I have solved and simulated a continuous thrust rendezvous problem recently. The maximum total acceleration is about 5 m/s^2 and the specific impulse is 1000 sec. I need to know:
Note: the mission is the rendezvous of a chaser from a 1000 km altitude circular orbit with a target in a 3000 km circular orbit. The chaser is a small 2000 kg typical spacecraft with continuous thrust propulsion system (Thrust magnitude is variable).
Hi, in general a propulsion system is characterized by two values:
F : maximum thrust modulus (N)
Isp : specific impulse (s)
The modulus of the acceleration produced by the thrust F/m depends on the mass m that varies according to:
dm/dt = -F/g0/Isp where g0 = 9.80665 m/s^-2
Many thanks for your answer Richard Epenoy . In my case, the mass of spacecraft is 2000 kg and the required acceleration is approximately 5 m/sec^2. So, it needs 10 kN of thrust. The mission time is approximately 10*T where T is the period of the target orbit (circular orbit with 3000 km altitude). The question is:
What kind of propulsion system should be used for this mission? Chemical? What is the appropriate Isp? I have chosen 1000 sec. Is that practical (as I know the Isp for regular chemical propulsion system is approximately 300 sec)?
Hi, 10kN is a very high thrust level. I don't know if such a level is available with a classical chemical engine. Usually the Isp of chemical engines is close to 300 s, you are right.
The current state of the art for high-power, continuous thrust electric propulsion systems yield about ~225-250 mN (0.225-0.25 N) of thrust per engine, operating at the maximum throttle setting. The Thrust/Power and Isp will vary depending on the exact type of electric propulsion technology. Additionally, throughput (maximum propellant expended before failure) is an important concern as you would need to plan on using more than one engine to expend your planned propellant. Here are a few examples:
Ion Thruster
ex) NEXT-C
Max Power: ~7 kW
Max Thrust: ~225 mN
Max Isp: ~4,000 seconds
Throughput: ~600 kg
Hall Thruster
ex) XR5
Max Power: ~4.5 kW
Max Thrust: ~250 mN
Max Isp: ~2,000 seconds
Throughput: ~450 kg
Check the literature for the latest values but these are in the neighborhood.
It looks like your specific application may be orders of magnitude away from current technology. At the end of the day the thrust limitation for electric propulsion is a power problem. Solar arrays can only be so large, and in-space use is currently pushing about ~40 kW for unmanned applications with longer term goals of getting to the 100's.
Alternatively, more power is attainable via nuclear sources, but the state of the art has lagged since the mid 2000's with the cancellation o JIMO. It's also controversial for use in space anywhere near the Earth and is rarely used other than to power satellites for deep space use (not for propulsion). This may change in time, but currently the focus for propulsion is driven by solar given the proliferation of electric propulsion applied to to Earth orbit raising and station keeping GEO to reduce cost.