# Can a solar sail be added to an ion engine and work better?

Has there been a concept vehicle that uses both solar sails and ion engines? Does the added mass of each system increase or reduce the spacecrafts total delta V capabilities?

Inspired by S.F.

As S.F. said the solar sail could be used as a parabolic mirror to direct light onto the electric engine along with providing propulsion. As the engines journey away from the Sun the sail would adjust keeping the ion engine at 100%, but the surface area of the sail will decrease.

• Solar sail is a slight misnomer, so "sails stay full" doesn't have a clear meaning in this context. – uhoh Jul 13 '18 at 0:15
• Both of these propulsion methods requrire heavy equipment and provide only very little thrust, thus the spacecraft's inertia would be increased. It makes more sense to choose whichever system is more mass-efficient for the mission and add enough of it to achieve the mission goals. – Rikki-Tikki-Tavi Jul 13 '18 at 1:15
• @CrisR Solar sails are heavy, and so are the power supplies for electrical engines. That doesn't mean that they can't provide the lowest overall system weight. If that was the case, there would be no point to them at all. – Rikki-Tikki-Tavi Jul 13 '18 at 6:03
• Personally, I see it this way: Make the sail concave. Working at full strength near Earth, acting as a focusing mirror on solar panels of the probe further away, making it possible to keep the ion engine powered. m^2 for m^2 the sail is much lighter than panels, and even if it absorbs a good part of the light, it would still focus enough to keep the panels lit similarly as they'd be lit directly near Earth. (the probe core would need to be able to move into/out of the focus area not to get fried near Earth.) – SF. Jul 13 '18 at 8:46
• @rikki you make you a valid point – ChrisR Jul 13 '18 at 18:55

Intro

This is a tricky question, primarily because solar sails benefit from a low mass spacecraft and with the addition of an electric propulsion system, you are increasing the mass decreasing the effective of acceleration and total delta V capabilities.

System Architecture and Supporting Systems

Solar Sails – Solar sails require photons to gain acceleration and as a solar sail propelled spacecraft moves further away from the sun, the sails effectiveness at propelling the spacecraft forward is greatly dimensioned. The supporting systems required to deploy a sail are relatively mass-light compared to chemical or electric propulsion systems. For the solar sail propulsion system, you have the mass of the deployable booms, Mylar sail, deployment mechanism, and sail control system.

Solar Sail ISP Metric:

Where:

• a0 = Sail characteristic acceleration
• T = Mission duration (usually mission acceleration duration)
• R = Payload Mass Fraction (mass_1/mass 2), where mass_1 is mass of sail and mass_2 is mass of payload or spacecraft.
• G = Gravity (-9.80665 m/s^2)

This quickly shows that as you increase the spacecraft mass (combining a solar sail + electric propulsion system together) decreases the overall systems ISP metric

Electric Propulsion System – These are extremely complex and elaborate systems as their performance is a function of several variables including power, spacecraft mass, engine IPS, etc… While I am by no means an expert in EP systems they do require a lot of supporting systems when compared to a chemical or solar propulsion system.

• Large power requirements – On average (depending EP system selected) 150W - 2kW of power – Aerojet’s Electric Propulsion Catalog. There are two main options of supplying power. First option is using solar arrays, however they’re power generation drops off at r^2. As an example, you generate 1/27 the power at Jupiter as you do at Earth. The second option is using a radioisotope thermal generator (RTG) which are heavy, extremely expensive, and only produce around 80-120 W and with NASA’s next generation RTG (MMRTG) only capable of producing around 110W, it’s unlikely that an ION RTG powered spacecraft will occur.
• Requires the addition of propellant tanks and propellant management systems (this includes heaters because fuel must remain at operational temperatures).
• Additional hardware and electronics to manage the complex electric propulsion system.

Conclusion

This brings me to my last point, combing these two systems together brings up new problems of how to manage them independently of each other. When you combine a solar sail and electric propulsion spacecraft, together you need a way to support the solar sails systems as well as the electric propulsion systems (structurally, hardware wise, electrically) and doing this greatly increases the total spacecraft mass and complexity involved. Ultimately it comes down to design complexity and does the added mass of each system increase or reduce the spacecrafts total detla V capabilities.

Python Code for ISP Relationships:

You can play around a view the relationships between the two systems.

'''
Requirements:
- Python: 2.7 or 3.6
- Python Numpy
'''

def solarSailISP(characteristic_acceleration, mission_duration_days, mass_ratio):
import numpy as np
gravity = -9.80665
delta_v = characteristic_acceleration*mission_duration_days*86400.0
isp = (delta_v/gravity)/(np.log(mass_ratio))
return isp

def engineISP(delta_V, mass_ratio):
import numpy as np
gravity = -9.80665
isp = (delta_V/gravity)/(np.log(mass_ratio))
return isp

solarsail_isp = solarSailISP(0.001, 500, 0.1)
electric_isp = engineISP(3200, 0.3)

print("Solar Sail Propulsion System ISP: {} sec").format(solarsail_isp)
print("Electric Propulsion System ISP: {} sec").format(electric_isp)

• Did you take in the added design of the picture in the question? – Muze Aug 5 '18 at 17:31

I am not aware of any designs combining the two concepts. As noted in the various comments there is some overlap in that both are low thrust/continuous operation designs but they have different optimal trajectories so a given craft would generally be better off being lighter and able to optimize for a single thrust type.

Ion drive control systems on a massive solar sail might make sense, or if there is some magic fabric that can be both a solar sail and a solar panel. Both would depend on large scale resource extraction in space and are well outside current planning.

With the second half of your question most solar sail designs are getting most of the thrust from light though solar wind can play a role was well, in either case thrust would only reduce at some measurable fraction of the speed of light and the very function of a solar sail means it cannot get much above solar system escape velocity (0.001% c) because it leaves the star providing the thrust.

A very poorly design mixed thrust craft might collapse the sail supporting structure with excessive forces, but the sail would still be producing thrust during this process.

Ikaros had solar panels embedded in the solar sail

The sail, shaped somewhat like a kite, also has solar cells embedded to generate electricity. It was not expected to make much power during this flight, but more to serve as a test bed for future ion propulsion-engines that not only use solar cells for electricity, but also are moved along by sails.

There's some effort to make thin film photovoltaics that enjoy lots of surface area for little mass thus improving the alpha. And for solar sails you also want to maximize surface area while minimizing mass.

So the two technologies have goals in common. The Japanese Ikaros was an exciting mission.

These are two different types of propulsion systems which are still in theoretical stage and have to go across lot of vigorous research.

In my opinion having solar sails and ion engines , provides good acceleration and increases the redundancy of the spacecraft and by the way , yes it is feasible , because ion engines works on the principle of positive charges , while solar sails works due to the solar wind , although there is a chance of interference.

• Neither solar sails nor ion engines are theoretical. Both have successfully flown spacecraft to their destinations. For the rest of the answer, it is pretty vague considering the number of concepts being covered. I think this answer needs more explanation and links to references. – kim holder Jul 13 '18 at 0:09
• Welcome to Stack Exchange! You can take the tour and read about how to write good answers in the help center or just take a look at other answers here. Generally "In my opinion" is not a good way to answer an SE question. – uhoh Jul 13 '18 at 0:17
• Ion engines, in particular, are used for station-keeping in a large fraction of currently-deployed satellites, and have been used for half a dozen or more interplanetary probes already. They are in no sense in "theoretical stage". Even solar sails have been used in at least one, I believe two, successfully deployed satellites. – Nathan Tuggy Jul 13 '18 at 2:48
• Solar sails have never been flown , only beam based propulsion has been used to adjust the trajectories and ion thrusters we're used by Nasa in the 1960s as an experiment and it is not at all feasible even with the present technology , the experiment did not fully utilize the ion thrusters concept , hence I can conclude they are still in the theoretical stage – Bharath Simha Reddy Jul 13 '18 at 7:37
• Ion engines are definitely TRL 9. Solar sails might be 7 (demonstrated in space but not in use except in demonstration missions). Neither are theoretical at this point. – Erin Anne Jul 14 '18 at 4:55