Blow torching space debris
While considering business case for using BFR as space debris cleanup vehicle, I was amazed by a number of different methods proposed for active debris removal. There are hooks, fishnets, robotic arms, foams, foils, lasers etc.
But in essence, all of active debris methods require some momentum change that will be used to lower the debris orbit so low that atmospheric drag can do the rest. For majority of these efforts the major risk lies in rendezvous and capture with uncooperative target which may (and probably will) spin uncontrollably. So the ideal way would be to perform some kind of momentum exchange between visiting vehicle (such as BFR) and uncooperative target without making actual physical contact.
Firing lasers or electrostatic charged dust on the target are some forms of contact-less momentum exchange. But what orbital vehicles like BFR have in abundance? A lot of spare propellant. Visiting vehicle can approach to close proximity in front of the uncooperative target and then fire its engines so that the exhaust plume pushes against the uncooperative target, effectively slowing it down. This is different from typical Earthly applications of blow torches, which are used to burn, melt or cut metal. In this application, main goal is to transfer momentum from the exhaust plume to the uncooperative target.
Since plume expands very fast in vacuum, engine exhaust must be placed very close to the target. Also overall efficiency will be quite small due to rapidly expanding plume with distance in vacuum. But if the visiting vehicle has a lot of spare fuel, efficiency is not such a great issue. Mass of the visiting vehicle should be much greater that the uncooperative target. Both of these assumptions hold true for BFR compared to typical large debris found in LEO.
How could the efficiency be improved? Well, it is not necessary to utilize main or RCS engines for such operation. Imagine instead having a propellant fuel lines on an extendable/retractable mast which will have a small rocket engine at the end. Visiting vehicle would position itself as close to the target as considered safe (lets assume ten meters from the target). After that the mast would be extended to close proximity of the target and small engine would be ignited. That would cause a slow acceleration of visiting vehicle and (even slower) deceleration of the target. At the same time, BFR would activate its RCS engines that would slow down BFR at the same rate as the target. Effectively, relative speed of BFR and uncooperative target would stay the same, and would thus allow BFR to change the perigee of the target in a single orbit.
This process would not put visiting vehicle in any risk of collision. Also, rotation of the target would not make much difference to the overall process. If there is a problem (such as damage to the mast or rocket engine), whole mast could be disconnected without affecting visiting vehicle. With enough testing, momentum exchange can reach 100 percent (for example, if mast can be safely extended to nearly touch the uncooperative target).
How efficient would it be? Lets assume that uncooperative target is in 800x800 SSO circular orbit, with 2000kg weight. Lets assume 10% momentum exchange efficiency and 2000 m/s exhaust speed of metholox RCS engines. To lower the perigee to 100km, whole system would need to be slowed down by approximately 270m/s. Now the additional impulse spent on the uncooperative target would be 10x times larger (due to 10% efficiency), so BFR would need to spend 2700kg of fuel to slow down uncooperative target (that would be fired prograde) and additional 2700kg of propellant to counter prograde impulse. So by spending additional 5-10t of propellant, (not taking into account initial rendezvous delta-V budget), BFR would be capable of de-orbiting larger satellites or Centaur upper stages from most LEO and SSO orbits. The same approach can be used for quick de-orbiting of GTO debris which might require even lower propellant reserves.
Although this concept can be used and tested on existing vehicles (for example, Cygnus or even Dragon), BFR is ideal primarily because of its large mass and propellant reserves. Since it would use the main propellant for attitude control and orbital maneuvers, the same engine can be used for deorbiting uncooperative targets without putting BFR at risk. Proposed approach is ideal for secondary missions where spare propellant is available. For example, lets assume that is used to launch a single satellite to GTO. BFR has around 20t payload capacity for direct to GTO launches. If we take that most current satellites are under 7t, that gives just enough reserves to perform a single de-orbit secondary mission. But lets assume a single GTO satellite mission, and four LEO refuel flights. After delivery of satellite to GTO, BFR could undertake multiple de-orbit missions before landing. The best part - it does not require much of hardware development. Major effort would be focused on mission planning and attitude control/rendezvous software.
But in essence, all of active debris methods require some momentum change that will be used to lower the debris orbit so low that atmospheric drag can do the rest. For majority of these efforts the major risk lies in rendezvous and capture with uncooperative target which may (and probably will) spin uncontrollably. So the ideal way would be to perform some kind of momentum exchange between visiting vehicle (such as BFR) and uncooperative target without making actual physical contact.
Firing lasers or electrostatic charged dust on the target are some forms of contact-less momentum exchange. But what orbital vehicles like BFR have in abundance? A lot of spare propellant. Visiting vehicle can approach to close proximity in front of the uncooperative target and then fire its engines so that the exhaust plume pushes against the uncooperative target, effectively slowing it down. This is different from typical Earthly applications of blow torches, which are used to burn, melt or cut metal. In this application, main goal is to transfer momentum from the exhaust plume to the uncooperative target.
Since plume expands very fast in vacuum, engine exhaust must be placed very close to the target. Also overall efficiency will be quite small due to rapidly expanding plume with distance in vacuum. But if the visiting vehicle has a lot of spare fuel, efficiency is not such a great issue. Mass of the visiting vehicle should be much greater that the uncooperative target. Both of these assumptions hold true for BFR compared to typical large debris found in LEO.
How could the efficiency be improved? Well, it is not necessary to utilize main or RCS engines for such operation. Imagine instead having a propellant fuel lines on an extendable/retractable mast which will have a small rocket engine at the end. Visiting vehicle would position itself as close to the target as considered safe (lets assume ten meters from the target). After that the mast would be extended to close proximity of the target and small engine would be ignited. That would cause a slow acceleration of visiting vehicle and (even slower) deceleration of the target. At the same time, BFR would activate its RCS engines that would slow down BFR at the same rate as the target. Effectively, relative speed of BFR and uncooperative target would stay the same, and would thus allow BFR to change the perigee of the target in a single orbit.
This process would not put visiting vehicle in any risk of collision. Also, rotation of the target would not make much difference to the overall process. If there is a problem (such as damage to the mast or rocket engine), whole mast could be disconnected without affecting visiting vehicle. With enough testing, momentum exchange can reach 100 percent (for example, if mast can be safely extended to nearly touch the uncooperative target).
How efficient would it be? Lets assume that uncooperative target is in 800x800 SSO circular orbit, with 2000kg weight. Lets assume 10% momentum exchange efficiency and 2000 m/s exhaust speed of metholox RCS engines. To lower the perigee to 100km, whole system would need to be slowed down by approximately 270m/s. Now the additional impulse spent on the uncooperative target would be 10x times larger (due to 10% efficiency), so BFR would need to spend 2700kg of fuel to slow down uncooperative target (that would be fired prograde) and additional 2700kg of propellant to counter prograde impulse. So by spending additional 5-10t of propellant, (not taking into account initial rendezvous delta-V budget), BFR would be capable of de-orbiting larger satellites or Centaur upper stages from most LEO and SSO orbits. The same approach can be used for quick de-orbiting of GTO debris which might require even lower propellant reserves.
Although this concept can be used and tested on existing vehicles (for example, Cygnus or even Dragon), BFR is ideal primarily because of its large mass and propellant reserves. Since it would use the main propellant for attitude control and orbital maneuvers, the same engine can be used for deorbiting uncooperative targets without putting BFR at risk. Proposed approach is ideal for secondary missions where spare propellant is available. For example, lets assume that is used to launch a single satellite to GTO. BFR has around 20t payload capacity for direct to GTO launches. If we take that most current satellites are under 7t, that gives just enough reserves to perform a single de-orbit secondary mission. But lets assume a single GTO satellite mission, and four LEO refuel flights. After delivery of satellite to GTO, BFR could undertake multiple de-orbit missions before landing. The best part - it does not require much of hardware development. Major effort would be focused on mission planning and attitude control/rendezvous software.
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