"The Sometimes Invisible Spaceship" by Charles Bowles (1988)
Solely on title and date
A review and details about the book can be found here: The Sometimes Invisible Spaceship by Charles Bowles
From the back cover:
During summer vacation from school, a brother and sister (Jeef and S.L. Strewbury) must work to help their widowed mother (Maria) make a living.
S.L. (Sabedora Louise) is younger than her brother; but, always
speaking with clarity and volume, she is equally blunt, daring, and
self-assertive. She is able, for instance, to drive a farm tractor
though her legs are too short for her feet to touch the floor pedals.
Jeef (Jefferson Francisco) would rather fish or wander the woods with
his dog Sam, but he recognizes the need to earn money. He has a
stronger urge toward adventure than he will readily admit.
The youngsters become involved with a brilliant, conceited, and
bumbling scientist (Professor Szrplqx) when they accidentally discover
his secret base of operations. The professor has developed an unusual
spacecraft, which he is testing before beginning an exploration of
outer space. Among his other achievements is a device (amatrivance) he
invented "on a whim," which can force any person or animal to like and
obey its operator. When the professor's former assistant (Zarch) gains
control of the amatrivance, the professor fears the entire universe
will come under the rule of an evil dictator.
A mixed crew, made up of the Strewbury youngsters, Sam, the professor,
and a greeble named Doctor Harkel, sets out to save the universe.
This book was published in 1987 by Aegina Press, a vanity publisher. It's well done. That is, it's been proofread for typos and such, and the paper and binding are fine. But the typesetting has very small margins for a children's book, and the cover is pretty spare. Still, not a bad effort.
The absent-minded Professor Szrplqx (pronounced Zerplix) is annoying, but not half so irritating as S.L., who always talks in ALL CAPITAL LETTERS, and is continually announces that SHE"S HONGRY. Not hungry, HONGRY.
Professor Szrplqx has built a spaceship named GLOMBUS, and he takes Jeef and S.L. out to explore the galaxy. They rescue a pink alien named Doctor Harkel. Szrplqx has also invented a device called an amatrivance, which compells love and obedience. Of course the professor's nemesis, Zama Z. Zarch, steals the device and escapes in his own spaceship.
Szrplqx and the crew chase Zarch to the planet Kimera. They meet Sir Gruel, a medieval knight on horseback. It's like a whole renaissance fair planet. Kimera is ruled by Queen Whoomit I, and she sends one of her knights--Sir Quembly, because why not add more characters?--to help Szrplqx. The Professor and the children and the knight eventually catch Zarch, who turns out a be a woman. Yes, Zarch was a woman all along. She used to be Szrplqx's assistant, and she only stole the amatrivance to teach him a lesson about being responsible. She has no desire for conquest. Um. OK.
Of course Zarch and Szrplqx get married.