Our Reusable Fly-back Rocket
Part of the mission of Pulsar Aerospace is to help bring sustainability to the space industry. We're doing that by developing fly back capability so that our rocket can be reused rather than discarded. It is designed to work either as a first stage booster for satellite launch systems or as a stand-alone suborbital research rocket.
One of the possible uses of our system is as a booster rocket plane for the SPARTAN system. This is a satellite launch system which uses a scramjet for its second stage. Hypersonix is a company that is commercialising scramjet technology as a satellite launch system. You can read more about the SPARTAN system here.
At the moment, with few exceptions such as SpaceX, first stage rockets are single use. This is one reason why getting to space is still so expensive. Discarding a rocket after one use is not only polluting, it's expensively wasteful. It's like taking a Ferrari for one drive and then dumping it. On the other hand, flying the rocket back and reusing it massively changes that. The cost of building that rocket can be spread over many launches. This then reduces the cost of each launch.
2015 scale flight test - University of Queensland
Simple yet challenging
Our design involves adding a fold out wing and deployable propeller motor to the rocket, which can control the return, in a similar way to a drone. It is conceptually simple, but it has some challenges.
Even though it will be able to carry out standalone suborbital missions, ultimately, we intend for the Pulsar rocket to be the first stage of the SPARTAN satellite launch system. Here is an overview of what a typical launch looks like when used this way.
Launch phase
It will launch vertically with the wing stowed along the length of the rocket, as shown in figure 1. When acting as a booster for the scramjet second stage, it will accelerate up to approximately Mach 5 or 5000km/h. Its trajectory will also curve toward horizontal until it is just a few degrees above level flight in order to release the scramjet at the right angle for it to begin flying. At approximately 35km altitude, rocket burn ends, the scramjet separates, and the booster rocket begins its return to earth.
Cutaway view showing wing stowed lengthways
Hypersonic re-entry
As it re-enters the atmosphere, it is travelling at enormous speeds and the temperatures and stresses during this phase would be too great to deploy the wings, which are shaped for low speed flight. First we have to slow it down using actuators on the tail. These will have acted as fixed stabilising fins in the launch phase, but now in the glide phase they will be able to move. We will use these actuators to control the direction and rate of descent by gradually flattening the flight path. When the atmosphere is dense enough, they will even cause the spacecraft to turn upwards, to bleed off more speed.
Being able to maintain control of the spacecraft in this phase is a significant challenge. It involves a vast range of airspeed change while at the same time going from almost no atmosphere, increasing toward normal atmospheric pressure. Along with this, there may be localised turbulence, particularly as it enters into the troposphere at around 10km-15km altitude. There are many changes happening all at once and it is the actuators that will need to respond to these. This is the phase of the flight that we will be focusing on in our upcoming launch.
The Speed of Sound
Throughout this article and most of this website, we will approximate the speed of sound as 1000km/h. In practice, the speed of sound in the atmosphere varies with the square root of the temperature of the air. At ground level, it's more like 1200km/h, but in the colder upper atmosphere where the rocket reaches the higher mach numbers that we discuss, it drops closer to 1000km/h.
Gradual wing deployment
When the spacecraft speed drops below Mach 1, the wing can gradually be deployed by swinging out in stages. Even when it becomes subsonic, the spacecraft will be going too fast to have the wing swivel out all at once. We open it out partially, which helps to further slow the spacecraft as well as give the control surfaces an opportunity to stabilise each new configuration before opening out further. When the wing is swivelled out partially, one wing is angling forward and the other backward. Even though there are some asymmetrical effects from this, the differences are controllable when the deployment is gradual.
As the speed drops, the wing folds out further, until eventually it is at right angles to the fuselage.
Engine deployment
When the spacecraft has slowed to under about 200km/hr, the lightweight propeller motor can also fold out and start up. In this configuration, it can fly back to the launch site and land automatically on the adjacent runway. After a system check and any necessary maintenance work, it can be refuelled and made ready for another launch.
Flight profile when delivering the scramjet to Mach 5. image: Hypersonix
Is this design more complex than current rocket systems?
Having wings and a propeller added to a rocket may seem to be complicating an already complex system. However, there is an enormous amount of knowledge and experience in the field of aeronautics and aircraft engines, which makes this aspect of the launch profile extremely reliable and relatively simple. Yes, we are pushing boundaries of technology with regard to flying back into the atmosphere at high speed and high altitude. However, our expertise is primarily aeronautics and there is a lot of value that can be added in a space system to make a launch more efficient. This is particularly true with the first stage, where a large part of its trajectory is within the atmosphere and temperatures of re-entry are lower. We have done a lot of ground testing in wind tunnels and shock tunnels and we are confident that we will be able to prove aspects of our design with our upcoming test launches.
The other alternative to reusability is the path that companies like SpaceX and Blue Origin have taken, where they use the existing rocket propulsion to land. This has been a remarkable achievement, and while it seems commonplace now, you may remember how it seemed surreal when you first watched a SpaceX rocket reversing down to the launch pad and setting down perfectly. This is a better option when the objective is to fly to the Moon or Mars, which has been the intention of SpaceX from the outset.
Our design won't work on Mars because the atmosphere there is only 1% of the pressure of earth's atmosphere, let alone the fact that it has insufficient oxygen for an air aspirated engine. So it makes sense for SpaceX to develop a system that works both here and on Mars and the Moon. On the other hand, for us, being able to design something specifically for the earth allows us to take advantage of our atmosphere and to make use of many decades of aeronautical technology to make our system cheaper and achieve a reliability and turnaround capability approaching that of commercial airlines.
Can it take humans?
No, not for now. We need to cut our teeth on a system that doesn't risk lives. The simpler we can make our system, the sooner we can become self-sufficient financially. From there we can continue to iterate and learn from each launch, and develop safer systems. Only when we have developed redundancy systems and proven the necessary level of reliability could we consider human flight.