Building from the outside in for all destinations
Building a complex vehicle like a Single Stage To Orbit interplanetary vehicle with multiple reuse cycles in mind is a challenge. There is are a lot of capable systems (life support, refueling, landing gear, reusable heat shields, navigation and communications) that have to be integrated into your final design before you can set of to your end goal destination, like MARS or one of the Moons of Jupiter and Saturn. I also don’t want to shed parts of my basic vehicle. The idea being to have one single object able to ferry to different parts of the solar system and land about 20 metric tons of payload on Mars and Moon.
When designing this, I noticed that only on Earth would you need to add some extra thrust capability to your vehicle. In our design case this means that we would add a central drop-tank at the bottom of the vehicle and in between the conformal engines of this upper stage, which remains a true SSTO when it lifts up from all moon and planetary surfaces we want to visit in the near future. On Earth however, the same engine package goes full thrust and uses the extra fuel from the drop-tank that can be either simple drop-tank or an engined drop-tank with some extra thrusting and even reuse capability, analogous to SpaceX’s F9 first stage or Jeff Bezos’s New Shepard design.
If you do all of this -and this is only an engineering project, not an impossible technical challenge- one comes to the conclusion that all destination can be served with just one vehicle concept.
Prevent long spiky investment phases
If you want to design and integrate all these basic functions into your vehicle from start, you end up with a huge investment spike at the start of your project, while you are forced to wait a very long time before you can fly your first mission or test a single of your subsystems in its operational environment or a similar environment in Earth Orbit. Long investment periods without a quick return would scare off most investors and that is not what you need if you want to successfully pitch in a round A investment meeting.
By using a smart design philosophy this can be prevented. One could choose to build from the outside in. As always one needs to design with the end goal in mind, but here we go a little bit further. You do not start with designing a sub-scale demonstrator, to test some parts, but you immediately start at the size of the final vehicle. Basically, in this design concept you start with the outside shell of the space vehicle, which is an almost empty cavernous structure, define its volumetric and weight parameters, examine what it would eventually need to be capable of enveloping or transporting, and determine how this would influence your center of gravity and some other basic parameters like thrust profile, drag, thrust requirements, fuel choice and engine mode (e.g. pressure fed, full flow combustion, aerospike, e.a.).
Step one: SSTO
After building a cavernous shell that will fulfill all your desired future volumetric requirements, you build the power package that will always consist of multiple engines, and you provide for enough room and design constraints for the modules of this power package (a single methalox engine) of which a given multiple will be installed in accordance with your end goal performance requirements (e.g. transport a load of 50 metric tons to LEO, or lift of 50 metric tons from the Moon, which could be either your empty vehicle or your vehicle including cargo and crew).
When you design an interplanetary SSTO, your vehicle actually is the crew shelter for the duration of the mission, both in transit and on the surface of your destination, e.g. the Mars or the Moon. This means that using part of your weight budget to strengthen the hull or provide for an extra safety margin, is always beneficial to the survival and safety of the crew. Off course the price being that sometimes you would have to loft some extra fuel, but it will be much cheaper to launch an extra mission of a reusable vehicle than to design a fleet of cooperating vehicles, each designed for optimal performance. To put it even more simply: An extra load of propellant is cheap, designing a new vehicle is very costly.
However, at this point in our design progression, you are still dealing with an empty shell that is outfitted with an engine package and your avionics, navigation and communications package. Thankfully, this means that you have already, with this act, developed a useful vehicle that can loft payloads into orbit. Without further ado you can start earning money from customer payloads and through selling some testing and other valuable data. Your company is on its way to turn into the black. Because we are looking to develop a SSTO vehicle, we must always design to fulfill that basic requirement. Because we are planning for a variety of missions, our basic design will most likely be overpowered. This means we can choose to shed some of the engines on these flights or simply turn them off during the ascent. For redundancy and crew safety purposes we project to have up to 18 engines on that stage. On descent, they are less exposed to the free air stream, and experience a more benign heat pulse, than is the case with the SpaceX and Blue Origin designs.
Step Two: reusable SSTO
As SpaceX demonstrated, the next stage consists in progressively making constituent parts and components of your vehicle fit for reuse up until the entire vehicle can be reused. In the case of the proposed MARS vehicle, reuse would always mean reuse of the entire vehicle, so right after the first customers have been served and the first cash is earned the same materials and coatings that are publicly available to SpaceX and that it will start experimenting for the reuse of their upper vehicle stages can also be bought and used from European providers.
The TPS requirements for MARS are much more benign than for EDL trips to Earth’s surface. This means that if you would plan on reusing your vehicle in the CISLUNAR Mars environment, but never land on EARTH on your ferry trips, you could save a lot of dead heat shield weight, while still benefiting of a heat shield that can be reused almost indefinitely if you don’t load the MARVELOUS too heavy. For this reason I see the MARVELOUS as a great solution for frequent local UP-DOWN-UP ferry requirements in the MARS system, or MARS-CISLUNER orbital envelope.
Adding these surface components will increase the weight system and the vehicle that initially, to the delight of the customers, was vastly overpowered, will start to loose some of its payload capability. In this phase of the project being able to deliver 5 metric tons to LEO and desirable orbits will be sufficient. The satellite customer community is screaming for more options to get their payloads into orbit quickly and start earning on their investment. Even if this would mean paying for a more costly ride, it would shorten the period that they are loosing money as they await their launch opportunity.
At this point we have now have designed a cavernous outer shell, in a single stage to orbit package that has been made reusable and we are earning cash.
Step Three: manned reusable SSTO
Now we want to add crew. Our vehicle needs to be able to keep alive and carry the crew in all circumstances while providing enough room, for all the life support systems, radiation protection and all the gear needed on the way to and at their destination.
This can be done in a variety of manners, but remember that we already have an outer shell, probably made of carbon fibre and cladded with heat resistant coating or some form of Metallic Thermal Protection System (TPS). This would already provide some level of safety to the crew. In our proposal we would now not add a tin can inside of the design, but go the inflatable route. This means that inside our hard shell, once we reach orbit, we would inflate a Bigelow Style inflatable habitat, complete with all the support systems like solar panels, reaction systems and radiators inside that shell. Of course, the solar power systems and radiators would extend on the outside of the vehicle.
Because the inflatable sits inside of the hull, it might be possible to shed some of the protective bladders it would need and save on weight.
Another advantage of this approach is that you do not have to redesign the outer shell. One can choose to either have the inside elements attached to that outer shell or be physically separated from it. Both approaches have advantages and disadvantages when lifting up from the outside, but it would be very advantageous to be able to decouple these systems once in orbit. It would add some flexibility in that you could either choose to drop off your inflatable in orbit or once arrived to the destination or to just leave it in place as a permanent part of your MARVELOUS SSTO.
Because at first we have a limited number of vehicles in our reusable fleet, we would do some duration tests but more reasonably drop off those inflatable habitats to create a destination in orbit.
Step Four: keep on adding weight
The SSTO would have no problem lifting off from the MOON or MARS in SSTO mode carrying this inflatable habitat. The vibration environment will always be very benign. Vibration would also not be a problem when lifting off from Earth or landing with this cargo.
However, once we add weight above the capacity of the SSTO-mode, we would need to add the droptank carrying extra fuel and if required an extra set of engines that provide some extra thrust. We will make sure that these engines are the same as the once used in the upper stage maybe only differing in the rocket nozzle (which in the conformal engines is has a croissant or moon sickle shape but would be perfectly round on the droptank.
The amazing advantage of this system is, that if you need extra fuel or extra thrust, the only part you would have to redesign or play with is the length of the drop tanks and the number of engines it carries. Since you have already developed your engines for the upper stage, this cuts the R&D time considerably.
However, no one today has experience with a shape that aerodynamically looks like a hammerhead and has most thrust coming from above and some thrust coming from below. The flight and vibration characteristics of this vehicle, and the interaction between the engine thrust regimes with the drop tank structure throughout the vehicle flight will become very important research areas. In fact, the feasibility of this approach must in some form be experimented with in advance, either by running virtual sims, and wind-tunnel tests and/or combining this with sub-scale vehicles. Because hammerhead configurations do exist, have been flown and will be flown (take for example the Boeing CST-100/Atlas V configuration which has a narrow neck in the middle of the vehicle stack), this would at most be a challenge with known solutions.
Depending on how long your droptank is and how heavy you want to engine it, you could end up not only with the MARVELOUS vehicle in orbit, but with the droptank attached to it in LEO.
With a refueling run from a second stack, we can either choose to refuel both the fuel tanks of the MARVELOUS and part of the Droptank, to give it the required fuel load to continue its mission to the Moon or MARS, or we could choose to fuel only the two MARVELOUS stages in orbit, and attach both vehicle stacks end-to-end, or from the bottom of one droptank to the bottom of the other droptank.
So what do we end up with?
We end up with a vehicle that is up to 160 meters long from end to end, of which the drop tank volumes, at 3.66 m to 5 meter in diameter can be turned into pressurized corridors and you have an instant space station with the required strength to be spun around as a rotating space station.
The only missing piece in the technology puzzle, is to actually be able to equip upper stages with airlocks and to turn them into accessible and usable volumes at low cost. But that is exactly a technology program funded by NASA and being carried out by IXION.
It would pay to have them, or anybody willing to develop a competitive technology solution, as a partner in the early stages of the drop tank design. By the time our project gets to that stage we will be able to learn from their experiences and maybe get them onboard.
So as always: If you have the brains, contacts or the money, don’t hesitate to contact us to discuss the options. Let’s do this! We would have a MARVELOUS future.
ONESTAGETOSPACE is a genuine space effort seeking funds to further develop and build its MARVELOUS reusable vehicle. An innovative design allows us to decimate the operational cost of both orbital and interplanetary crewed missions, from surface to surface, over even the best industry competitors. This maturing site allows us to talk about our vehicle and to share stories about projects and people that or who inspire us on our long journey. If there is a story you feel we should cover, or a person that would like to be interviewed, let us know. Subscribe, like, share or support us with time, money or advice to help bring this project to fruition. It is ONESTAGETOSPACE, and you can become part of the adventure.