Dragon. Cygnus. What’s the Difference?

Uber, Lyft. Both are rideshare services. Why do some people use one or the other? It’s typically differences like service availability, pricing variations,  or car/driver personalities.

Dragon, Cygnus. Both are rides to the International Space Station (ISS), so why do some payloads fly on one and not the other? Are they really that different? Does one have a furry pink mustache on the grill and the other doesn’t?

For a researcher flying a microgravity payload to the ISS, it’s good to know the similarities and differences when working with a payload developer. Here are some background, stats, and capabilities that can help clarify which may be the best one for your rideshare to space!


Orbital ATK Cygnus
Cygnus is an expendable cargo spacecraft currently launching to the ISS and was developed by Orbital Sciences ATK headquartered in Dulles, Virginia. Orbital ATK is a commercial company contracted by NASA through its Commercial Orbital Transportation Services (COTS) and Commercial Resupply Services (CRS) agreements to design, build and fly Cygnus. Orbital ATK was recently awarded a portion of the second phase of CRS, called CRS-2, to fly cargo to the ISS at least until 2014.

Nine Cygnus spacecraft have been launched since September 2013, with eight arriving at the ISS and seven delivering science cargo. The initial flight was a demo flight and in October 2014 one vehicle was destroyed when the the booster rocket failed shortly after launch.

Having the capacity to launch 3,500 kg in it’s ~25 m3 cylindrical (some say it looks like a beer keg) pressurized section, Cygnus can haul a lot of mail. While it has mostly been using only its pressurized payload volume for cargo, Cygnus has recently been flying cubesat deployers on the outside.


Grappling the Swan. The ISS robotic arm grabs or “grapples” the spacecraft shortly before attaching it to the ISS. The pressurized portion of Cygnus is the large silver cylinder. Connected below that are the service module and solar arrays. The white box is a NanoRacks cubesat deployer. Photo: NASA.


Cygnus can provide a standard suite of temperature control hardware, typically called “cold stowage,” for your samples or payload on the ride up. Temperatures ranging from -95C to +40C can be accommodated. They can even provide specialty temperatures, if needed.


Winter is Coming to the ISS. When flying your project, you have many options for temperature control. This service is contracted by NASA and available to any payload developer and are suited to fly on Cygnus or Dragon. Click to enlarge. Picture is from NASA Cold Stowage Brochure.


When it comes time to hand over your payload to NASA or your payload developer, two time slots are available: nominal load or late load time. These deadlines are written as “L (for launch) – (minus) (days/hours/weeks/months.)”

For example, the nominal load time for Cygnus is L-8 weeks, meaning you turn over your nominal payload 8 weeks before the scheduled launch date.

A nominal load time typically means your payload doesn’t have any special time critical storage requirements before or during the ride to the ISS. This loading time is commonly used for instruments, tools, crew clothing, or spare ISS parts for example.


Down the Hatch! In this pic, (looking down the long cylinder axis of Cygnus) the loading crew is using a crane to transport heavier payloads through the hatch and into the vehicle. Photo: Orbital ATK.


Late load time is the latest a payload can be delivered before launch for stowage on the spacecraft. Payloads needing this option have specific requirements, typically due to perishable items that have an expiration date or require a temperature controlled environment.

Examples of this may be life science experiments or food items. These are special cases and, because of that, are space limited. For many life science applications, late load is almost a given, but you still must articulate a need for it before receiving this service.

For Cygnus, the late load time is currently at ~L-5 days. The cargo loading process for Cygnus is done while the spacecraft is horizontal and loading is done front to back, either by hand or sometimes with a small crane to carry larger or heavier payloads inside. The cargo crew then attaches your project or clean underwear for the astronauts or whatever to the interior wall of the vehicle.


Interior View of Cygnus During Loading. This view is from Cygnus’ main (and only) hatch. After a long day of loading, these two cargo loading experts are kneeling and praying for a good launch. Photo credit: NASA/Ben Smegelsky.


Next, Cygnus is enclosed in a fairing that surrounds and protects it from aerodynamic, acoustic and thermal forces during launch. This 4 m x 10 m structure is normally jettisoned ~4 minutes after launch.


This Fairing Mating GIF Needs Narration by Sir David Attenborough. The fairing on the right is slowly rolled into position by hand (see the guy underneath!) around Cygnus (center). Cygnus is already mated to its service module (silver thingy), it’s second stage (black tube) and just beyond that, the first stage rocket. Animated GIF from this video.

Lastly, there is what’s called the loiter phase. After launch, there is a time between reaching orbit and catching up with the ISS. So the spacecraft “loiters” during this time. Average loiter time for Cygnus is 3.4 (± 1.2) days over seven missions.

The challenge for researchers is that the late load and loiter times for Cygnus are currently the longest for both vehicles. This can add up quickly. If you have a life science project with a specific shelf life and you handover at L-5 days, plus the 3-4 days loiter time, then another day before the astronauts unload and begin your experiment, it can be 9-10 days before it is started on the ISS.

Breaking Up is Not Hard To Do. Cygnus and its cargo of dirty astronaut underwear and other garbage becomes a beautiful streaking meteor upon re-entry. Photo: NASA

While Cygnus may burn up on re-entry, it is far from being wasteful and can carry a lot of ISS garbage along with it. Orbital ATK has also been working with companies and researchers that want to utilize the vehicle after it leaves the ISS.

The Houston based aerospace company NanoRacks has been deploying cubesats from Cygnus after it leaves the ISS since November of 2016 and NASA researchers have used it for a series of microgravity fire projects called Spacecraft Fire Experiment (SAFFIRE).


Cubsats! Pew! Pew! An external view of Cygnus with its solar array on the left and the NanoRacks deployer on the bottom right as it kicks out a cubesat. Full video here.


Overall, in it’s current configuration, Cygnus can haul a lot of pressurized cargo up to the ISS. It’s lengthy pre-processing flow doesn’t allow for many perishable science items to fly on it, but it has gotten better.

It also seems this spacecraft has plenty of potential uses for researchers and tech demos in the future and I look forward to seeing how it adapts to fit customer’s needs over the coming years.


Space-X Dragon
The Dragon spacecraft is constructed and flown by Space Exploration Technologies Corporation (SpaceX) of Hawthorne, California under the same NASA COTS and CRS/CRS-2 contracts. Like Cygnus, SpaceX is obligated to fly additional cargo flights to the ISS at least until 2024.

Dragon first arrived at the ISS with cargo in May of 2012 and there have been 10 more since, with one failure and loss of vehicle during its ascent in June of 2015.

Dragon has a gumdrop shaped pressurized capsule for internal payloads and a cylindrical external un-pressurized cargo section called a trunk. It can carry a whopping total of 6,000 kg of cargo in combination of internal and external payloads. The usable internal and external volumes are 11 m3 and 14 m3, respectively, for a combined usable payload volume of 25 m3.


Dragon Berthed to the ISS. The gumdrop shaped white part is the pressurized portion, while the trunk is just below with solar arrays extended. Photo: NASA


Science Junk in the Trunk. Underneath and inside the Dragon truck are several external projects ( Defense Department’s STP-H5,  and SAGE 3 instrument, its hexapod attach mechanism flown on CRS-10) mounted to the trunk. Once berthed to the ISS, these payloads will be extracted by the ISS’s robotic arm and then attached to the ISS. Photo: NASA


Dragon is loaded with nominal payloads while it is vertical, then late load is installed while horizontal. Its nominal load time is L-6 weeks.


I Like to Move It, Move It! This view is from the side hatch. Heavier payloads are brought in using a crane, through the main hatch seen above. Another shove and nudge will do it guys. Animated gif from this NASA video


Dragon doesn’t have a fairing like Cygnus, allowing for payloads to be packed relatively soon before it’s rolled out to the pad. Sometimes, when the weather allows, Dragon/Falcon are horizontal at the pad while loading late cargo.

Also in contrast to Cygnus, Dragon has relatively reasonable late load times of L-72h, L-48h and L-28h. The only vehicle that ever came close to that was the Space Shuttle at L-24h and with some extreme cases even closer to launch time.


Dragon and Falcon Basking in the Sun. A scissor lift truck parked next to to the Dragon clean room delivers late load payloads for loading onto Dragon CRS-8 flight in April 2016. This process is typically completed 12-15 hrs before launch. Photo: SpaceX.


Loiter time for Dragon is also, on average, better than Cygnus, at 2.3 (± 0.7) days over 11 flights. Temperature control for your samples/payload is available for all phases of flight.

The other important service Dragon performs is that it’s currently the only cargo vehicle capable of returning a significant about of items back to Earth. Returning up to 3,000 kg of payloads, samples and hardware, Dragon performs a vital service to the ISS microgravity research community.

There is a gotcha with this, though: after splashdown off the southern coast of California, the spacecraft is loaded onto a boat and rides to port. Lately, it has been arriving in port at about 24-30h after splashdown, but it can be up to three days, depending on weather and sea state. For many projects, especially live rodents, this can possibly alter your micro-g results.


Dragon Splashdown. This occurs around 3-4 hours after unberthing from the ISS. Photo: SpaceX.


I’m on a Dragon Boat! After retrieval, the ship motors to a port near Los Angeles to disperse its time critical Dragon treasure. The rest of the payloads will travel to a SpaceX facility in McGregor, TX for unloading there. Photo SpaceX


The good news is, once Dragon arrives in port you have the option of picking up your payload right there or have your payload developer overnight ship it to you. If you don’t need it that quickly, your project can be returned in 4-6 weeks, sometimes sooner. I have handed researchers their payload at the port and had them texting me pictures and data within a couple hours of pick up. I love seeing that.

Dragon has some overlapping capacities as Cygnus, but has some distinct differences as well. The late load time and return services make it attractive to many researchers. The external trunk portion of Dragon is great for hauling projects that need to be mounted on the outside of the ISS. So far, Bigelow Aerospace has made the most use of the trunk with it’s Bigelow Expandable Activity Module (BEAM), taking up the entire trunk volume and weighing in at 1, 413 kg.


One of the most difficult parts of planning for a microgravity research project is when a launch scrubs. This all too common occurrence adds precious time to an already long work flow and is unfortunately one of the many deal breakers for life science payloads.

Some life science projects like seeds or samples that can be frozen indefinitely are well suited for this, but others like cell samples, microbiology, or protein crystal growth just don’t like sitting around for a long duration.

The main distinction between Dragon and Cygnus for scrub scenarios is, as mentioned previously, the Dragon capsule is accessible through the side hatch when the rocket is horizontal. After a launch attempt or two, the rocket is made safe, brought horizontal and sensitive samples are swapped out. 12-15 hours after the scrub the investigator or payload developer gets 15-30 min to swap hardware or samples and they are reloaded onto Dragon.

The fairing around Cygnus lacks an access port, so there is no way to enter the vehicle with it installed. The rocket has to be rolled back to the processing facility and the whole fairing has to be removed.

How long does this take? I don’t know. While there have been delays and scrubs for Cygnus, in eight launch attempts a full rollback and re-load has only ever been done once during Orb-1. And that roll back and de-stow was ollowed by a 23 day delay until it launched.


Which Spacecraft Is Right for Your Research Payload?
When you work with a payload developer, they will help you with this decision process. There is a whole suite of paperwork and matrices that NASA and its contractors use to determine what vehicle is best for you and when there is room on the manifest for it.

So, all of the above was just to say: it really comes down to how delicate your samples are.

Are they stable or can be stabilized for at least 10 days or more? Then Cygnus or Dragon will work for you.

If you absolutely require the latest load time available or need your samples back, then Dragon is the only ship for you.

Here is a nice side by side comparison of the vehicles:


Side-by-Side Graphical Representation of Dragon and Cygnus to Scale. Currently, only the extended versions are used. Red = pressurized volume. Yellow = un-pressurized. Graphic by Craigboy, modified from original creative commons  Wiki pic.

Comparison Table for Cygnus and Dragon.


But Wait, There’s More! New ISS Cargo Transport Option on the Horizon!
One of my biggest gripes with the current capabilities of these cargo ships is that there is currently only one that has reasonable late load times and can return a substantial amount of pressurized cargo back to Earth.

Well, I’m hoping those challenges of the research community will be eased in a new cargo vehicle: Sierra Nevada Corporation’s Dream Chaser Cargo System. Allow me to swoon a bit.

Awarded as part of the second phase of the CRS funding selections in January 2016, Sierra Nevada is obligated to provide six flights to the ISS.

Dream Chaser is a lifting body type spacecraft that looks like a baby shuttle. It will be even be reusable like a shuttle in that it can return from space with a gentle 1.5g flight path and land on a 3, 000 m runway.

With wings that fold up, it will be able to fit into the fairing of several types of rockets. To get around the shortcoming that Cygnus has, the fairing will have an access port so that late load payloads can be added far into the launch preparations, even possibly while on the pad.


Dream Chaser Cargo. Cargo carrying mini-space plane in the front, with a service module and solar arrays in the back. Photo: Sierra Nevada Corporation

Sierra Nevada is advertising Dream Chaser will have the capacity to lift up to 5,500 kg of pressurized and 500 kg un-pressurized cargo to the ISS. It will also be able to return 1,750 kg of payloads from the ISS and since the spacecraft will also use non-toxic chemicals for its systems, so safing the vehicle after landing will allow for very quick recovery of time critical payloads.


Dream Chaser After Landing During an Un-powered Glide Test. It’s so cute! Good pic to give you a sense of its size. Photo: Sierra Nevada Corporation


Dream Chaser Cargo just had its first successful glide test and is on target for its first flight to the ISS in 2020. The value of having a cargo vehicle that can do all of that will be a great asset to the microgravity research community and I simply cannot wait.



More on NASA CRS

Orbital ATK Website

SpaceX Website

Data for cargo mass averages from NASA CRS Media Releases

How Many Research Payloads Have Flown to the ISS?

I get asked this question every once in a while and I finally got some free time over Christmas break to try and track an answer down. I quickly found a NASA brochure online titled “International Space Station Utilization Statistics Expedition 0 – 44 December 1998 – September 2015” . Seemed like a good place to start and right on the first page I found this:

Easy peasy. “Total investigations from ISS Expeditions 0-44 is 2,060.” There. Finally, an answer I can give those who ask. But is an “investigation” the same as a payload? They give the definition of investigation in the paragraph above the table, but it’s kind of loose and doesn’t really say something is specifically a physical research payload or not.

NASA also maintains a website with a list of experiments by expedition (and a few other categories) and I thought of comparing the number of payloads on that site to the number above. After some Excel wrangling I came up with the following:


Example of raw data from NASA site. Pink cells indicate duplicate entry. Using these data, the total number of payloads (red circle) is 3, 818 since Expedition 0 in 2000. (Click pic to make larger)


Well, the total of 3,818 from the data on that page is a bit larger than the 2,060 reported in the brochure. Even if you include the three additional expeditions since that publication, it only adds up to 2,609. Also, the values I have don’t match the values they have for “Total Investigations” during each expedition increment.

A quick check for duplicate values in the data (pink in the Excel sheet above) showed that many of the experiments are repeated through expeditions. For example, Made In Space’s 3D printer is listed three times in the data (shown as “3D Printing in Zero-G”) because it ran multiple times during several expeditions. I learned from a NASA friend that they call each payload use an “interaction” and as the data above shows, there can be several interactions with a payload over multiple increments. Still don’t know why the totals don’t match up, though. NASA’s data in their brochure doesn’t even match up with the data shown on their website. Alrighty.

Curiouser and curiouser (or more like: nerdier and nerdier), I edited out the duplicates and came up with what I think is a much more reasonable number.


Example of data from NASA site with duplicates removed. Total (in red circle) shows 1,062 payloads since Expedition 0 in 2000. (Click pic to make larger)


OK, 1,062 payloads ~16 years seems like a more reasonable estimate to me. Plotted as a bar graph,

Data from above Excel sheet plotted as a bar graph. Expedition 0 (September 2000-November 2000) on far left of the horizontal axis to Expedition 49/50 (September 2016-February 2017) on the right.  Note that early expeditions were shorter duration. (Click pic to make larger)

It’s great to see a slight upward trend in new payloads since late 2013-early 2014. Since I was already in the thralls of Excel ecstasy, I was wondering what caused the spikes of new payloads, especially between March 2014-September 2015. I plotted this data along with the number of visiting payload vehicles during the increment.

Plot of number of new payloads during increment (blue bars, values on left vertical axis) with number of visiting vehicles during the increment (orange line, values on right vertical axis). Red arrows show significant events during an increment. (Click pic to make larger)

The peaks correlate nicely to a very busy time on the ISS between March 2014 and September 2015 with 12 vehicles arriving: SpaceX had 4 Dragon visits, Orbital had one Cygnus visit, there was 1 ESA ATV and JAXA HTV visit each, and finally 5 Progress vehicles (see below).  The other peaks seen at October 2007-April 2008, April 2009-October 2009 and March 2011-September 2011 correlate to ATV-1, HTV-1, and STS-134, STS-135 ATV-3, respectively.

This certainly shows the value of Dragon and Cygnus at being able to carry a significant amount of science payloads. Dragon is particularity useful for life sciences, not only because of its ability to return samples, but because researchers can deliver their samples to be loaded in the capsule at ~30 hours before launch.


Overall, it looks like a reasonable number for the amount of research payloads flown to the ISS in 16 years is ~1,000. I hesitate to say the number is exactly 1,062 as calculated, because I may be missing something (maybe NASA has an explanation for the numbers they published) and some of the payloads listed on the NASA site are not really research payloads (such as “Crew Earth Observation” or “Story Time From Space”).

We can also see from these data the significant value Dragon and Cygnus bring to the research community. This value will no doubt increase again hopefully in 2019 when Sierra Nevada flies their Dream Chaser cargo vehicle to the ISS, adding more late sample loading and return capabilities. Now if we could just get more crew up there to work on all of this science…