CREATE TABLE table_203_823 (id DECIMAL, "model name" TEXT, "date" TEXT, "type" TEXT, "thrust  ( kg )  / power  ( eshp ) " TEXT, "fitted to" TEXT)

what is the difference in thrust between the al-7 and al-21 ?
SELECT ABS((SELECT "thrust (kg) / power (eshp)" FROM table_203_823 WHERE "model name" = 'al-7') - (SELECT "thrust (kg) / power (eshp)" FROM table_203_823 WHERE "model name" = 'al-21'))
SELECT ABS((SELECT "thrust (kg) / power (eshp)" FROM "table_203_823" WHERE "model name" = 'al-7') - (SELECT "thrust (kg) / power (eshp)" FROM "table_203_823" WHERE "model name" = 'al-21'))