1. This is excellent work. I agreed with you up until seeing your design for the BA-1150(1250) model. It would also be possibly to get the BA-2100 simply by stretching the BA-1150(1250). Without designing the bigger diameter BA-2100.

  2. Rich, thanks for your comment.

    I created the BA-2100 model in this post by shrinking the ISPCS module model I made along its axis (essentially cutting away the barrel length) until it fit the 2100 m^3 parameters. However, to shrink it further to the volume of a BA-1250 would require a length-to-diameter ratio of less than 1, almost on the edge of the 0.667 level where the "barrel" portion goes to non-existent.

    If they were to actually build a BA-2100 and BA-1250, I do suspect that they would be based on a common diameter around 13 m as you say. Thanks for your interest. Have you seen my Transhab Module Calculator?

  3. Very interesting-

    I do believe though Bigelow should stick with their Sundancer and BA-330. Once there launched and we have a heavy launch vehical, then they could proceed with the bigger models.

    I'm waiting to see how every thing is layed out on the Sundancer and BA-330